Re: [Highlight] Performance improvements

["Martin Liška" <mliska@suse.cz>]

Hello.

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?
- Are you interested in the xxhash patch? Do you want it as a conditional build
   or may I replace the currently existing hash function?

Cheers,
Martin