ZheNing HuMy eleventh week blog finished: The web version is here: https://adlternative.github.io/GSOC-Git-Blog-11/
### Attempt to optimize performance
This week, at the prompt of my mentor Christian, I used `gprof` for some performance tests about `git cat-file --batch`: [Re: [GSOC] How to improve the performance of git cat-file --batch] (https://lore.kernel.org/git/CAOLTT8TdL7UhfVSOzbpmo-WFNrcKwmy=E720tNt4KM9o_p=keg@mail.gmail.com/)
``` 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 ```
Before, I thought that the proportion of `lookup_object()` is not very large(11.47%), so I didn't care about it. But Christian strongly recommends that I use `trace_printf()` to observe the number of calls to `lookup_object()`.
Here is an amazing fact:
The number of calls to `lookup_object()` before and after using my patch are 0 and 522709 respectively. Therefore, I am very surprised, why do we have these additional calls?
```
(gdb) bt
#0 lookup_object (r=r@entry=0x5555558b8cc0 <the_repo>,
oid=oid@entry=0x5555558b8980 <oi>) at object.c:92
#1 0x0000555555665572 in lookup_commit (r=0x5555558b8cc0 <the_repo>,
oid=0x5555558b8980 <oi>) at commit.c:62
#2 0x00005555556edff5 in parse_object_buffer (r=0x5555558b8cc0
<the_repo>, oid=oid@entry=0x5555558b8980 <oi>, type=OBJ_COMMIT,
size=788, buffer=0x5555558d0080, eaten_p=eaten_p@entry=0x7fffffffcc0c)
at object.c:214
#3 0x000055555571da42 in get_object (ref=ref@entry=0x7fffffffcf30,
deref=deref@entry=0, obj=obj@entry=0x7fffffffcc90,
oi=oi@entry=0x5555558b8980 <oi>, err=err@entry=0x7fffffffcf10)
at ref-filter.c:1774
#4 0x000055555571fdc2 in populate_value
(ref=ref@entry=0x7fffffffcf30, err=err@entry=0x7fffffffcf10) at
ref-filter.c:1999
#5 0x00005555557202eb in get_ref_atom_value
(ref=ref@entry=0x7fffffffcf30, atom=0, v=v@entry=0x7fffffffcea8,
err=err@entry=0x7fffffffcf10) at ref-filter.c:2033
#6 0x00005555557212d6 in format_ref_array_item
(info=info@entry=0x7fffffffcf30, format=format@entry=0x7fffffffd0f0,
final_buf=final_buf@entry=0x7fffffffd060,
error_buf=error_buf@entry=0x7fffffffcf10) at ref-filter.c:2627
#7 0x00005555555859d8 in batch_object_write (scratch=0x7fffffffd060,
opt=0x7fffffffd0d0, data=<optimized out>, obj_name=0x0) at
builtin/cat-file.c:224
```After printing the call stack of `lookup_object()`, we can know that the `parse_buffer()` is calling them. A very straightforward idea, can we avoid calling this function?
In `parse_object_buffer()`, `parse_blob_buffer()`, ``parse_tree_buffer()`, `parse_commit_buffer()`, and `parse_tag_buffer()` parse the object data, and then store it in `struct object *obj`, finally return it to the caller.
`get_object()` will feed the `obj` to `grab_values()`, and then `grab_values()` will feed the `obj` to `grab_tag_values()`, `grab_commit_values()`, which can fill the object info in `obj` to implement some atom, e.g. `%(tag)`, `%(type)`, `%(object)`, `%(tree)`, `%(numparent)`,`%(parent)`. It is worth noting that `%(objectname)`, `%(objecttype)`, `%(objectsize)`, `%(deltabase)`, `%(rest)`, `%(raw)` are did not appear in them, this means that we can avoid parsing object buffer when we don't use those atoms which require `obj`'s information!
After some processing and adaptation, I made the patch which can skip `parse_object_buffer()` in some cases, this is the result of the performance test of `t/perf/p1006-cat-file.sh`:
``` Test HEAD~ HEAD ------------------------------------------------------------------------------------ 1006.2: cat-file --batch-check 0.10(0.09+0.00) 0.11(0.10+0.00) +10.0% 1006.3: cat-file --batch-check with atoms 0.09(0.08+0.01) 0.09(0.06+0.03) +0.0% 1006.4: cat-file --batch 0.62(0.58+0.04) 0.57(0.54+0.03) -8.1% 1006.5: cat-file --batch with atoms 0.63(0.60+0.02) 0.52(0.49+0.02) -17.5% ```
We can see that the performance of `git cat-file --batch` has been a certain improvement!
Tell a joke: removing 1984531500 if checking can reduce the startup time of GTA5 by 70%. :-D
Currently the patch has not been submitted to the mailing list, let us wait a bit...
-- ZheNing Hu
