1213 lines
55 KiB
HTML
1213 lines
55 KiB
HTML
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<meta name="generator" content="Asciidoctor 1.5.8">
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<title>Concerning Git’s Packing Heuristics</title>
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</style>
|
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</head>
|
|
<body class="article">
|
|
<div id="header">
|
|
<h1>Concerning Git’s Packing Heuristics</h1>
|
|
</div>
|
|
<div id="content">
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>Oh, here's a really stupid question:</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> Where do I go
|
|
to learn the details
|
|
of Git's packing heuristics?</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Be careful what you ask!</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Followers of the Git, please open the Git IRC Log and turn to
|
|
February 10, 2006.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Let’s listen in!</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><pasky> yes, the packing-related delta stuff is somewhat
|
|
mysterious even for me ;)</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Ah! Modesty after all.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> I guess the next step is "read the source again", but I
|
|
have to build up a certain level of gumption first :-)</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Indeed! On both points.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> The packing heuristic is actually really really simple.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Bait…​</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> But strange.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>And switch. That ought to do it!</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The traditional understatement:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> I suspect that what I'm missing is the precise definition of
|
|
the word "magic"</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The traditional insight:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><pasky> yes</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>And Babel-like confusion flowed.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> oh, hmm, and I'm not sure what this sliding window means either</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><pasky> iirc, it appeared to me to be just the sha1 of the object
|
|
when reading the code casually ...</pre>
|
|
</div>
|
|
</div>
|
|
<div class="olist lowerroman">
|
|
<ol class="lowerroman" type="i">
|
|
<li>
|
|
<p>which simply doesn’t sound as a very good heuristics, though ;)</p>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> .....and recency order. okay, I think it's clear I didn't
|
|
even realize how much I wasn't realizing :-)</pre>
|
|
</div>
|
|
</div>
|
|
</li>
|
|
</ol>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Ah, grasshopper! And thus the enlightenment begins anew.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>Before talking about the ordering for the sliding delta
|
|
window, let's talk about the recency order. That's more
|
|
important in one way.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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...</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Waaait for it…​.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> okay</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Read that again, because it is important.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> But recency ordering is totally useless for deciding how
|
|
to actually generate the deltas, so the delta ordering is
|
|
something else.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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).</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>That's not exactly it, but it's very close.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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?</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>And as if to clarify:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>The penultimate rule is for "thin" packs. It is used when
|
|
the other side is known to have such objects.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>There it is again. "Thin" packs. I’m thinking to myself, "What
|
|
is a <em>thin</em> pack?" So I ask:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><jdl> What is a "thin" pack?</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><gitster> Use of --objects-edge to rev-list as the upstream of
|
|
pack-objects. The pack transfer protocol negotiates that.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Woo hoo! Cleared that <em>right</em> up!</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><gitster> There are two directions - push and fetch.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>There! Did you see it? It is not <em>"push" and "pull"</em>! How often the
|
|
confusion has started here. So casually mentioned, too!</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> 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).</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>OK then.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Uh.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Let’s return to the previous conversation still in progress.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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?</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Luckily, that too is a point that gitster clarified for us!</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>If I might add, the trick is to make files that <em>might</em> 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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The algorithm allows not just for the <em>same</em> bucket, but for <em>close</em>
|
|
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.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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").</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Nice Engineering Touch, there. Find when it doesn’t matter, and
|
|
proclaim it a non-issue. Good style too!</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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)).</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Vastly simplified, but the essence is there!</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> Correct.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> Yup. Some other small details:</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>And of course there is the "Other Shoe" Factor too.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> - We limit the delta depth to another magic value (right
|
|
now both the window and delta depth magic values are just "10")</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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")</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> So the point is just that in practice, delta order and
|
|
recency order match each other quite well.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <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!</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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 :-)</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> See?</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Yeah, no. I missed that on the first two or three readings myself.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> 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'"</pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>njs` has quit: Read error: 104 (Connection reset by peer)</p>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> Uhhuh. I hope I didn't blow njs` mind.</pre>
|
|
</div>
|
|
</div>
|
|
</li>
|
|
<li>
|
|
<p>njs` has joined channel #git</p>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><pasky> :)</pre>
|
|
</div>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The silent observers are amused. Of course.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>And as if njs` was expected to be omniscient:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> njs - did you miss anything?</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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!</p>
|
|
</div>
|
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<div class="literalblock">
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<div class="content">
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<pre><njs`> Stupid router. Or gremlins, or whatever.</pre>
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</div>
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</div>
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<div class="paragraph">
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<p>It’s a cheap shot at Cisco. Take 'em when you can.</p>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre><njs`> Yes and no. Notice the rule: we only write out the base
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object first if the delta against it was more recent.</pre>
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</div>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre> I'm getting lost in all these orders, let me re-read :-)
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So the write-out order is from most recent to least recent?
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(Conceivably it could be the opposite way too, I'm not sure if
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we've said) though my connection back at home is logging, so I
|
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can just read what you said there :-)</pre>
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</div>
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</div>
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<div class="paragraph">
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<p>And for those of you paying attention, the Omniscient Trick has just
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been detailed!</p>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre><linus> Yes, we always write out most recent first</pre>
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</div>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre><njs`> And, yeah, I got the part about deeper-in-history stuff
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having worse IO characteristics, one sort of doesn't care.</pre>
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</div>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre><linus> With the caveat that if the "most recent" needs an older
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object to delta against (hey, shrinking sometimes does
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happen), we write out the old object with the delta.</pre>
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</div>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre><njs`> (if only it happened more...)</pre>
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</div>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre> <linus> Anyway, the pack-file could easily be denser still, but
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|
because it's used both for streaming (the Git protocol) and
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|
for on-disk, it has a few pessimizations.</pre>
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</div>
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</div>
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<div class="paragraph">
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|
<p>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:</p>
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|
</div>
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<div class="literalblock">
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<div class="content">
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<pre><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.</pre>
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</div>
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</div>
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<div class="literalblock">
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<div class="content">
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<pre><njs`> I'm assuming the real win for delta-ing large->small is
|
|
more homogeneous statistics for gzip to run over?</pre>
|
|
</div>
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|
</div>
|
|
<div class="literalblock">
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<div class="content">
|
|
<pre>(You have to put the bytes in one place or another, but
|
|
putting them in a larger blob wins on compression)</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
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<div class="content">
|
|
<pre>Actually, what is the compression strategy -- each delta
|
|
individually gzipped, the whole file gzipped, somewhere in
|
|
between, no compression at all, ....?</pre>
|
|
</div>
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|
</div>
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<div class="literalblock">
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<div class="content">
|
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<pre>Right.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Reality IRC sets in. For example:</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Heh.</p>
|
|
</div>
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<div class="literalblock">
|
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<div class="content">
|
|
<pre><linus> pasky: g'nite</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
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<div class="content">
|
|
<pre><njs`> pasky: 'luck</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> Right: large->small matters exactly because of compression
|
|
behaviour. If it was non-compressed, it probably wouldn't make
|
|
any difference.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> yeah</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Gasp! OK, saved. That’s a fair Engineering trade off. Close call!
|
|
In fact, Linus reflects on some Basic Engineering Fundamentals,
|
|
design options, etc.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><linus> More importantly, they allow Git to still _conceptually_
|
|
never deal with deltas at all, and be a "whole object" store.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> 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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>It's all been quite stable.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> Yeah.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Nuff said.</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><njs`> :-)</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> <njs`> appreciate the infodump, I really was failing to find the
|
|
details on Git packs :-)</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>And now you know the rest of the story.</p>
|
|
</div>
|
|
</div>
|
|
<div id="footer">
|
|
<div id="footer-text">
|
|
Last updated 2019-02-26 19:31:11 UTC
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