On 6/8/2020 4:36 AM, SZEDER Gábor wrote:
> On Mon, Jun 08, 2020 at 11:18:27AM +0530, Abhishek Kumar wrote:
>> On Sun, Jun 07, 2020 at 09:53:47PM +0200, SZEDER Gábor wrote:
>>> On Thu, Jun 04, 2020 at 10:22:27AM -0400, Derrick Stolee wrote:
>>>> On 6/4/2020 3:27 AM, Abhishek Kumar wrote:
>>>>> The struct commit is used in many contexts. However, members generation
>>>>> and graph_pos are only used for commit-graph related operations and
>>>>> otherwise waste memory.
>>>>>
>>>>> This wastage would have been more pronounced as transistion to
>>>>> generation number v2, which uses 64-bit generation number instead of
>>>>> current 32-bits.
>>>>
>>>> Thanks! This is an important step, and will already improve
>>>> performance in subtle ways.
>>>
>>> While the reduced memory footprint of each commit object might improve
>>> performance, accessing graph position and generation numbers in a
>>> commit-slab is more expensive than direct field accesses in 'struct
>>> commit' instances. Consequently, these patches increase the runtime
>>> of 'git merge-base --is-ancestor HEAD~50000 HEAD' in the linux
>>> repository from 0.630s to 0.940s.
>>>
>>
>> Thank you for checking performance. Performance penalty was something we
>> had discussed here [1].
>>
>> Caching the commit slab results in local variables helped wonderfully in v2 [2].
>> For example, the runtime of 'git merge-base --is-ancestor HEAD~50000 HEAD'
>> in the linux repository increased from 0.762 to 0.767s. Since this is a
>> change of <1%, it is *no longer* a performance regression in my opinion.
>
> Interesting, I measured 0.870s with v2, still a notable increase from
> 0.630s.
This is an interesting point. The --is-ancestor is critical to the
performance issue (as measured on my machine).
For "git merge-base HEAD~50000 HEAD" on the Linux repo, I get
v2.27.0:
real 0m0.515s
user 0m0.467s
sys 0m0.048s
v2 series:
real 0m0.534s
user 0m0.481s
sys 0m0.053s
With "--is-ancestor" I see the following:
v2.27.0:
real 0m0.591s
user 0m0.539s
sys 0m0.052s
v2 series:
real 0m0.773s
user 0m0.733s
sys 0m0.040s
The --is-ancestor option [1] says
Check if the first <commit> is an ancestor of the second
<commit>, and exit with status 0 if true, or with status
1 if not. Errors are signaled by a non-zero status that
is not 1.
[1] https://git-scm.com/docs/git-merge-base#Documentation/git-merge-base.txt---is-ancestor
This _should_ be faster than "git branch --contains HEAD~50000",
but it is much much slower:
$ time git branch --contains HEAD~50000
real 0m0.068s
user 0m0.061s
sys 0m0.008s
So, there is definitely something going on that slows the
"--is-ancestor" path in this case. But, the solution is not
to halt the current patch (which likely has memory footprint
benefits when dealing with a lot of tree and blob objects)
and instead fix the underlying algorithm.
Let's add that to the list of things to do.
>>> create mode 100644 contrib/coccinelle/generation.cocci
>>> create mode 100644 contrib/coccinelle/graph_pos.cocci
>>
>> I appreciate the Coccinelle scripts to help identify
>> automatic fixes for other topics in-flight. However,
>> I wonder if they would be better placed inside the
>> existing commit.cocci file?
>
> We add Coccinelle scripts to avoid undesirable code patterns entering
> our code base. That, however, is not the case here: this is a
> one-time conversion, and at the end of this series 'struct commit'
> won't have a 'generation' field anymore, so once it's merged the
> compiler will catch any new 'commit->generation' accesses. Therefore
> I don't think that these Coccinelle scripts should be added at all.
I disagree. We _also_ add Coccinelle scripts when doing one-time
refactors to avoid logical merge conflicts with other topics in
flight. If someone else is working on a parallel topic that adds
references to graph_pos or generation member, then the scripts provide
an easy way for the maintainer to update those references in the merge
commit. Alternatively, the contributor could rebase on top of this
series and run the scripts themselves to fix their patches before
submission.
For example, this was done carefully in the sha->object_id
conversion using contrib/coccinelle/object_id.cocci.
Thanks,
-Stolee
