Re: [Highlight] Performance improvements

["Martin Liška" <mliska@suse.cz>]

On 1/3/22 23:06, Mark Wielaard wrote:
> Hi Martin,
> 
> I noticed that this is a reply to a thread from 2 years ago. Is it
> related to the work mentioned by Tom in that thread?

Hello.

It's related only a bit as it's also connected to Performance improvements :)

> 
> On Thu, Dec 23, 2021 at 12:57:48PM +0100, Martin Liška wrote:
>> I've made couple of experiments with dwz speed. I've taken the following packages:
>> gcc, krita, libetonyek, rtags, sysdig and run dwz -m x ... for them.
>>
>> There are numbers I collected for the following configurations:
>> dwz (system package, built with LTO and -O2), dwz-O2_lto is supposed
>> to be the same (built from source), then I experimented with -O3 and PGO
>> (based on tramp3d copies 4 times). And the final run is experimental patch
>> I have that replaces the iterative_hash with xxhash:
>> https://github.com/Cyan4973/xxHash
>>
>> # 1/5: sysdig (60M)
>> dwz                   : 10.0
>> dwz                   : 9.8 (98.7%)
>> dwz-O2_lto            : 9.5 (95.6%)
>> dwz-O3_lto            : 9.2 (91.9%)
>> dwz-O3_lto_pgo        : 8.1 (81.3%)
>> dwz-O3_lto_pgo_xxhash : 7.3 (72.9%)
>> # 2/5: rtags (148M)
>> dwz                   : 19.6
>> dwz                   : 19.6 (99.9%)
>> dwz-O2_lto            : 17.4 (89.0%)
>> dwz-O3_lto            : 16.7 (85.4%)
>> dwz-O3_lto_pgo        : 14.4 (73.6%)
>> dwz-O3_lto_pgo_xxhash : 13.2 (67.6%)
>> # 3/5: libetonyek (112M)
>> dwz                   : 10.5
>> dwz                   : 10.5 (100.6%)
>> dwz-O2_lto            : 10.8 (102.8%)
>> dwz-O3_lto            : 10.1 (96.7%)
>> dwz-O3_lto_pgo        : 9.1 (87.4%)
>> dwz-O3_lto_pgo_xxhash : 8.1 (77.1%)
>> # 4/5: krita (685M)
>> dwz                   : 133.7
>> dwz                   : 134.3 (100.5%)
>> dwz-O2_lto            : 95.3 (71.3%)
>> dwz-O3_lto            : 91.2 (68.2%)
>> dwz-O3_lto_pgo        : 78.9 (59.0%)
>> dwz-O3_lto_pgo_xxhash : 71.6 (53.5%)
>> # 5/5: gcc (1.2G)
>> dwz                   : 61.9
>> dwz                   : 61.9 (99.9%)
>> dwz-O2_lto            : 58.5 (94.5%)
>> dwz-O3_lto            : 56.6 (91.3%)
>> dwz-O3_lto_pgo        : 54.1 (87.4%)
>> dwz-O3_lto_pgo_xxhash : 51.7 (83.4%)
>>
>> So as seen, using -O3 really help, one gets a bigger binary, but as dwz is small
>> it's negligible:
>>
>> bloaty dwz-O3_lto -- dwz-O2_lto
>>      FILE SIZE        VM SIZE
>>   --------------  --------------
>>     +28% +50.3Ki  [ = ]       0    .debug_loclists
>>     +18% +25.3Ki   +18% +25.3Ki    .text
>>     +12% +24.6Ki  [ = ]       0    .debug_info
>>     +16% +17.3Ki  [ = ]       0    .debug_line
>>     +31% +6.19Ki  [ = ]       0    .debug_rnglists
>>     +11%    +689  [ = ]       0    .debug_abbrev
>>    +7.1%    +633  [ = ]       0    .strtab
>>    +5.5%    +504  +5.5%    +504    .eh_frame
>>    +1.3%    +453  [ = ]       0    .debug_str
>>    +0.8%    +375  +0.8%    +375    .rodata
>>    +2.8%    +336  [ = ]       0    .symtab
>>     +11%     +64  [ = ]       0    .debug_aranges
>>    +4.2%     +64  +4.4%     +64    .eh_frame_hdr
>>    [ = ]       0  +1.8%     +32    .bss
>>    -3.1%     -21  -3.1%     -21    [LOAD #2 [RX]]
>>   -61.0% -2.20Ki  [ = ]       0    [Unmapped]
>>     +16%  +124Ki   +13% +26.2Ki    TOTAL
>>
>> Then, PGO also helps significantly. And finally, using xxhash one can get 5-10% percent
>> improvement.
>>
>> For now I'm suggesting using -O3 and PGO for our openSUSE package:
>> https://build.opensuse.org/request/show/942235
>>
>> Upstream questions I have:
>> - What about changing -O2 with -O3 by default?
> 
> Did you test that without -flto? If it still gets a ~5% speedup then I

Yep:

# 1/5: sysdig (60M)
dwz_O2                : 9.7
dwz_O2_xxhash         : 8.5 (87.7%)
# 2/5: rtags (58M)
dwz_O2                : 17.6
dwz_O2_xxhash         : 15.8 (89.5%)
# 3/5: libetonyek (91M)
dwz_O2                : 10.8
dwz_O2_xxhash         : 9.4 (86.7%)
# 4/5: krita (685M)
dwz_O2                : 96.0
dwz_O2_xxhash         : 85.6 (89.1%)
# 5/5: gcc (1.2G)
dwz_O2                : 58.6
dwz_O2_xxhash         : 54.1 (92.4%)


> like that idea. Or maybe we should also include -flto by default?

Well, it's probably something that can be decided by distributions. Maybe, we can
add a default dwz.spec file?

> 
>> - Are you interested in the xxhash patch? Do you want it as a conditional build
>>    or may I replace the currently existing hash function?
> 
> I think it is best to simply replace the existing hash function
> instead of making it a conditional thing.

Fine, I'm going to prepare a patch.

> 
> Does it rely on having the libxxhash dynamic library available or
> would we simply embed a copy (replacing the hashtab.[ch] files)?

I would not do that as it may become obsolete quite fast. I would rather use a standard
shared library (similarly to libelf).

Martin

> 
> Cheers,
> 
> Mark
>