Re: [GSoC Patch 0/3] Move generation, graph_pos to a slab

On Mon, Jun 08, 2020 at 09:45:12AM -0400, Derrick Stolee wrote:

On 6/8/2020 4:36 AM, SZEDER Gábor wrote:

quoted

On Mon, Jun 08, 2020 at 11:18:27AM +0530, Abhishek Kumar wrote:

quoted

On Sun, Jun 07, 2020 at 09:53:47PM +0200, SZEDER Gábor wrote:

quoted

On Thu, Jun 04, 2020 at 10:22:27AM -0400, Derrick Stolee wrote:

quoted

On 6/4/2020 3:27 AM, Abhishek Kumar wrote:

quoted

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

I, too, see similarly small differences in this case.

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.

Other, more common cases are affected as well, notably the simple 'git
rev-list --topo-order':

  performance: 1.226479734 s: git command: /home/szeder/src/git/BUILDS/v2.27.0/bin/git rev-list --topo-order HEAD
  max RSS: 162400k
  
  performance: 1.741309536 s: git command: /home/szeder/src/git/git rev-list --topo-order HEAD
  max RSS: 169556k

Is the supposed memory footprint reduction that large to justify this
runtime increase?

Let's add that to the list of things to do.

And to the commit messages.

quoted

quoted

quoted

 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.

'object_id.cocci' is not about sha->object_id conversions, but about
avoiding undesirable code patterns, e.g. we prefer oideq() over
!oidcmp(), and the compiler, of course, can't help to catch that.
Coccinelle scripts used for actual sha->object_id transformations were
not added to 'object_id.cocci', but were recorded only in the commit
messages for reference, see e.g.  9b56149996 (merge-recursive: convert
struct merge_file_info to object_id, 2016-06-24) and a couple of its
ancestors.