MALE SPEAKER: Thank you for coming, everybody.
Some of you have probably already
heard of Linus Torvalds.
Those of you who haven't, you're the people with
Macintoshes on your laps.
He's a guy who delights in being cruel to people.
His latest cruel act is to create a revision control
system which is expressly designed to make you feel less
intelligent than you thought you were.
Thank you for coming down today, Linus.
I've been getting emails for the past few days from people
saying, where's Linus?
Why hasn't he measured my tree?
Doesn't he love me anymore?
And he walked into my office this afternoon.
What are you doing here?
But thank you for taking the time off.
So Linus is here today to explain to us why on Earth he
would write a software tool which only he is smart enough
to know how to use.
Thanks, Linus.
LINUS TORVALDS: So I have a few words of warning, which is
I don't actually do speaking very much, partly because I
don't like speaking, partly because over the last few
years everybody actually wants me to talk about nebulous
visions for the next century about Linux.
And I'm a tech geek, so I actually prefer talking about
technology.
So that's why I am not talking about the kernel, because it's
just too big to cram into a one-hour talk.
Although apparently, Andrew did that two days ago.
And I'm instead talking about Git, which is the source
control management system that we use for the kernel.
I'm really, really, really bad at doing slides, which means
that if we actually end up following these slides, you
will be bored out of your mind and the talk will probably not
be very good anyway.
So I am the kind of speaker who really
enjoys getting questions.
And if that means that we kind of veer off in a tangent,
you'll be happier, I'll be happier, the talk will
probably be more interesting anyway.
I don't know how you do things here at the Google talks, but
I'm just saying don't feel shy as far as I'm concerned.
If your manager will shoot you, that's your problem.
So next slide.
I want to give a few credits before I start.
Credit CVS in a very, very negative way because in many
ways when I designed Git, it's the what would Jesus do?
Except it's what would CVS never, ever do kind of
approach to source control management.
I've never actually used CVS for the kernel.
For the first 10 years of kernel maintenance, we
literally used tarballs and patches, which is a much
superior source control management system than CVS is.
But I did end up using CVS for seven years at a commercial
company and I hated it with a passion.
When I say I hate CVS with a passion, I have to also say
that if there are any SVN users in Subversion, users in
the audience, you might want to leave because my hatred of
CVS has meant that I see Subversion as being the most
pointless project ever started, because the slogan
for Subversion for a while was, CVS done right or
something like that.
And if you start with that kind of slogan, there's
nowhere you can go.
There is no way to do CVS right.
So that's the negative kind of credit.
The positive credit is BitKeeper.
And I realize that a lot of people thought there was a lot
of strife over BitKeeper and that the parting was very
painful in many ways.
As far as I'm concerned, the parting was amicable, even
though it looked very non-amical to outsiders.
And BitKeeper was not only the first source control system
that I ever felt was worth using at all, it was also the
source control system that taught me why there's a point
to them and how you actually can do things.
So Git in many ways, even though from a technical angle
it is very, very different from BitKeeper, which was
another design goal because I wanted to make it clear that
it wasn't a BitKeeper clone, a lot of the flows we use with
Git come directly from the flows we
learned from BitKeeper.
And I don't think you use BitKeeper here inside Google.
As far as I know, BitKeeper is the only commercial source
control management system that actually does distribution.
And if you need a commercial run, that's the one you should
use, for that reason.
I'd also like to point out that I've been doing Git now
for slightly over two years, but while I started it and I
made all the initial coding design, it's actually being
maintained by a much more pleasant person, Junior
Hermano, for the last year and a half.
And he's really the person who actually made it more
approachable for mere mortals.
Early versions of Git did require a certain amount of
brainpower to really wrap your mind around.
It's gotten much, much easier since.
Obviously the way I always do everything is I try to get
everybody else to do as much as possible so that I can sit
back and sip my pina colada, so there's been a lot of other
people involved, too.
That's the credits.
With those out of the way.
So this slide is now one day old, and I didn't actually do
the slides last night because last night I was out carousing
and eating sushi.
But the slides will talk about implementation of a high
performance distributed content management thing.
And the keyword here is actually the distributed part.
I will start off trying to explain why
distribution is so important.
If we never get past that point, I
will actually be happy.
If we never get to actually what Git implementation
internally is, it's fine.
I am not also trying to teach you how to use Git.
There is this thing called google.com.
You may have seen it.
It has this thing you can type things into.
You type Git and then you press the I'm Feeling Lucky
button, and you will actually get the home page.
The home page has tutorials, it has the user manual,
they're all in HTML.
If you actually want to learn to use Git, that's where you
should start, not at this talk.
But as mentioned, if we actually start veering off
topic into other tangents because of
questions, it's all good.
I already gave you kind of a heads up warning on this.
I use the SCM, which I consider to mean Source Code
Management, that is, revision control.
Some other people think SCM means Software Configuration
Management and see it as a much bigger feature, including
release management and stuff like that.
That's not what I'm talking about, although Git is clearly
relevant in that setting, too.
CVS, we already went there.
You can disagree with me as much as you want, but during
this talk, by definition anybody who disagrees is
stupid and ugly.
So keep that in mind.
When I'm done speaking, you can go on with their lives.
Right now, yes.
I have strong opinions and CVS users, if you actually like
using CVS, you shouldn't be here.
You should be in some mental institution somewhere else.
So before actually go and talk about the whole distribution
thing, which I think is the most important part, I'll talk
a bit about the background because it invariably comes up
because people, if they have heard about Git, a lot of the
things they've heard about is the background for doing it in
the first place.
One piece of background information is I really am not
an SCM person.
I have never been very
interested in revision control.
I thought it was evil until I met BitKeeper.
I actually credit that to some degree for why Git is so much
better than everything else.
It's because my brain did not rot from years and years of
thinking CVS did something sane.
I needed a replacement for BitKeeper.
The reason for that was BitKeeper is a commercial
product, but BitMover and Larry McVoy allowed it to be
used freely for open source projects, as
some of you may know.
The only restriction was you were not supposed to reverse
engineer it and you weren't supposed to try to create a
competing product.
And I was happy with that because, quite frankly, as far
as I'm concerned I do open source because I think it's
the only right way to do software.
But at the same time, I'll use the best tool for the job and,
quite frankly, BitKeeper was it.
However, not everybody agreed with me.
They are ugly and stupid.
But they cause problems and it resulted in the fact that
Larry and I had several telephone conversations which
ended up saying we'll all be much happier if we just part
ways and don't make this any worse.
So we did.
And I made the Linux 2.6.12-rc2 release about two
years ago and said, I'm not going to touch Linux until I
have a replacement for BitKeeper for doing source
code maintenance.
And one of the replacement options was going back to
tarballs and patches, but nobody
really liked that anymore.
So I actually looked at a lot of alternatives.
Most of them I could discard without even trying them out.
If you're not distributed, you're not worth using.
It's that simple.
If you perform badly, you're not worth using.
It's that simple.
And if you cannot guarantee that the stuff I put into an
SCM comes out exactly the same, you're not worth using.
Quite frankly, that pretty much took care of
everything out there.
There's a lot of SCM systems that do not guarantee that
what you get out of it again is the same thing you put in.
If you have memory corruption, if you have disk corruption,
you may never know.
The only way you'll know is you notice that there's
corruption in the files when you check them out.
The source control management system does not protect you at
all, and this is not even uncommon.
It is very, very common.
The performance issue, one of the things I kind of liked was
a system called monotone, which actually, I think there
was a talk at Google about them some time
ago, I'm not sure.
It had a lot of interesting ideas, but performance was so
horrendously bad that I tried it for a day and realized that
I cannot use it.
The end result was I decided I can write something better
than anything out there in two weeks, and I was right.
So now we get to distribution.
And this is the worst slide of them all, and I'm not very
proud of it.
And the problem is distribution is really, really
important, but when I tried to make slides about it I
could not do it.
And part of it is my obvious artistic talents, which are on
display for all of you, but part of it is that it's really
hard to explain.
So before you can start, I'd like to know how many people
are used to the notion of a truly distributed source
control management system?
Are most of you kernel developers?
No, OK.
So there were maybe 10 hands coming up.
Being distributed very much means that you do not have one
central location that keeps track of your data.
No single place is more important than any other
single place.
So for example, this is why I would never touch Subversion
with a 10 foot pole.
There is a massive Subversion repository, and it's where
everybody has to write.
The centralized model just doesn't work when you want to
be-- let's look at a few of the cases.
I say it's so much more than just offline work, but the
offline work part is actually maybe the most obvious thing,
which is that you can take a truly distributed source
control management system, you can take it on a plane and
even if they don't offer Wi-Fi and satellite hookups, you
just continue working, you can look at all your logs, you can
commit, you can do everything you would do even if you were
connected to a nice gigabit ethernet
directly to the backbone.
And that is really important.
It is doubly important when you have hundreds or thousands
of people working on the same project and they may not be
literally disconnected, but in practice they aren't really
well-connected either.
So part of distribution is this offline work theme.
Even if it's not completely offline, it is important to be
able to do everything you want to do from any location
without having to be able to access the server.
What that basic fact actually results in is that you
effectively have a lot more branching because everybody
who has a complete repository and can do commits on his own
will effectively have his own branch, even if you don't
realize it.
Even if you think of your project as just having a
single branch, every single time you disconnect your
laptop and start working with it, you
are on your own branch.
And this is really, really important and is very
different from anybody who's used CVS, where branching is
considered something that only true gurus do.
How many of you have ever used CVS?
OK, everybody.
How many of you have really done a branch and ever
merged it in CVS?
Good job.
I mean, it wasn't everybody but it was actually more than
I expected.
How many of you enjoyed the experience?
OK, so there were a couple.
But it is considered hard.
In CVS, when you merge a branch--
I've done it as little as possible, but
I've had to do it--
what you do is you plan ahead for a week and then you
basically set aside one day for doing it.
Am I wrong?
I'm not seeing a lot of people say no, it was easy.
I liked it.
It's horrible.
If you're distributed, you have to realize that every
single person has his own branch.
It's horrible.
It's not something you even have to set up.
It just is.
In fact, in Git, we like branches so much that a lot of
people just have five or ten or fifteen of them.
Just because once you realize that you have to have a
special branch anyway, you might as well have many.
And one of the branches you do some experimental work on and
one of the branches you do maintenance on.
So branching is much more inherent when you do
distribution.
One of the other things that, to me, is very important is
that by being distributed, you also automatically get to be
slightly more trustworthy.
I have a theory of backup switches.
I don't do them.
I put stuff up on one side and everybody else mirrors it.
And if I crash my own machine I don't really care, because I
can just download my own work right back.
And it works beautifully well, and I don't have to have an
MIS department.
I heartily suggest everybody else do the same.
But this only really works in a distributed environment.
If you use CVS, you can't do this.
What do you use here?
Perforce?
Perforce.
I'm sorry.
I'm sure it's better than CVS. [WHISPERS].
So that's part of it.
One of the really nice things which is also--
maybe you don't have this issue inside a company, but we
certainly have it in every single open source community
I've ever seen that uses CVS or Subversion or something
like that-- is you have this notion of commit access.
Because you have a central repository, it means that
everybody who is working on that project needs to write to
the central repository, which means that since you don't
want everybody to write to the central repository because
most people are morons, you create this class of people
who are ostensibly not morons.
And most of the time, what happens is you make that class
too small because it's really hard to know if a person is
smart or not, and even when you make it too small, you
will have problems. So this whole commit access issue,
which some companies are able to ignore by just giving
everybody commit access, is a huge psychological barrier and
causes endless hours of politics in
most open source projects.
If you have a distributed model, it goes away.
Everybody has commit access.
You can do whatever you want to your project.
You just get your own branch, you do great work or you do
stupid work.
Nobody cares.
It's your copy, it's your branch.
And later on, if it turns out you did a good job, you can
tell people hey, here's my branch.
And by the way, it performs 10 times faster than anybody
else's branch, so nyah nyah nyah, how about
pulling from me?
And people do.
And that's actually how it works, and we
never have any politics.
That's not quite true, but we have other politics.
We don't have to worry about the commit access thing.
And I think this is a huge issue and that alone should
mean that every single open source system should never use
anything but a distributed model.
You get rid of a lot of issues.
One of the things that commercial companies,
distributed models actually help also
with the release process.
You can have a verification team that has its own tree,
and they pull from people and they verify it.
And when they've verified it, they can push it to the
release team and say, hey, we have now verified our version.
And the development people, they can go on playing with
their head.
Instead of having to create tagged branches, whatever you
do to try to keep off each other's toes, again, you keep
off each other's toes by just every single group can have
its own tree and track its work and what they want done.
So distributed is really, really central to any SCM you
should ever use.
So get rid of Perforce now.
[APPLAUSE]
LINUS TORVALDS: It's sad, but it is so, so true.
That was my only real slide about distribution.
I'd love to get questions, because we're now moving into
other areas that--
AUDIENCE: So how would you do it?
If you had this monstrously awesomely big code base, and
you wanted to use this without stopping business for six
months, how would you do it?
LINUS TORVALDS: Stay by the mic because I couldn't quite
make out your question.
OK, he went away.
How would you do this?
AUDIENCE: [INAUDIBLE].
LINUS TORVALDS: So an example of actual distribution is you
have a group of five people working on one small,
particular feature.
And that means that for a while, that feature will be
very, very broken, right?
Because nobody actually creates perfect code the first
time around except me, but there's only one of me.
So what happens is they need to have their own tree that
they can work in without affecting other people.
You can do this many different ways.
In CVS, one of the most common ways, because branches are so
painful, is that you don't actually commit.
You never commit until it passes every single test. For
example, at your company you have a very strict committing
rule saying you will never, ever commit until it's past
the whole test suite.
And by the way, the fact that the test suite takes two hours
to run, tough.
You cannot afford to commit.
And this is something that happens at
every single company.
I bet it happens even here at Google.
You probably have a strict test suite, and you are not
supposed to commit unless it passes.
And then in practice, people make one-liner changes and
ignore the test suite because they know the one-liner
changes can't possibly break.
This happens.
This is a horrible, horrible model.
It just means that you make huge commits because you
commit something after you've worked on it for two weeks,
and you have three people working in the same sandbox
because before they commit, they can't see the changes
that the other people made.
This is common.
It happens everywhere, it's scary.
The other alternative is to use branches even in a
centralized environment.
But branches always end up being pretty expensive to do,
so you can't do them for experimental features.
You don't know beforehand if it's something that's going to
take one day or two weeks, but most of the time most
programmers say, hey, I can do this in 48 hours.
And it turns out, yeah, no you couldn't.
But because you feel you can do it in 48 hours, creating a
branch, even in systems that are better at creating
branches than CVS, is a big pain.
So you don't do it because you think you can get it resolved
and you're back to case number one.
But if you decide to create a branch, you will affect
everybody else's repository because in a centralized
environment, branches are global.
So you're kind of screwing with everybody else, but at
least you're not screwing with their main, head branch.
You are adding stuff to their repositories, but hopefully in
a way that they won't notice.
But it does make everybody's repositories bigger.
So either way, you can't win.
In contrast, in a distributed environment, what you do is
you have five people, they pull the current head, which
is hopefully good and tested, and they start working on it
and they start committing on it.
And you don't need to wait for two weeks until your commits
are stable because your commits are always local.
And what happens is within that group of five people, you
can pull from each other.
That's what distributed means.
There's no central location, it means everybody's the same.
So you can merge between yourself.
So not only can you commit every single line if you want
to without having to run the two-hour test suite, but you
can then communicate by pulling and merging each
other's work and one person finds the bug again commits it
and tells the other four people, hey, my repository has
a fix for this.
And then when that group is done two weeks later, they can
tell their manager, hey, we've done this.
Can you ask the main group to pull, and they'll get this new
feature and by the way, we've tested it over two weeks and
it works and it performs this much better because we have
actually been able to time it before we even ask anybody
else to look at it.
And that's a hugely better model for doing development.
And this is the model that the kernel uses.
It turns out in many places, we don't need all that power,
even in the kernel.
So people usually don't pull within one group, but does it
does happen.
For example, the networking people sometimes affect the
NFS people, and the fact that they can
synchronize actually helps.
So this is a real, practical advantage.
Somebody else has a question.
AUDIENCE: So it feels like the politics has just been moved
to an indirect political question.
If everyone's got access and they're all playing with their
branches and they have their sandbox and they're having
fun, at the end of the day there has to be merging and
resolving unless you have 80 billion flavors
of every Linux kernel.
LINUS TORVALDS: Absolutely.
There will be 1,000 or maybe 20,000 different branches, but
in practice you won't ever see them because they won't care.
You will see like a few main branches, maybe
you'll see only one.
In the case of the kernel, a lot of people they only really
look at my branch.
So even though there are lots of branches,
you can ignore them.
What happens is the way merging is done is the way
real security is done, by a network of trust. If you have
ever done any security work and it did not involve the
concept of network of trust, it wasn't security work.
It was masturbation.
I don't know what you were doing, but trust me, it's the
only way you can do security, it's the only way you can do
development.
The way I work, I don't trust everybody.
In fact, I am a very cynical and untrusting person.
I think most of you are completely incompetent.
The whole point of being distributed is I don't
have to trust you.
I don't have to give you commit access.
But I know that among the multitude of average people,
there are some people that just stand out, that I trust
because I've been working with them.
I only need to trust 5, 10, 15 people.
If I have a network of trust that covers those 5, 10, 15
people that are outstanding and I know they're
outstanding, I can pull from them.
I don't have to spend a lot of brain power on the question.
When Andrew sends me patches-- he doesn't actually use Git,
it's some kind of defect--
other than that, he's a very solid person.
When he asks me to pull, he does it by sending me a
million patches.
Instead, I just do it.
Sometimes I disagree with some of these patches, but at some
point, trust means never having to say you're sorry.
I don't know.
It basically means you have to accept
other people's decisions.
The nice thing about trust is it does network, that's where
the network of trust comes in.
I only need to trust a few people that much.
They have other people, they have determined, hey, that guy
is actually smarter than I am.
That's actually a really good measure of who
you should pull from.
If you have determined that somebody else is smarter than
you, go for it.
You can't lose, right?
Even if it turns out you pulled crap and somebody else
starts complaining, you know who you pulled from and you
can just point to the other person and
say, hey, I just pulled.
Go to him, he knows what he's doing.
So that's how I work.
That's probably most of my lieutenants work.
I pull the networking changes from one person, he gets them
from many other people that he's worked with over time.
So this is how it all comes together.
It doesn't have to come together to one point.
In the kernel, it comes together to one point largely
I think for historical reasons.
And actually, I've always tried to encourage people to
have more trees.
So we do have vendor trees, we do have -mm trees, we have
multiple one points, and it happens to be that my one
point is getting maybe more attention
than it always should.
But even if it doesn't come down to one point, it means
that you can take these thousands of branches and
ignore 99.9% of them.
And you know that, hey, there are five branches that are
really interesting to follow because I'm interested in
those sub-areas.
And it all works very naturally.
One of the nice things about this whole network of trust is
it's not just easy to do technically, it's actually how
every single person in this room is very fundamentally
wired to work.
It is how we think.
We don't know 100 people.
We have five, seven, ten close, personal friends.
Well, we're geeks, so we have two.
But I mean, that's basically how humans work, is that we
have these people that we really trust. It's family,
it's close friends.
And it really fits.
You don't even have to have a mental model.
It fits how we are wired up.
So there's huge advantages to this whole model network of
trust.
AUDIENCE: Do you know any companies that are using
distributed systems internally?
It seems like there might be a risk of vulcanizing the code
base as people not being in the same sandbox don't
contribute back.
LINUS TORVALDS: So quite frankly, there aren't that
many distributed systems. There is BitKeeper.
It is clearly being used at commercial companies.
We might have somebody in the audience who actually knows.
What?
AUDIENCE: [INAUDIBLE].
LINUS TORVALDS: So HP is using things like BitKeeper for the
printer project.
I'm sure they have a lot more companies.
In the open source world, there are two distributed
systems that are worth looking at right now.
One of them is obviously Git and you really should pick
that one, but the other one is Mercurial, which actually has
pretty much the same time design.
There are huge differences in implementation and there are
some differences in details, but it boils down to a very
similar model.
Git just does it better.
Everything else, it's either centralized or it is too
unstable or too slow to use for anything big.
AUDIENCE: Right, but is there an advantage for a company to
have everybody playing in the same sandbox?
LINUS TORVALDS: I think a lot of companies think there is an
advantage to that.
I know that inside companies, I don't think a lot of
companies use Git knowingly in the sense that it
is a company decision.
I know several companies who use Git internally, not
knowing that they do so because they actually have
their main repository in Subversion and a lot of
developers then import it into Git because Git can actually
merge things for you.
So you can take a Subversion tree, just import it into Git,
let Git do the merge, which would be a major headache to
do in Subversion, create a merge commit, and actually
export it back to Subversion, and nobody else even
knew you used Git.
It's kind of sad, but we have cases of people talking about
doing exactly that inside companies.
Git has not been around in a form where a lot of people
will be comfortable using it for more than
half a year or so.
We have had so huge improvements to the user
interfaces that realistically, a year ago at a commercial
company a lot of people would just have said
it's too hard to use.
I think we're way past that hump.
Git is much easier to use than CVS, really.
It's easier to use than anything else.
Just get over it.
You don't have to use all the powerful tools.
Some of them might be things you want to explain and
introduce to people only after they got over the initial hump
of understanding what distribution really means.
But the basic stuff is really easy to do.
AUDIENCE: One characteristic of a centralized system is
that it's the original developer who has to resolve
any merges, who has to fix merges.
How do you do that in Git and how do you
minimize merge conflicts?
LINUS TORVALDS: Thank you for asking me that question.
Did I tell you to ask that question first?
One of the really nice parts of Git is A, it does make
things much easier to merge than a lot of other systems.
Merging a branch in CVS tends to be really painful.
One of my main statistics is the kernel is actually one of
the biggest open source projects.
We have 22,000 files.
We've used Git for two years.
During those two years, we have averaged 4.5 merges a
day, every single day.
That's not something you'd do in something
where merging was hard.
So Git makes merging easy, but you will in inevitably have
cases where two maintainers send me the question to please
pull my stuff.
And I pick one of them at random usually, because their
mail happened to be first in my mailbox, and I have pull
their stuff.
And another person had made changes that--
it doesn't happen that often, but it does happen--
just clashed so much that I said, I could fix this up but
I really don't want to.
I didn't write the code, it's not my area of expertise, its
networking or something like that, I can't really judge it,
I can't test it, so asking me to resolve the
merge is just crazy.
It's not how you should do things.
OK, the Windows machine flaked out again.
Remember, distribution means nobody is special.
So instead of me merging, I just push out my first tree
that didn't have any merge issues and I tell the second
person, hey, I tried to pull from you but I had merge
conflicts and they weren't completely trivial, so I
decided you get to do the honors instead.
And they do.
And they know what they're doing
because it's their changes.
So they can do the merge and they probably think I'm a
moron because the merge was so easy and it was obvious I
should have taken their code.
But they do the merge and then they update their tree and
say, hey, can you pull from me now?
And I pull from them and they did all the work for me.
That's what it's all about.
They did all the work for me.
And I take the credit.
Now I just need to figure out step three, profit.
But that's another thing that comes very naturally from
being distributed.
It's not something that is special to Git.
Git makes merging easier than anything else, but Git does it
exactly because Git is distributed.
Yes.
AUDIENCE: So I guess I don't entirely understand why you
think that its necessary to have a distributed system--
it seems like you get a lot of the good effects, at least for
corporate development.
For open source development, it seems very useful that
everybody can work on their own.
But when you really have a centralized, corporate tree,
then a centralized system with really cheap branches,
wouldn't that give you pretty much the same effect?
Or is that just impossible to do?
LINUS TORVALDS: No.
I will argue that centralized systems can't work.
But it is clearly true that if you're in a tightly controlled
corporate environment, centralized
systems work better.
And it's unquestionably true that people have been able to
use centralized system for the last 35 years.
Nobody's really arguing that centralized
systems cannot work.
They cannot work as well as distributed systems. One of
the issues you tend to have is centralized systems inevitably
have problems when you have groups in different locations.
It tends to work really well if you have a really beefy
background fiber.
And I guess for Google, you probably do have some kind of
network going.
I don't know.
And maybe it's not as big of an issue as it is for other
projects, but trust me.
Not having to go over the network for everything is a
huge performance saver.
I can't show you demonstrations and it's not a
very interesting demonstration anyway, but this is a laptop
that is what, four or five years old.
It's like a Pentium M 1.6 gigahertz thing.
I could show you me doing a full diff of the kernel on
that laptop in whatever, just over a second.
On my main machine, it takes less than 1/10 of a second.
That's the kind of performance you simply cannot get if you
have to go over a network.
We're talking a couple of packets going over the network
and you just blew the performance.
So if you have a decentralized system and you're used to
having something like commit or diffing the whole source
tree taking 30 seconds.
Maybe 30 seconds doesn't sound that bad to you.
Trust me, when you're used to taking 1/10 of a second, 30
seconds sounds pretty bad.
So there are huge performance issues even if you have a good
network, nevermind the fact that most people don't have a
good network.
The other thing is branches, even if you make them
technically very cheap to create, just the fact that you
create them and everybody sees them because everybody will
see them since they're centralized, basically means
that you don't want to make branches willy nilly.
You will have namespace issues.
What do you call your branch?
Would you call it Test?
Oh by the way, there's 5,000 other branches called Test 1
through 5,000.
So now you have to make up all these naming rules for your
branches because you have a centralized system that has a
centralized branch namespace, which is kind of inevitable
when you have a centralized system.
How does that work in distributed environments?
You call your branch test, and it's that easy.
Actually, you shouldn't call it tests.
You should basically name your branches the way
you name your functions.
You should call them something short and
sweet and to the point.
What is that branch doing?
Git, by default, gives you one branch that is called master.
It's short and sweet and to the point.
It's the master branch.
But you can make a branch that is called Experimental Feature
X and it will be obvious.
But this is something you simply cannot do in a
centralized environment.
You cannot call branches Experimental Feature X. You
have to make up stupid, idiotic names.
I worked for a company that had nice--
as nice as you probably can make them-- scripts around CVS
that helped you make branches.
You could actually make branches
with a simple command.
It didn't take that long.
It picked a name for you, exactly because it would pick
the number.
So you'd give it a base name and you would say, this is my
branch for doing so and so and it would call your
branch So and So-56.
And it would tag where you started that branch because in
CVS you need to do that, too.
It took a while, but it worked.
You can do these things in centralized systems, but you
don't need to.
If your system is decentralized, it just works.
That is how it should work.
So I'm not going to force you to switch over to
decentralized, I'm just going to call you
you ugly and stupid.
That's the deal.
Anyway, we are on the performance slide.
AUDIENCE: Can I ask a question?
LINUS TORVALDS: Yes.
AUDIENCE: Two questions, actually.
So one is how many files will Git take.
And then the second one, let's say if you have a humongous
tree under Git, would it be possible to check
out part of the tree?
LINUS TORVALDS: Great questions.
Those questions actually kind of dovetail into a different
issue, even though they are performance related.
One of the things that Git is really special about, and this
special even with regards to things like Mercurial which is
otherwise fairly similar, Git tracks your content.
It never, ever tracks a single file.
You cannot track a file in Git.
What you can do is you can track a project that has a
single file, but if your project has a single file,
sure do that, and you can do it.
But if you track 10,000 files, Git never, ever sees those as
individual files.
Git thinks of everything as the full content.
All history in Git is based on the content of all of the
history of the whole project.
This has implications for performance.
When you use CVS it's perfectly fine.
It's stupid, but it's perfectly fine to have one
huge repository that has a million files in it because at
the end of the day, CVS actually thinks of all those
million files as a single file.
And you can actually ask CVS to only update that one file
because CVS really thinks in those terms. And that's
actually true of pretty much everything else too.
It is actually even true of BitKeeper.
That was one of the mistakes in BitKeeper.
The problem with thinking in terms of single files is that
quite often, especially if you're a high level maintainer
like me, I have 22,000 files to track, I don't care about
one of them.
I might care about a sub-collection of them that
contains maybe 1,000 thousand files.
I might care about the USB subsystem, but I never care
about the single file.
So Git tracks everything as a collection of files, and if
you ask for the history of a single file, Git will
literally start from the global history
and simplify it.
It's a fairly efficient system.
It's a very efficient system.
You would normally not even realize that it does that.
But it does mean that if you try to track a million files
in one repository, when you then ask for a single file
history it's going to be slower.
So it has different scaling properties than a lot of other
systems for this very fundamental design reason.
We have used big repositories.
We've imported things like something like 3/4 of the
Subversion history of the whole KDE project.
And the KDE people are--
I like KDE but trust me, they put every single component in
one repository.
Not very smart.
What you ended up with, you had a repository that took I
think eight gigabytes under the CVS tree and Subversion
blew it up to like three times that size.
Maybe it wasn't quite eight gigabytes in
CVS, but it was big.
It was more than four gigabytes.
Git would actually compress it down to
something like 1.3 gigabytes.
So Git is actually very efficient at taking this
project and just smushing it together and most things
perform very well.
But certain things did not.
The things that do not perform very well, if you put a
million files in one repository, initial clones,
when you get it, you get it all.
You put it in one repository, Git thinks of it as one thing.
Don't do that.
If you have multiple components, do them as
separate repositories.
You can actually have what we call a super project that
contains pointers to other projects and the user
interfaces there are some lacking.
But you keep separate projects separate, and then you avoid
the problem of, OK, you have to get it all.
Because with Git, you do have to get it all.
AUDIENCE: Why don't they all share code?
[INAUDIBLE]?
LINUS TORVALDS: If they all shared code.
What you can do with Git, if you actually have a lot of
shared stuff, since Git actually internally uses a
content-addressable file system, if there are files
with identical content, Git will actually use the exact
same object for them and save you tons of space.
You can have these shared objects and still have them as
separate entities.
You can still see them at separate repositories that
just have a shared file system backing the
data, you can do that.
If you actually have shared code in the sense that you,
for example, have a library that is used by five different
things, that's when you use the super project support,
where you have one Git repository that just tracks
all the other Git repositories.
It may contain stuff like a shared build
infrastructure, too.
But then the individual pieces are individual.
These are like CVS modules.
In CVS, modules aren't really individual but that's because
in CVS, the directory is a thing of its own anyway.
So CVS modules are kind of a combination of this and just
tracking them all.
But you can basically think of it as CVS modules.
And we do support it but I do have to admit, that code is
fairly recent and that's one area where our user interfaces
right now are definitely lacking some.
There was probably some other part to that question that I
completely forgot.
AUDIENCE: [INAUDIBLE].
LINUS TORVALDS: I can't hear that.
AUDIENCE: The question was, can you have just part of the
files pulled out of the repository, not the entire
repository?
LINUS TORVALDS: You can export things as tarballs, you can
export things as individual files.
You can rewrite the whole history to say, I want a new
version of that repository that only contains that part.
You can do that.
It's a fairly expensive operation.
It's something you would do, for example, if you import an
old repository into one huge Git repository and then you
can split it later on to be multiple, smaller ones.
You can do it.
What I'm trying to say, you should generally
try to avoid it.
It's not that Git can't handle huge projects, it's that Git
won't perform as well as it would otherwise and you will
have issues that you wish you didn't have.
I'm skipping this and going back to the performance issue.
One of the things I want to say about performance is a lot
of people seem to think that performance is about doing the
same thing, just doing it faster.
And that's not true.
That's not what performance is all about.
If you can do something really fast really well, people start
using it differently.
One of the things I wanted to make sure is that merges go
really, really quickly because I want people to merge often
and merge early because it turns out it
becomes easier to merge.
If you merge every day, suddenly you never get to the
point where you have huge conflicts
that are hard to resolve.
If you actually make branching and merging easy, you actually
avoid a whole class of problems that you otherwise
have a really, really hard time avoiding.
So for example, let's go back to one of the things where I
think the designers of Subversion
were complete morons.
Strong opinions.
That's me, right?
There's a few of them in the room today, I suspect.
You're stupid.
Subversion, for example, talks very loudly about how they do
CVS right by making branching really cheap.
It's probably on their main web page where they probably
say that branching in Subversion is an 01 operation.
You can do as many cheap branches as you want.
Nevermind that the 01 is actually a
pretty large 0 I think.
But even if it takes a millionth of a second to do
branching, who cares?
It's the wrong thing you're measuring.
Nobody is interest in branching.
Branches are completely useless unless you merge them,
and CVS cannot merge anything at all.
You can merge things once, but because CVS then forgets what
you did, you can never, ever merge anything again without
getting horrible, horrible conflicts.
Merging in Subversion is a complete disaster.
The Subversion people kind of acknowledge this and they have
a plan and their plan sucks, too.
It is incredible how stupid these people are.
They've been looking at the wrong problem all the time.
Branching is not the issue, merging is, and merging they
didn't do squat for five years after the fact.
That is sad.
So performance is important, but you need to
look at what matters.
Performance for making a branch under Git, literally
you create a new file that is 41 bytes in size.
How fast do you think that is?
I don't think you can measure it.
If you use Windows you can probably
measure it because file--
but whatever.
It is so fast you can't really measure it.
That's creating a branch.
Nobody cares.
It's not an issue.
That's not it.
The only thing that matters is how fast can you merge?
In Git I merge 22,000 several times a day and I get unhappy
if a merge takes more than five seconds.
And all of those five seconds are just the downloading of
the deltas between the two trees.
The merge itself takes less than half a second, and I
don't have to think about it.
What takes longer than the merge is after every merge by
default, Git will do a diff stat of everything that
changed as a result of that merge because I
do care about that.
When I merged from somebody, I trust them.
But on the other hand, hey, they might have stopped using
their medication.
I mean, I trust them, but let's just be honest here.
They might have been OK yesterday,
today not a good day.
So I do a diff stat and Git does that by default.
You can turn it off if you really want to, but you
probably shouldn't.
It's fast enough anyway.
If it's a big merge, the diff stat usually takes a second or
two because creating a diff and actually doing all the
stats on how many lines changed, that actually is much
more expensive than doing the merge itself.
That is the kind of performance that actually
changes how you work.
It's no longer doing the same thing faster, it's allowing
you to work in a completely different manner, and that is
why performance matters and why you really shouldn't look
at anything but Git.
Hg's Mercurial is pretty good, but Git is better.
I think I'm running out of time.
OK, this one is still interesting.
We never got to the implementation part, you
really don't care.
I will say so much about implementation is the
implementation is really simple.
The code data structures are really, really, really simple.
If you then look at the source code and realize it's 80,000
lines and mostly in C. And the kind of C I write most people
don't understand, but I commented.
The source code may sometimes look complicated because we
are very performance-centric.
I am, I really care.
And sometimes to make things go really fast, you have to
use more complicated algorithms than just checking
one file at the time.
When you're doing 22,000-file merges, you don't want to
check one file at a time.
You want to check the whole three in one go and say,
they're the same, I didn't need to do anything.
So Git does things like that and that kind of blows the
source code up a bit because doing it well is complicated.
But the basics are really, really simple, and one of the
basics is this trust and reliability thing.
Every single piece of data, when Git tracks your content,
we compress it, we delta it against everything else.
But we also do a SHA-1 hash of the content, and we actually
check it when we use it.
If you have disk corruption, if you have DRAM corruption,
if you have any kind of problems at all, Git will
notice them.
It's not a question of if, it's a guarantee.
You can have people who try to be malicious.
They won't succeed.
You need to know exactly 20 bytes, you need to know the
160-bit SHA-1 name of your top of tree, and if you know that,
you can trust your tree all the way
down, the whole history.
You can have 10 years of history, you can have 100,000
files, you can have millions of revisions, and you can
trust every single piece of it because Git is so reliable and
all the basic data structures are really, really simple.
And we check checksums.
And we don't just check some piddly UDP packet checksum
that is a 16-bit sum of all the bytes.
We check a checksum that is considered
cryptographically secure.
Nobody has been able to break SHA-1, but the point is the
SHA-1, as far as Git is concerned, isn't even a
security feature.
It's purely a consistency check.
The security parts are elsewhere, so a lot of people
assume that since Git uses SHA-1 and SHA-1 is used for
cryptographically secure stuff, they think that, OK,
it's a huge security feature.
It has nothing at all to do with security, it's just the
best hash you can get.
Having a good hash is good for being able to trust your data.
It happens to have some other good features, too.
It means that when we hash objects, we know that the
hashes are actually well-distributed and we don't
have to worry about certain distribution issues.
So internally, it means from an implementation standpoint
we can trust that the hashes are so good that we can use
hashing algorithms and know that there are no bad cases.
So there are some reasons to like the
cryptographic site, too.
But it's really about the ability to trust your data.
I guarantee you, if you put your data in Git, you can
trust the fact that five years later, after it was converted
from your hard disk to DVD to whatever new technology and
you copied it along, five years later you can verify
that the data you get back out is the exact same
data you put in.
And that's something you really should look for in a
source control management system.
One of the reasons I care is for the kernel, we had a break
in on one of the BitKeeper sites where people tried to
corrupt the kernel source code repositories.
And BitKeeper actually caught it.
BitKeeper did not have a really fancy hash at all.
I think it's a 16-bit CRC, something like that.
But it was good enough that you could actually see clumsy.
It was not cryptographically secure, but it was hard enough
in practice to overcome that it was caught immediately.
When that happens once to you, you got burnt once, you don't
ever want to get burnt again.
Maybe your projects aren't that important.
My projects, they're important.
There's a reason I care.
This is also one of the reasons to go back to the
distribution angle a bit.
When you do Google, for example, Google code, you have
your source repositories that you help people maintain, and
I think you do so under Subversion.
I would never, ever trust Google to maintain my source
code for me.
I'm sorry.
You're just not that trustworthy.
The reason I really prefer a distributed system is I can
keep my source code behind three firewalls on a system
that does not allow SSH in at all.
When I'm here, I cannot read my email because my email goes
onto my machine, and the only way I can get into that
machine is when I'm physically on that network.
So maybe I'm cuckoo, maybe I'm a bit crazy and I care about
security more than most people do.
But this whole notion that I would give the master copy of
source code that I trust and I care about so much, and I
would give it to a third party is ludicrous.
Not even Google, not a way in hell would I do that.
I allow Google to have a copy of it, but I want to have
something that I know nobody touched.
And by the way, I'm not a great MIS person, so the disk
corruption issue is definitely a case that I might worry
about because I don't do backups.
So it's OK if I can then download it again from
multiple trusted parties.
I can verify them against each other, that
part is really easy.
I can verify them against hopefully that 20 bytes that I
really, really cared about.
Hopefully I have that in a few places.
20 bytes is easier to track than 180 megabytes and
corruption is less likely to hit those 20 bytes.
If I have those 20 bytes, I can download a Git repository
form a completely untrusted source and I can guarantee
that they didn't do anything bad to it.
That's a huge thing, and that's something that when you
do hosted repositories for other people, if you use
Subversion you're just not doing it right.
You're not allowing them to sleep well at night.
Of course, if you do it for 75,000 projects, most of them
are probably pretty small and not very
important, so it's OK.
That should make people feel better.
I have a few more slides.
I think we're over time.
I'm not even going to bother showing them, they're not that
interesting I think.
I talked a bit about this, about content versus
individual files.
Git tracks content.
There is the only sample command line in the whole
presentation.
Gitk is the graphical viewer of history of a Git project.
It's a [UNINTELLIGIBLE]
script that is really only doing viewing of stuff that
Git is really good at showing you.
And this is the kind of command line I use as a
top-level maintainer.
I want to be able to say what changed since a particular
version, maybe since a particular date, I can do that
easily, in those two directories or in those two
directories and that file.
And what this will show me is the global history as it
pertains to those parts of the repository.
It is more expensive to compute than the global,
global history, but if my laptop was actually connected
to the A/V system, I could show you.
Even on that laptop, it comes up in seconds.
It is that expensive, but we are that good.
This is something that is really, really unique to get.
Nobody else can do it.
And it's a hugely important feature.
Maybe it's not so important to individual developers because
individual developers often do think in terms of single
files, but it is important for the people who merge stuff, it
is important for people like me and the people I work with
directly because they never basically care
about a single file.
And they do care about these kinds of features.
Somebody sends a bug report, which bug reports are usually
not very good.
But maybe the bug report is good enough that you can
pinpoint, OK, scuzzy subsystem.
That's the command line.
You can't say which file, but you can do this and say, OK,
that will cut it down from the 15,000 commits we've had since
last week, it will cut it down to 50.
That's a huge deal.
That is something that nobody else can do, I guarantee you.
So that's the reason you want to use Git.
That's what it all boils down to.
It's safe, it is so fast that you can do things that nobody
else can do, it does things that nobody else can do, even
slowly, and it's distributed.
So go and spread the word.
We have one more question, I guess.
What is the timing like?
I don't know.
AUDIENCE: Quickly.
So one of the reasons why we would switch from Perforce is
release capability and performance.
Otherwise, people would just say, keep using it.
Would we be exchanging one set of scalability performance
problems for other scalability performance problems?
LINUS TORVALDS: I already mentioned the fact that I
don't know how you maintain stuff in Perforce, but when
and if you do a switchover to Git, what you want to make
sure is because of this content model, you need to do
it at same content boundaries.
The content boundaries usually are actually pretty
self-obvious.
I mean, they really are.
You have the compiler, you have the main source, you have
the documentation.
Well, you probably have the documentation spread out, but
you may have some user visible documentation.
Or maybe Google doesn't.
But a lot of companies have a separate set of documentation
that they give to customers, and then they have the
documentation that goes into each individual package, is
package-based.
So one of the things you do have to think about with Git
is you want to make sure it is in a somewhat sane hierarchy.
Git can easily handle larger projects.
You can have 10,000 files and that's not a problem.
The kernel is 22, we've done tests with 100, it's fine.
It's faster than anything else.
With a million files, I suspect other systems will be
faster at some things.
And that's the kind of situation I don't
want you to get into.
But if you do that basic setup correctly, it will be
basically faster at pretty much everything, than anything
anybody else will.
I am very confident about Git performance.
One of the things we don't necessarily do really well is
the CVS Annotate.
People use CVS Annotate a lot if they use CVS. I'm told it
sucks under Perforce, too, so you probably don't use the
Perforce version of Annotate, I'm not sure.
But CVS users are used to CVS Annotate.
It's the one operation that CVS can do faster than Git,
because CVS does track things one file at a time.
Git doesn't.
Git has an annotate, but if you moved a function from one
file to another, Git will literally tell you the history
of that function even across that move.
Not to file move, a function within a file.
It will go and dig back and say, hey, those two lines
actually came from that other file five years ago.
That is, again, something nobody else can do and it
boils down to the same thing.
It's the content that matter, it's not actually the files.
But it does make it a much more expensive operation, so
if you go back five years maybe it takes 30 seconds.
On the kernel, it takes a second for any file I have. We
started from no history two years ago because we just made
the decision that let's not make it more complicated than
it needs to be.
So right now, we only have two years of
history in the kernel.
We have more history in other projects that we've done
timings on.
So we've done timings on importing the KDE and things
like that with more history.
There are performance issues, but most of them are, Git is
one or two orders of magnitude faster.
So most of them are the good kind.
And if you find something, we actually have a really, really
good community.
The Git mailing list is fairly high signal to noise.
It does get a fair amount of emails, but it's actually a
very pleasant mailing list. If anybody is interested, read
the sources first, but start looking at the
mailing list archives.
We have our flames, we have are pointless discussions, but
most of it is actually very good.
OK.
Thanks.