mirror of
https://github.com/git/git
synced 2024-08-27 19:49:21 +00:00
![Thomas Ackermann](/assets/img/avatar_default.png)
AsciiDoc wants these header-lines left-aligned. Signed-off-by: Thomas Ackermann <th.acker@arcor.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>
461 lines
18 KiB
Plaintext
461 lines
18 KiB
Plaintext
Concerning Git's Packing Heuristics
|
|
===================================
|
|
|
|
Oh, here's a really stupid question:
|
|
|
|
Where do I go
|
|
to learn the details
|
|
of Git's packing heuristics?
|
|
|
|
Be careful what you ask!
|
|
|
|
Followers of the Git, please open the Git IRC Log and turn to
|
|
February 10, 2006.
|
|
|
|
It's a rare occasion, and we are joined by the King Git Himself,
|
|
Linus Torvalds (linus). Nathaniel Smith, (njs`), has the floor
|
|
and seeks enlightenment. Others are present, but silent.
|
|
|
|
Let's listen in!
|
|
|
|
<njs`> Oh, here's a really stupid question -- where do I go to
|
|
learn the details of Git's packing heuristics? google avails
|
|
me not, reading the source didn't help a lot, and wading
|
|
through the whole mailing list seems less efficient than any
|
|
of that.
|
|
|
|
It is a bold start! A plea for help combined with a simultaneous
|
|
tri-part attack on some of the tried and true mainstays in the quest
|
|
for enlightenment. Brash accusations of google being useless. Hubris!
|
|
Maligning the source. Heresy! Disdain for the mailing list archives.
|
|
Woe.
|
|
|
|
<pasky> yes, the packing-related delta stuff is somewhat
|
|
mysterious even for me ;)
|
|
|
|
Ah! Modesty after all.
|
|
|
|
<linus> njs, I don't think the docs exist. That's something where
|
|
I don't think anybody else than me even really got involved.
|
|
Most of the rest of Git others have been busy with (especially
|
|
Junio), but packing nobody touched after I did it.
|
|
|
|
It's cryptic, yet vague. Linus in style for sure. Wise men
|
|
interpret this as an apology. A few argue it is merely a
|
|
statement of fact.
|
|
|
|
<njs`> I guess the next step is "read the source again", but I
|
|
have to build up a certain level of gumption first :-)
|
|
|
|
Indeed! On both points.
|
|
|
|
<linus> The packing heuristic is actually really really simple.
|
|
|
|
Bait...
|
|
|
|
<linus> But strange.
|
|
|
|
And switch. That ought to do it!
|
|
|
|
<linus> Remember: Git really doesn't follow files. So what it does is
|
|
- generate a list of all objects
|
|
- sort the list according to magic heuristics
|
|
- walk the list, using a sliding window, seeing if an object
|
|
can be diffed against another object in the window
|
|
- write out the list in recency order
|
|
|
|
The traditional understatement:
|
|
|
|
<njs`> I suspect that what I'm missing is the precise definition of
|
|
the word "magic"
|
|
|
|
The traditional insight:
|
|
|
|
<pasky> yes
|
|
|
|
And Babel-like confusion flowed.
|
|
|
|
<njs`> oh, hmm, and I'm not sure what this sliding window means either
|
|
|
|
<pasky> iirc, it appeared to me to be just the sha1 of the object
|
|
when reading the code casually ...
|
|
|
|
... which simply doesn't sound as a very good heuristics, though ;)
|
|
|
|
<njs`> .....and recency order. okay, I think it's clear I didn't
|
|
even realize how much I wasn't realizing :-)
|
|
|
|
Ah, grasshopper! And thus the enlightenment begins anew.
|
|
|
|
<linus> The "magic" is actually in theory totally arbitrary.
|
|
ANY order will give you a working pack, but no, it's not
|
|
ordered by SHA-1.
|
|
|
|
Before talking about the ordering for the sliding delta
|
|
window, let's talk about the recency order. That's more
|
|
important in one way.
|
|
|
|
<njs`> Right, but if all you want is a working way to pack things
|
|
together, you could just use cat and save yourself some
|
|
trouble...
|
|
|
|
Waaait for it....
|
|
|
|
<linus> The recency ordering (which is basically: put objects
|
|
_physically_ into the pack in the order that they are
|
|
"reachable" from the head) is important.
|
|
|
|
<njs`> okay
|
|
|
|
<linus> It's important because that's the thing that gives packs
|
|
good locality. It keeps the objects close to the head (whether
|
|
they are old or new, but they are _reachable_ from the head)
|
|
at the head of the pack. So packs actually have absolutely
|
|
_wonderful_ IO patterns.
|
|
|
|
Read that again, because it is important.
|
|
|
|
<linus> But recency ordering is totally useless for deciding how
|
|
to actually generate the deltas, so the delta ordering is
|
|
something else.
|
|
|
|
The delta ordering is (wait for it):
|
|
- first sort by the "basename" of the object, as defined by
|
|
the name the object was _first_ reached through when
|
|
generating the object list
|
|
- within the same basename, sort by size of the object
|
|
- but always sort different types separately (commits first).
|
|
|
|
That's not exactly it, but it's very close.
|
|
|
|
<njs`> The "_first_ reached" thing is not too important, just you
|
|
need some way to break ties since the same objects may be
|
|
reachable many ways, yes?
|
|
|
|
And as if to clarify:
|
|
|
|
<linus> The point is that it's all really just any random
|
|
heuristic, and the ordering is totally unimportant for
|
|
correctness, but it helps a lot if the heuristic gives
|
|
"clumping" for things that are likely to delta well against
|
|
each other.
|
|
|
|
It is an important point, so secretly, I did my own research and have
|
|
included my results below. To be fair, it has changed some over time.
|
|
And through the magic of Revisionistic History, I draw upon this entry
|
|
from The Git IRC Logs on my father's birthday, March 1:
|
|
|
|
<gitster> The quote from the above linus should be rewritten a
|
|
bit (wait for it):
|
|
- first sort by type. Different objects never delta with
|
|
each other.
|
|
- then sort by filename/dirname. hash of the basename
|
|
occupies the top BITS_PER_INT-DIR_BITS bits, and bottom
|
|
DIR_BITS are for the hash of leading path elements.
|
|
- then if we are doing "thin" pack, the objects we are _not_
|
|
going to pack but we know about are sorted earlier than
|
|
other objects.
|
|
- and finally sort by size, larger to smaller.
|
|
|
|
In one swell-foop, clarification and obscurification! Nonetheless,
|
|
authoritative. Cryptic, yet concise. It even solicits notions of
|
|
quotes from The Source Code. Clearly, more study is needed.
|
|
|
|
<gitster> That's the sort order. What this means is:
|
|
- we do not delta different object types.
|
|
- we prefer to delta the objects with the same full path, but
|
|
allow files with the same name from different directories.
|
|
- we always prefer to delta against objects we are not going
|
|
to send, if there are some.
|
|
- we prefer to delta against larger objects, so that we have
|
|
lots of removals.
|
|
|
|
The penultimate rule is for "thin" packs. It is used when
|
|
the other side is known to have such objects.
|
|
|
|
There it is again. "Thin" packs. I'm thinking to myself, "What
|
|
is a 'thin' pack?" So I ask:
|
|
|
|
<jdl> What is a "thin" pack?
|
|
|
|
<gitster> Use of --objects-edge to rev-list as the upstream of
|
|
pack-objects. The pack transfer protocol negotiates that.
|
|
|
|
Woo hoo! Cleared that _right_ up!
|
|
|
|
<gitster> There are two directions - push and fetch.
|
|
|
|
There! Did you see it? It is not '"push" and "pull"'! How often the
|
|
confusion has started here. So casually mentioned, too!
|
|
|
|
<gitster> For push, git-send-pack invokes git-receive-pack on the
|
|
other end. The receive-pack says "I have up to these commits".
|
|
send-pack looks at them, and computes what are missing from
|
|
the other end. So "thin" could be the default there.
|
|
|
|
In the other direction, fetch, git-fetch-pack and
|
|
git-clone-pack invokes git-upload-pack on the other end
|
|
(via ssh or by talking to the daemon).
|
|
|
|
There are two cases: fetch-pack with -k and clone-pack is one,
|
|
fetch-pack without -k is the other. clone-pack and fetch-pack
|
|
with -k will keep the downloaded packfile without expanded, so
|
|
we do not use thin pack transfer. Otherwise, the generated
|
|
pack will have delta without base object in the same pack.
|
|
|
|
But fetch-pack without -k will explode the received pack into
|
|
individual objects, so we automatically ask upload-pack to
|
|
give us a thin pack if upload-pack supports it.
|
|
|
|
OK then.
|
|
|
|
Uh.
|
|
|
|
Let's return to the previous conversation still in progress.
|
|
|
|
<njs`> and "basename" means something like "the tail of end of
|
|
path of file objects and dir objects, as per basename(3), and
|
|
we just declare all commit and tag objects to have the same
|
|
basename" or something?
|
|
|
|
Luckily, that too is a point that gitster clarified for us!
|
|
|
|
If I might add, the trick is to make files that _might_ be similar be
|
|
located close to each other in the hash buckets based on their file
|
|
names. It used to be that "foo/Makefile", "bar/baz/quux/Makefile" and
|
|
"Makefile" all landed in the same bucket due to their common basename,
|
|
"Makefile". However, now they land in "close" buckets.
|
|
|
|
The algorithm allows not just for the _same_ bucket, but for _close_
|
|
buckets to be considered delta candidates. The rationale is
|
|
essentially that files, like Makefiles, often have very similar
|
|
content no matter what directory they live in.
|
|
|
|
<linus> I played around with different delta algorithms, and with
|
|
making the "delta window" bigger, but having too big of a
|
|
sliding window makes it very expensive to generate the pack:
|
|
you need to compare every object with a _ton_ of other objects.
|
|
|
|
There are a number of other trivial heuristics too, which
|
|
basically boil down to "don't bother even trying to delta this
|
|
pair" if we can tell before-hand that the delta isn't worth it
|
|
(due to size differences, where we can take a previous delta
|
|
result into account to decide that "ok, no point in trying
|
|
that one, it will be worse").
|
|
|
|
End result: packing is actually very size efficient. It's
|
|
somewhat CPU-wasteful, but on the other hand, since you're
|
|
really only supposed to do it maybe once a month (and you can
|
|
do it during the night), nobody really seems to care.
|
|
|
|
Nice Engineering Touch, there. Find when it doesn't matter, and
|
|
proclaim it a non-issue. Good style too!
|
|
|
|
<njs`> So, just to repeat to see if I'm following, we start by
|
|
getting a list of the objects we want to pack, we sort it by
|
|
this heuristic (basically lexicographically on the tuple
|
|
(type, basename, size)).
|
|
|
|
Then we walk through this list, and calculate a delta of
|
|
each object against the last n (tunable parameter) objects,
|
|
and pick the smallest of these deltas.
|
|
|
|
Vastly simplified, but the essence is there!
|
|
|
|
<linus> Correct.
|
|
|
|
<njs`> And then once we have picked a delta or fulltext to
|
|
represent each object, we re-sort by recency, and write them
|
|
out in that order.
|
|
|
|
<linus> Yup. Some other small details:
|
|
|
|
And of course there is the "Other Shoe" Factor too.
|
|
|
|
<linus> - We limit the delta depth to another magic value (right
|
|
now both the window and delta depth magic values are just "10")
|
|
|
|
<njs`> Hrm, my intuition is that you'd end up with really _bad_ IO
|
|
patterns, because the things you want are near by, but to
|
|
actually reconstruct them you may have to jump all over in
|
|
random ways.
|
|
|
|
<linus> - When we write out a delta, and we haven't yet written
|
|
out the object it is a delta against, we write out the base
|
|
object first. And no, when we reconstruct them, we actually
|
|
get nice IO patterns, because:
|
|
- larger objects tend to be "more recent" (Linus' law: files grow)
|
|
- we actively try to generate deltas from a larger object to a
|
|
smaller one
|
|
- this means that the top-of-tree very seldom has deltas
|
|
(i.e. deltas in _practice_ are "backwards deltas")
|
|
|
|
Again, we should reread that whole paragraph. Not just because
|
|
Linus has slipped Linus's Law in there on us, but because it is
|
|
important. Let's make sure we clarify some of the points here:
|
|
|
|
<njs`> So the point is just that in practice, delta order and
|
|
recency order match each other quite well.
|
|
|
|
<linus> Yes. There's another nice side to this (and yes, it was
|
|
designed that way ;):
|
|
- the reason we generate deltas against the larger object is
|
|
actually a big space saver too!
|
|
|
|
<njs`> Hmm, but your last comment (if "we haven't yet written out
|
|
the object it is a delta against, we write out the base object
|
|
first"), seems like it would make these facts mostly
|
|
irrelevant because even if in practice you would not have to
|
|
wander around much, in fact you just brute-force say that in
|
|
the cases where you might have to wander, don't do that :-)
|
|
|
|
<linus> Yes and no. Notice the rule: we only write out the base
|
|
object first if the delta against it was more recent. That
|
|
means that you can actually have deltas that refer to a base
|
|
object that is _not_ close to the delta object, but that only
|
|
happens when the delta is needed to generate an _old_ object.
|
|
|
|
<linus> See?
|
|
|
|
Yeah, no. I missed that on the first two or three readings myself.
|
|
|
|
<linus> This keeps the front of the pack dense. The front of the
|
|
pack never contains data that isn't relevant to a "recent"
|
|
object. The size optimization comes from our use of xdelta
|
|
(but is true for many other delta algorithms): removing data
|
|
is cheaper (in size) than adding data.
|
|
|
|
When you remove data, you only need to say "copy bytes n--m".
|
|
In contrast, in a delta that _adds_ data, you have to say "add
|
|
these bytes: 'actual data goes here'"
|
|
|
|
*** njs` has quit: Read error: 104 (Connection reset by peer)
|
|
|
|
<linus> Uhhuh. I hope I didn't blow njs` mind.
|
|
|
|
*** njs` has joined channel #git
|
|
|
|
<pasky> :)
|
|
|
|
The silent observers are amused. Of course.
|
|
|
|
And as if njs` was expected to be omniscient:
|
|
|
|
<linus> njs - did you miss anything?
|
|
|
|
OK, I'll spell it out. That's Geek Humor. If njs` was not actually
|
|
connected for a little bit there, how would he know if missed anything
|
|
while he was disconnected? He's a benevolent dictator with a sense of
|
|
humor! Well noted!
|
|
|
|
<njs`> Stupid router. Or gremlins, or whatever.
|
|
|
|
It's a cheap shot at Cisco. Take 'em when you can.
|
|
|
|
<njs`> Yes and no. Notice the rule: we only write out the base
|
|
object first if the delta against it was more recent.
|
|
|
|
I'm getting lost in all these orders, let me re-read :-)
|
|
So the write-out order is from most recent to least recent?
|
|
(Conceivably it could be the opposite way too, I'm not sure if
|
|
we've said) though my connection back at home is logging, so I
|
|
can just read what you said there :-)
|
|
|
|
And for those of you paying attention, the Omniscient Trick has just
|
|
been detailed!
|
|
|
|
<linus> Yes, we always write out most recent first
|
|
|
|
<njs`> And, yeah, I got the part about deeper-in-history stuff
|
|
having worse IO characteristics, one sort of doesn't care.
|
|
|
|
<linus> With the caveat that if the "most recent" needs an older
|
|
object to delta against (hey, shrinking sometimes does
|
|
happen), we write out the old object with the delta.
|
|
|
|
<njs`> (if only it happened more...)
|
|
|
|
<linus> Anyway, the pack-file could easily be denser still, but
|
|
because it's used both for streaming (the Git protocol) and
|
|
for on-disk, it has a few pessimizations.
|
|
|
|
Actually, it is a made-up word. But it is a made-up word being
|
|
used as setup for a later optimization, which is a real word:
|
|
|
|
<linus> In particular, while the pack-file is then compressed,
|
|
it's compressed just one object at a time, so the actual
|
|
compression factor is less than it could be in theory. But it
|
|
means that it's all nice random-access with a simple index to
|
|
do "object name->location in packfile" translation.
|
|
|
|
<njs`> I'm assuming the real win for delta-ing large->small is
|
|
more homogeneous statistics for gzip to run over?
|
|
|
|
(You have to put the bytes in one place or another, but
|
|
putting them in a larger blob wins on compression)
|
|
|
|
Actually, what is the compression strategy -- each delta
|
|
individually gzipped, the whole file gzipped, somewhere in
|
|
between, no compression at all, ....?
|
|
|
|
Right.
|
|
|
|
Reality IRC sets in. For example:
|
|
|
|
<pasky> I'll read the rest in the morning, I really have to go
|
|
sleep or there's no hope whatsoever for me at the today's
|
|
exam... g'nite all.
|
|
|
|
Heh.
|
|
|
|
<linus> pasky: g'nite
|
|
|
|
<njs`> pasky: 'luck
|
|
|
|
<linus> Right: large->small matters exactly because of compression
|
|
behaviour. If it was non-compressed, it probably wouldn't make
|
|
any difference.
|
|
|
|
<njs`> yeah
|
|
|
|
<linus> Anyway: I'm not even trying to claim that the pack-files
|
|
are perfect, but they do tend to have a nice balance of
|
|
density vs ease-of use.
|
|
|
|
Gasp! OK, saved. That's a fair Engineering trade off. Close call!
|
|
In fact, Linus reflects on some Basic Engineering Fundamentals,
|
|
design options, etc.
|
|
|
|
<linus> More importantly, they allow Git to still _conceptually_
|
|
never deal with deltas at all, and be a "whole object" store.
|
|
|
|
Which has some problems (we discussed bad huge-file
|
|
behaviour on the Git lists the other day), but it does mean
|
|
that the basic Git concepts are really really simple and
|
|
straightforward.
|
|
|
|
It's all been quite stable.
|
|
|
|
Which I think is very much a result of having very simple
|
|
basic ideas, so that there's never any confusion about what's
|
|
going on.
|
|
|
|
Bugs happen, but they are "simple" bugs. And bugs that
|
|
actually get some object store detail wrong are almost always
|
|
so obvious that they never go anywhere.
|
|
|
|
<njs`> Yeah.
|
|
|
|
Nuff said.
|
|
|
|
<linus> Anyway. I'm off for bed. It's not 6AM here, but I've got
|
|
three kids, and have to get up early in the morning to send
|
|
them off. I need my beauty sleep.
|
|
|
|
<njs`> :-)
|
|
|
|
<njs`> appreciate the infodump, I really was failing to find the
|
|
details on Git packs :-)
|
|
|
|
And now you know the rest of the story.
|