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




On 1/7/2018 5:42 PM, Ævar Arnfjörð Bjarmason wrote:

On Sun, Jan 07 2018, Derrick Stolee jotted:

     git log --oneline --raw --parents

Num Packs | Before MIDX | After MIDX |  Rel % | 1 pack %
----------+-------------+------------+--------+----------
         1 |     35.64 s |    35.28 s |  -1.0% |   -1.0%
        24 |     90.81 s |    40.06 s | -55.9% |  +12.4%
       127 |    257.97 s |    42.25 s | -83.6% |  +18.6%

The last column is the relative difference between the MIDX-enabled repo
and the single-pack repo. The goal of the MIDX feature is to present the
ODB as if it was fully repacked, so there is still room for improvement.

Changing the command to

     git log --oneline --raw --parents --abbrev=40

has no observable difference (sub 1% change in all cases). This is likely
due to the repack I used putting commits and trees in a small number of
packfiles so the MRU cache workes very well. On more naturally-created
lists of packfiles, there can be up to 20% improvement on this command.

We are using a version of this patch with an upcoming release of GVFS.
This feature is particularly important in that space since GVFS performs
a "prefetch" step that downloads a pack of commits and trees on a daily
basis. These packfiles are placed in an alternate that is shared by all
enlistments. Some users have 150+ packfiles and the MRU misses and
abbreviation computations are significant. Now, GVFS manages the MIDX file
after adding new prefetch packfiles using the following command:

     git midx --write --update-head --delete-expired --pack-dir=<alt>

(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.

The reason we do that at all is because it makes for a prettier UI.

We tried setting core.abbrev=40 on GVFS enlistments to speed up performance and the users rebelled against the hideous output. They would rather have slower speeds than long hashes.

Are there things that both want the pretty SHA-1 and also care about the
throughput? I'd have expected machine parsing to just use
--no-abbrev-commit.

The --raw flag outputs blob hashes, so the --abbrev=40 covers all hashes.

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.

Thanks,
-Stolee