Subject: Re: [PATCH v3 3/5] sha1_name: Unroll len loop in find_unique_abbrev_r

From: Derrick Stolee <[email-masked]>
Date: Wed, 4 Oct 2017 09:06:32 -0400

On 10/4/2017 2:10 AM, Junio C Hamano wrote:
> Derrick Stolee <[email-masked]> writes:
> ...
>> I understand that this patch on its own does not have good numbers. I
>> split the
>> patches 3 and 4 specifically to highlight two distinct changes:
>>
>> Patch 3: Unroll the len loop that may inspect all files multiple times.
>> Patch 4: Parse less while disambiguating.
>>
>> Patch 4 more than makes up for the performance hits in this patch.
> Now you confused me even more.  When we read the similar table that
> appears in [Patch 4/5], what does the "Base Time" column mean?
> Vanilla Git with [Patch 3/5] applied?  Vanillay Git with [Patch 4/5]
> alone applied?  Something else?
In PATCH 3, 4, and 5, I used the commit-by-commit diff for the perf 
numbers, so the "Base Time" for PATCH 4 is the time calculated when 
PATCH 3 is applied. The table in the [PATCH 0/5] message includes the 
relative change for all commits.

I recalculated the relative change for each patch related to the 
baseline (PATCH 2). Looking again, it appears I misspoke and PATCH 4 
does include a +8% change for a fully-repacked Linux repo relative to 
PATCH 2. Since PATCH 5 includes an optimization targeted directly at 
large packfiles, the final performance gain is significant in the 
fully-packed cases.

It is also worth looking at the absolute times for these cases, since 
the fully-packed case is significantly faster than the multiple-packfile 
case, so the relative change impacts users less.

One final note: the improvement was clearer in test p0008.1 when the 
test included "sort -R" to shuffle the known OIDs. Providing OIDs in 
lexicographic order has had a significant effect on the performance, 
which does not reflect real-world usage. I removed the "sort -R" because 
it is a GNU-ism, but if there is a good cross-platform alternative I 
would be happy to replace it.

p0008.1: find_unique_abbrev() for existing objects
--------------------------------------------------

For 10 repeated tests, each checking 100,000 known objects, we find the
following results when running in a Linux VM:

| Repo  | Baseline | Patch 3 | Rel % | Patch 4 | Rel % | Patch 5 | Rel % |
|-------|----------|---------|-------|---------|-------|---------|-------|
| Git   | 0.09     | 0.06    | -33%  | 0.05    | -44%  | 0.05    | -44%  |
| Git   | 0.11     | 0.08    | -27%  | 0.08    | -27%  | 0.08    | -27%  |
| Git   | 0.09     | 0.07    | -22%  | 0.06    | -33%  | 0.06    | -33%  |
| Linux | 0.13     | 0.32    | 146%  | 0.14    | + 8%  | 0.05    | -62%  |
| Linux | 1.13     | 1.12    | - 1%  | 0.94    | -17%  | 0.88    | -22%  |
| Linux | 1.08     | 1.05    | - 3%  | 0.86    | -20%  | 0.80    | -26%  |
| VSTS  | 0.12     | 0.23    | +92%  | 0.11    | - 8%  | 0.05    | -58%  |
| VSTS  | 1.02     | 1.08    | + 6%  | 0.95    | - 7%  | 0.95    | - 7%  |
| VSTS  | 2.25     | 2.08    | - 8%  | 1.82    | -19%  | 1.93    | -14%  |

(Each repo has three versions, in order: 1 packfile, multiple packfiles, 
and multiple packfiles and loose objects.)

Thanks,
-Stolee