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Re: [RFC PATCH 00/18] Multi-pack index (MIDX)




On 1/8/2018 5:20 AM, Jeff King wrote:
On Sun, Jan 07, 2018 at 07:08:54PM -0500, Derrick Stolee wrote:

(Not a critique of this, just a (stupid) question)

What's the practical use-case for this feature? Since it doesn't help
with --abbrev=40 the speedup is all in the part that ensures we don't
show an ambiguous SHA-1.
The point of including the --abbrev=40 is to point out that object lookups
do not get slower with the MIDX feature. Using these "git log" options is a
good way to balance object lookups and abbreviations with object parsing and
diff machinery. And while the public data shape I shared did not show a
difference, our private testing of the Windows repository did show a
valuable improvement when isolating to object lookups and ignoring
abbreviation calculations.
Just to make sure I'm parsing this correctly: normal lookups do get faster
when you have a single index, given the right setup?

I'm curious what that setup looked like. Is it just tons and tons of
packs? Is it ones where the packs do not follow the mru patterns very
well?

The way I repacked the Linux repo creates an artificially good set of packs for the MRU cache. When the packfiles are partitioned instead by the time the objects were pushed to a remote, the MRU cache performs poorly. Improving these object lookups are a primary reason for the MIDX feature, and almost all commands improve because of it. 'git log' is just the simplest to use for demonstration.

I think it's worth thinking a bit about, because...

If something cares about both throughput and e.g. is saving the
abbreviated SHA-1s isn't it better off picking some arbitrary size
(e.g. --abbrev=20), after all the default abbreviation is going to show
something as small as possible, which may soon become ambigous after the
next commit.
Unfortunately, with the way the abbreviation algorithms work, using
--abbrev=20 will have similar performance problems because you still need to
inspect all packfiles to ensure there isn't a collision in the first 20 hex
characters.
...if what we primarily care about speeding up is abbreviations, is it
crazy to consider disabling the disambiguation step entirely?

The results of find_unique_abbrev are already a bit of a probability
game. They're guaranteed at the moment of generation, but as more
objects are added, ambiguities may be introduced. Likewise, what's
unambiguous for you may not be for somebody else you're communicating
with, if they have their own clone.

Since we now scale the default abbreviation with the size of the repo,
that gives us a bounded and pretty reasonable probability that we won't
hit a collision at all[1].

I.e., what if we did something like this:

diff --git a/sha1_name.c b/sha1_name.c
index 611c7d24dd..04c661ba85 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -600,6 +600,15 @@ int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
  	if (len == GIT_SHA1_HEXSZ || !len)
  		return GIT_SHA1_HEXSZ;
+ /*
+	 * A default length of 10 implies a repository big enough that it's
+	 * getting expensive to double check the ambiguity of each object,
+	 * and the chance that any particular object of interest has a
+	 * collision is low.
+	 */
+	if (len >= 10)
+		return len;
+
  	mad.init_len = len;
  	mad.cur_len = len;
  	mad.hex = hex;

If I repack my linux.git with "--max-pack-size=64m", I get 67 packs. The
patch above drops "git log --oneline --raw" on the resulting repo from
~150s to ~30s.

With a single pack, it goes from ~33s ~29s. Less impressive, but there's
still some benefit.

There may be other reasons to want MIDX or something like it, but I just
wonder if we can do this much simpler thing to cover the abbreviation
case. I guess the question is whether somebody is going to be annoyed in
the off chance that they hit a collision.

No only are users going to be annoyed when they hit collisions after copy-pasting an abbreviated hash, there are also a large number of tools that people build that use abbreviated hashes (either for presenting to users or because they didn't turn off abbreviations).

Abbreviations cause performance issues in other commands, too (like 'fetch'!), so whatever short-circuit you put in, it would need to be global. A flag on one builtin would not suffice.

-Peff

[1] I'd have to check the numbers, but IIRC we've set the scaling so
     that the chance of having a _single_ collision in the repository is
     less than 50%, and rounding to the conservative side (since each hex
     char gives us 4 bits). And indeed, "git log --oneline --raw" on
     linux.git does not seem to have any collisions at its default of 12
     characters, at least in my clone.

     We could also consider switching core.disambiguate to "commit",
     which makes even a collision less likely to annoy the user.