[GSOC] How to improve the performance of git cat-file --batch

[GSOC] How to improve the performance of git cat-file --batch

[ZheNing Hu]

After reusing ref-filter logic for cat-file --batch, the function of
cat-file --batch has been enhanced,
but the performance of cat-file is severely degraded. So we need a
better solution to solve it.
its last version is here:
https://lore.kernel.org/git/pull.993.v2.git.1626363626.gitgitgadget@gmail.com/

Use google doc to show some of my recent ideas:
https://docs.google.com/document/d/1hyM-ecMd8C_TJ3OsOn46nMr8ZxCUxeK7HKj-bRkq3sk/edit?usp=sharing

Anyone is welcome to comment and suggest better solutions!

Thanks!
--
ZheNing Hu

Re: [GSOC] How to improve the performance of git cat-file --batch

[Ævar Arnfjörð Bjarmason]

On Sat, Jul 24 2021, ZheNing Hu wrote:

> After reusing ref-filter logic for cat-file --batch, the function of
> cat-file --batch has been enhanced,
> but the performance of cat-file is severely degraded. So we need a
> better solution to solve it.
> its last version is here:
> https://lore.kernel.org/git/pull.993.v2.git.1626363626.gitgitgadget@gmail.com/
>
> Use google doc to show some of my recent ideas:
> https://docs.google.com/document/d/1hyM-ecMd8C_TJ3OsOn46nMr8ZxCUxeK7HKj-bRkq3sk/edit?usp=sharing
>
> Anyone is welcome to comment and suggest better solutions!

Having looked at that Google Doc it seems to just discuss "Plan A", or
there some multi-doc magic and there's a "Plan B", or...? I'm not very
familiar with Google Docs.

Anyway, I'd encourage you to just send this as an E-Mail to the mailing
list.

Having skimmed it I'm a bit confused about this in reference to
performance generally. I haven't looked into the case you're discussing,
but as I noted in
https://lore.kernel.org/git/87im1p6x34.fsf@evledraar.gmail.com/ the
profiling clearly shows that the main problem is that you've added
object lookups we skipped before.

I think any plan to improve performance should start with a perf test,
see which of your patches have performance regressions, if some don't
and are otherwise OK perhaps those can go in earlier.

Then we'll have the smallest possible set of changes that's correct, but
introduces performance regressions, and can look at what those are doing
to slow things down...

Re: [GSOC] How to improve the performance of git cat-file --batch

[ZheNing Hu]

Ævar Arnfjörð Bjarmason <avarab@gmail.com> 于2021年7月25日周日 上午5:23写道：
>
>
> On Sat, Jul 24 2021, ZheNing Hu wrote:
>
> > After reusing ref-filter logic for cat-file --batch, the function of
> > cat-file --batch has been enhanced,
> > but the performance of cat-file is severely degraded. So we need a
> > better solution to solve it.
> > its last version is here:
> > https://lore.kernel.org/git/pull.993.v2.git.1626363626.gitgitgadget@gmail.com/
> >
> > Use google doc to show some of my recent ideas:
> > https://docs.google.com/document/d/1hyM-ecMd8C_TJ3OsOn46nMr8ZxCUxeK7HKj-bRkq3sk/edit?usp=sharing
> >
> > Anyone is welcome to comment and suggest better solutions!
>
> Having looked at that Google Doc it seems to just discuss "Plan A", or
> there some multi-doc magic and there's a "Plan B", or...? I'm not very
> familiar with Google Docs.
>

There may be several plan that have not been written to it for the time being.

> Anyway, I'd encourage you to just send this as an E-Mail to the mailing
> list.
>

Ok, I can migrate them here.

> Having skimmed it I'm a bit confused about this in reference to
> performance generally. I haven't looked into the case you're discussing,
> but as I noted in
> https://lore.kernel.org/git/87im1p6x34.fsf@evledraar.gmail.com/ the
> profiling clearly shows that the main problem is that you've added
> object lookups we skipped before.
>

Yeah, you showed me last time that lookup_object() took up a lot of time.

> I think any plan to improve performance should start with a perf test,
> see which of your patches have performance regressions, if some don't
> and are otherwise OK perhaps those can go in earlier.
>

Makes sense. Last time the test performance of commits is here:
https://lore.kernel.org/git/CAOLTT8RR4+tUuT2yc2PDL9NwCburW8bM_Sht6nhKJ_fYV8fGsQ@mail.gmail.com/

The main commit with performance degradation is "[GSOC] cat-file:
reuse ref-filter logic",
we need to focus on it.

> Then we'll have the smallest possible set of changes that's correct, but
> introduces performance regressions, and can look at what those are doing
> to slow things down...

Ok.

Thanks.
--
ZheNing Hu

Re: [GSOC] How to improve the performance of git cat-file --batch

[Christian Couder]

On Sun, Jul 25, 2021 at 2:04 PM ZheNing Hu <adlternative@gmail.com> wrote:
> Ævar Arnfjörð Bjarmason <avarab@gmail.com> 于2021年7月25日周日 上午5:23写道：

> > Having skimmed it I'm a bit confused about this in reference to
> > performance generally. I haven't looked into the case you're discussing,
> > but as I noted in
> > https://lore.kernel.org/git/87im1p6x34.fsf@evledraar.gmail.com/ the
> > profiling clearly shows that the main problem is that you've added
> > object lookups we skipped before.
>
> Yeah, you showed me last time that lookup_object() took up a lot of time.

Could the document explain with some details why there are more calls
to lookup_object()? For example it could take an example `git cat-file
--batch ...` command (if possible a simple one), and say which
functions like lookup_object() it was using (and how many times) to
get the data it needs before using the ref-filter logic, and then the
same information after using the ref-filter logic.

It could be nice if there were also some data about how much time used
to be spent in lookup_object() and how much time is now spent there,
and how this compares with the whole slowdown we are seeing. If Ævar
already showed that, you can of course reuse what he already did.

The GIT_TRACE_PERFORMANCE, GIT_TRACE2_PERF env variables and the
associated trace_*() and trace2_*() functions might help you with
measuring how much time is spent in different functions.

Re: [GSOC] How to improve the performance of git cat-file --batch

[ZheNing Hu]

Christian Couder <christian.couder@gmail.com> 于2021年7月26日周一 下午5:38写道：
>
> On Sun, Jul 25, 2021 at 2:04 PM ZheNing Hu <adlternative@gmail.com> wrote:
> > Ævar Arnfjörð Bjarmason <avarab@gmail.com> 于2021年7月25日周日 上午5:23写道：
>
> > > Having skimmed it I'm a bit confused about this in reference to
> > > performance generally. I haven't looked into the case you're discussing,
> > > but as I noted in
> > > https://lore.kernel.org/git/87im1p6x34.fsf@evledraar.gmail.com/ the
> > > profiling clearly shows that the main problem is that you've added
> > > object lookups we skipped before.
> >
> > Yeah, you showed me last time that lookup_object() took up a lot of time.
>
> Could the document explain with some details why there are more calls
> to lookup_object()? For example it could take an example `git cat-file
> --batch ...` command (if possible a simple one), and say which
> functions like lookup_object() it was using (and how many times) to
> get the data it needs before using the ref-filter logic, and then the
> same information after using the ref-filter logic.
>

Sorry but this time I use gprof but can’t observe the same effect as before.
lookup_object() is indeed a part of the time overhead, but its proportion is
not very large this time.

> It could be nice if there were also some data about how much time used
> to be spent in lookup_object() and how much time is now spent there,
> and how this compares with the whole slowdown we are seeing. If Ævar
> already showed that, you can of course reuse what he already did.
>

This is my test for git cat-file --batch --batch-all-objects >/dev/null:

daab8a564: The fifth batch (upstream/master)

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 38.13      0.61     0.61  1968866     0.00     0.00  patch_delta
 13.75      0.83     0.22  6568488     0.00     0.00
unpack_object_header_buffer
 11.25      1.01     0.18   344036     0.00     0.00  unpack_entry
  7.50      1.13     0.12  1964667     0.00     0.00  hashmap_remove
  6.88      1.24     0.11  6153182     0.00     0.00  hashmap_get
  1.88      1.27     0.03  7746299     0.00     0.00  zlib_post_call
  1.88      1.30     0.03   842731     0.00     0.00  bsearch_hash
  1.88      1.33     0.03   827663     0.00     0.00  nth_packed_object_offset
  1.25      1.35     0.02 15385422     0.00     0.00  use_pack
  1.25      1.37     0.02  6236120     0.00     0.00  get_delta_base
  1.25      1.39     0.02  2581859     0.00     0.00  git_inflate_end
  1.25      1.41     0.02   826650     0.00     0.00
do_oid_object_info_extended
  1.25      1.43     0.02   826650     0.00     0.00  find_pack_entry
  1.25      1.45     0.02   825692     0.00     0.00  packed_to_object_type
  1.25      1.47     0.02   378521     0.00     0.00  get_size_from_delta


d3b5272a94: [GSOC] cat-file: reuse ref-filter logic

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 27.06      0.59     0.59  1968866     0.00     0.00  patch_delta
 16.51      0.95     0.36  2202293     0.00     0.00
unpack_object_header_buffer
 13.76      1.25     0.30  5327015     0.00     0.00  hashmap_get
 11.47      1.50     0.25   344036     0.00     0.00  unpack_entry
  8.72      1.69     0.19   521278     0.00     0.00  lookup_object
  4.13      1.78     0.09  1964667     0.00     0.00  hashmap_remove
  2.75      1.84     0.06   348709     0.00     0.00  get_object
  2.29      1.89     0.05        1     0.05     2.17  oid_array_for_each_unique
  1.38      1.92     0.03  6373452     0.00     0.00  use_pack
  0.92      1.94     0.02  2202293     0.00     0.00  unpack_compressed_entry
  0.92      1.96     0.02  1394836     0.00     0.00  grab_sub_body_contents
  0.92      1.98     0.02   348709     0.00     0.00  create_object
  0.92      2.00     0.02   348709     0.00     0.00  format_ref_array_item
  0.92      2.02     0.02    74557     0.00     0.00  fill_commit_graph_info

Because we called parse_object_buffer() in get_object(), lookup_object()
is called indirectly...

We can see that some functions are called the same times: patch_delta(),
unpack_entry(), hashmap_remove()... But after using my patch,
format_ref_array_item(), grab_sub_body_contents(), get_object(), lookup_object()
begin to occupy a certain proportion.

This is my test for git cat-file --batch-check --batch-all-objects >/dev/null:

daab8a564: The fifth batch (upstream/master)

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 47.83      0.11     0.11  3385670     0.00     0.00
unpack_object_header_buffer
 13.04      0.14     0.03  6941590     0.00     0.00  use_pack
  8.70      0.16     0.02  1046130     0.00     0.00  expand_format
  4.35      0.17     0.01   349013     0.00     0.00  oid_array_append
  4.35      0.18     0.01   348710     0.00     0.00  strbuf_expand
  4.35      0.19     0.01   348709     0.00     0.00  find_pack_entry
  4.35      0.20     0.01   348230     0.00     0.00  packed_to_object_type
  4.35      0.21     0.01   259719     0.00     0.00  git_inflate
  4.35      0.22     0.01        1    10.00   210.00  oid_array_for_each_unique
  4.35      0.23     0.01                             pack_basename

d3b5272a94: [GSOC] cat-file: reuse ref-filter logic

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 52.00      0.13     0.13  3385670     0.00     0.00
unpack_object_header_buffer
 16.00      0.17     0.04  6941590     0.00     0.00  use_pack
  8.00      0.19     0.02  3296680     0.00     0.00  get_delta_base
  4.00      0.20     0.01   348709     0.00     0.00  find_pack_entry
  4.00      0.21     0.01   348709     0.00     0.00  oid_to_hex
  4.00      0.22     0.01   348709     0.00     0.00  populate_value
  4.00      0.23     0.01   348230     0.00     0.00  packed_object_info
  4.00      0.24     0.01   348230     0.00     0.00  packed_to_object_type
  4.00      0.25     0.01                             void_hashcmp

Similarly, unpack_object_header_buffer(), use_pack(),
find_pack_entry(), packed_to_object_type(), they are still called same
times as before; populate_value(),
packed_object_info(), oid_to_hex() are new.

> The GIT_TRACE_PERFORMANCE, GIT_TRACE2_PERF env variables and the
> associated trace_*() and trace2_*() functions might help you with
> measuring how much time is spent in different functions.

Thanks.
--
ZheNing Hu

Re: [GSOC] How to improve the performance of git cat-file --batch

[Christian Couder]

On Tue, Jul 27, 2021 at 3:37 AM ZheNing Hu <adlternative@gmail.com> wrote:
>
> Christian Couder <christian.couder@gmail.com> 于2021年7月26日周一 下午5:38写道：
> >
> > On Sun, Jul 25, 2021 at 2:04 PM ZheNing Hu <adlternative@gmail.com> wrote:
> > > Ævar Arnfjörð Bjarmason <avarab@gmail.com> 于2021年7月25日周日 上午5:23写道：
> >
> > > > Having skimmed it I'm a bit confused about this in reference to
> > > > performance generally. I haven't looked into the case you're discussing,
> > > > but as I noted in
> > > > https://lore.kernel.org/git/87im1p6x34.fsf@evledraar.gmail.com/ the
> > > > profiling clearly shows that the main problem is that you've added
> > > > object lookups we skipped before.
> > >
> > > Yeah, you showed me last time that lookup_object() took up a lot of time.
> >
> > Could the document explain with some details why there are more calls
> > to lookup_object()?

Please note that here we are looking for the number of times the
lookup_object() function is called. This means that to measure that
properly, it might actually be better to have some way to count this
number of times the lookup_object() function is called, rather than
count the time spent in the function.

For example you could add a trace_printf(...) call in the
lookup_object() function, set GIT_TRACE=/tmp/git_trace.log, and then
just run `git cat-file --batch ...` and count the number of times the
new trace from lookup_object() appears in the log file.

> > For example it could take an example `git cat-file
> > --batch ...` command (if possible a simple one), and say which
> > functions like lookup_object() it was using (and how many times) to
> > get the data it needs before using the ref-filter logic, and then the
> > same information after using the ref-filter logic.
>
> Sorry but this time I use gprof but can’t observe the same effect as before.
> lookup_object() is indeed a part of the time overhead, but its proportion is
> not very large this time.

I am not sure gprof is a good tool for this. It looks like it tries to
attribute spent times to functions by splitting time between many low
level functions, and it doesn't seem like the right approach to me.
For example if lookup_object() is called 5% more often, it could be
that the excess time is attributed to some low level functions and not
to lookup_object() itself.

That's why we might get a more accurate view of what happens by just
counting the number of time the function is called.

> > It could be nice if there were also some data about how much time used
> > to be spent in lookup_object() and how much time is now spent there,
> > and how this compares with the whole slowdown we are seeing. If Ævar
> > already showed that, you can of course reuse what he already did.

Now I regret having wrote the above, sorry, as it might not be the
best way to look at this.

> This is my test for git cat-file --batch --batch-all-objects >/dev/null:

[...]

> Because we called parse_object_buffer() in get_object(), lookup_object()
> is called indirectly...

It would be nice if you could add a bit more details about how
lookup_object() is called (both before and after the changes that
degrade performance).

> We can see that some functions are called the same times:

When you say "the same times" I guess you mean that the same amount of
time is spent in these functions.

> patch_delta(),
> unpack_entry(), hashmap_remove()... But after using my patch,
> format_ref_array_item(), grab_sub_body_contents(), get_object(), lookup_object()
> begin to occupy a certain proportion.

Thanks!

Re: [GSOC] How to improve the performance of git cat-file --batch

[ZheNing Hu]

Christian Couder <christian.couder@gmail.com> 于2021年7月28日周三 下午3:34写道：
>
> On Tue, Jul 27, 2021 at 3:37 AM ZheNing Hu <adlternative@gmail.com> wrote:
> >
> > Christian Couder <christian.couder@gmail.com> 于2021年7月26日周一 下午5:38写道：
> > >
> > > On Sun, Jul 25, 2021 at 2:04 PM ZheNing Hu <adlternative@gmail.com> wrote:
> > > > Ævar Arnfjörð Bjarmason <avarab@gmail.com> 于2021年7月25日周日 上午5:23写道：
> > >
> > > > > Having skimmed it I'm a bit confused about this in reference to
> > > > > performance generally. I haven't looked into the case you're discussing,
> > > > > but as I noted in
> > > > > https://lore.kernel.org/git/87im1p6x34.fsf@evledraar.gmail.com/ the
> > > > > profiling clearly shows that the main problem is that you've added
> > > > > object lookups we skipped before.
> > > >
> > > > Yeah, you showed me last time that lookup_object() took up a lot of time.
> > >
> > > Could the document explain with some details why there are more calls
> > > to lookup_object()?
>
> Please note that here we are looking for the number of times the
> lookup_object() function is called. This means that to measure that
> properly, it might actually be better to have some way to count this
> number of times the lookup_object() function is called, rather than
> count the time spent in the function.
>

Ok, therefore we need an accurate number of call times about lookup_object(),
although the conclusion is obvious: 0 (upstream/master) and a big
number (with my patch).

> For example you could add a trace_printf(...) call in the
> lookup_object() function, set GIT_TRACE=/tmp/git_trace.log, and then
> just run `git cat-file --batch ...` and count the number of times the
> new trace from lookup_object() appears in the log file.
>

After adding a trace_printf() in lookup_object(), here is the result:

Checkout to d3b5272a94 [GSOC] cat-file: reuse ref-filter logic

$ GIT_TRACE=/tmp/git_trace.log ggg cat-file --batch --batch-all-objects
$  cat /tmp/git_trace.log | wc -l
522710

Checkout to eb27b338a3e7 (upstream/master)

$ rm /tmp/git_trace.log
$ GIT_TRACE=/tmp/git_trace.log ggg cat-file --batch --batch-all-objects
$  cat /tmp/git_trace.log | wc -l
1

This is the only 1 time left is printed by git.c, which show that after using
my patch, we additionally call  lookup_object() when we use --batch option.
According to the results of the previous gprof test: lookup_object()
occupies 8.72%
of the total time. (Though below you seem to think that the effect of
gprof is not
reliable enough.) This may be a place worthy of optimization.

> > > For example it could take an example `git cat-file
> > > --batch ...` command (if possible a simple one), and say which
> > > functions like lookup_object() it was using (and how many times) to
> > > get the data it needs before using the ref-filter logic, and then the
> > > same information after using the ref-filter logic.
> >
> > Sorry but this time I use gprof but can’t observe the same effect as before.
> > lookup_object() is indeed a part of the time overhead, but its proportion is
> > not very large this time.
>
> I am not sure gprof is a good tool for this. It looks like it tries to
> attribute spent times to functions by splitting time between many low
> level functions, and it doesn't seem like the right approach to me.
> For example if lookup_object() is called 5% more often, it could be
> that the excess time is attributed to some low level functions and not
> to lookup_object() itself.
>

Maybe you are talking about "cumulative seconds" part of gprof?
it's "self seconds" part  is the number of seconds accounted for by
this function alone.

> That's why we might get a more accurate view of what happens by just
> counting the number of time the function is called.
>

According to https://sourceware.org/binutils/docs/gprof/Flat-Profile.html :

calls
This is the total number of times the function was called. If the
function was never called,
or the number of times it was called cannot be determined (probably
because the function
was not compiled with profiling enabled), the calls field is blank.

Therefore, there are accurate numbers in the previous gprof test results.

> > > It could be nice if there were also some data about how much time used
> > > to be spent in lookup_object() and how much time is now spent there,
> > > and how this compares with the whole slowdown we are seeing. If Ævar
> > > already showed that, you can of course reuse what he already did.
>
> Now I regret having wrote the above, sorry, as it might not be the
> best way to look at this.
>

Yes, because his previous results are based on a version of my patch set,
and some changes have taken place with my patch later.

> > This is my test for git cat-file --batch --batch-all-objects >/dev/null:
>
> [...]
>
> > Because we called parse_object_buffer() in get_object(), lookup_object()
> > is called indirectly...
>
> It would be nice if you could add a bit more details about how
> lookup_object() is called (both before and after the changes that
> degrade performance).
>

After we letting git cat-file --batch reuse the logic of ref-filter,
we will use get_object()
to grab the object's data. Since we used atom %(raw), it will require
us to grab the raw data
of the object, oi->info.contentp will be set, parse_object_buffer() in
get_object() will be
called, parse_object_buffer() calls lookup_commit(), lookup_blob(),
lookup_tree(),
and lookup_tag(), they call lookup_object(). As we have seen,
lookup_object() seems to
take a lot of time.

So let us think, can we skip this parse_object_buffer() in some scenarios?
parse_object_buffer() parses the data of the object into a "struct
object *obj", and then we use this
obj feed to grab_values(), and then grab_values() feed obj to
grab_tag_values() or grab_commit_values()
to handle some logic about %(tag), %(type), %(object), %(tree),
%(parent), %(numparent).

But git cat-file --batch can avaid handle there atoms with default format.

Therefore, maybe we can skip parsing object buffer if we really don't
care about these atoms.

> > We can see that some functions are called the same times:
>
> When you say "the same times" I guess you mean that the same amount of
> time is spent in these functions.

No... What I want to express is that the number of calls to these
functions is same,
see gprof "calls part" with  "patch_delta()", both are 1968866.

>
> > patch_delta(),
> > unpack_entry(), hashmap_remove()... But after using my patch,
> > format_ref_array_item(), grab_sub_body_contents(), get_object(), lookup_object()
> > begin to occupy a certain proportion.
>
> Thanks!

Thanks for hint!!!
--
ZheNing Hu

Re: [GSOC] How to improve the performance of git cat-file --batch

[Christian Couder]

On Wed, Jul 28, 2021 at 3:38 PM ZheNing Hu <adlternative@gmail.com> wrote:

> Ok, therefore we need an accurate number of call times about lookup_object(),
> although the conclusion is obvious: 0 (upstream/master) and a big
> number (with my patch).

[...]

> This is the only 1 time left is printed by git.c, which show that after using
> my patch, we additionally call  lookup_object() when we use --batch option.
> According to the results of the previous gprof test: lookup_object()
> occupies 8.72%
> of the total time. (Though below you seem to think that the effect of
> gprof is not
> reliable enough.) This may be a place worthy of optimization.

First, yeah, I hadn't seen the "calls" columns in your gprof reports.
Sorry! It's nice to see that your manual check with a trace_printf()
function gives the same result as gprof about this though.

Anyway if you agree that it might be a place worthy of optimization,
then it might be a good idea to explain the reason for the numerous
lookup_object() calls when using the ref-filter code.

[...]

> > It would be nice if you could add a bit more details about how
> > lookup_object() is called (both before and after the changes that
> > degrade performance).
>
> After we letting git cat-file --batch reuse the logic of ref-filter,
> we will use get_object()
> to grab the object's data. Since we used atom %(raw), it will require
> us to grab the raw data
> of the object, oi->info.contentp will be set, parse_object_buffer() in
> get_object() will be
> called, parse_object_buffer() calls lookup_commit(), lookup_blob(),
> lookup_tree(),
> and lookup_tag(), they call lookup_object(). As we have seen,
> lookup_object() seems to
> take a lot of time.

Not sure why you are talking about %(raw). Is the root of the issue
that we now use atom %(raw), or how we implemented it?

> So let us think, can we skip this parse_object_buffer() in some scenarios?
> parse_object_buffer() parses the data of the object into a "struct
> object *obj", and then we use this
> obj feed to grab_values(), and then grab_values() feed obj to
> grab_tag_values() or grab_commit_values()
> to handle some logic about %(tag), %(type), %(object), %(tree),
> %(parent), %(numparent).
>
> But git cat-file --batch can avaid handle there atoms with default format.
>
> Therefore, maybe we can skip parsing object buffer if we really don't
> care about these atoms.

Yeah, maybe oi->info.contentp should be set only if the user specified
one of the atoms that really needs the content provided by
parse_object_buffer().

Thanks!