[llvm-dev] gnu X sysv hash performance

[Rafael Avila de Espindola via llvm-dev]

Rui Ueyama <ruiu at google.com> writes:

> On Fri, Dec 1, 2017 at 1:26 PM, Rafael Avila de Espindola <
> rafael.espindola at gmail.com> wrote:
>
>>
>> I got curious how the lld produced gnu hash tables compared to gold. To
>> test that I timed "perf record ninja check-llvm" (just the lit run) in a
>> BUILD_SHARED_LIBS build.
>>
>> The performance was almost identical, so I decided to try sysv versus
>> gnu (both produced by lld). The results are interesting:
>>
>> % grep -v '^#' perf-gnu/perf.report-by-dso-sym | head
>>     38.77%  ld-2.24.so                              [.] do_lookup_x
>>      8.08%  ld-2.24.so                              [.] strcmp
>>      2.66%  ld-2.24.so                              [.]
>> _dl_relocate_object
>>      2.58%  ld-2.24.so                              [.]
>> _dl_lookup_symbol_x
>>      1.85%  ld-2.24.so                              [.] _dl_name_match_p
>>      1.46%  [kernel.kallsyms]                       [k] copy_page
>>      1.38%  ld-2.24.so                              [.] _dl_map_object
>>      1.30%  [kernel.kallsyms]                       [k] unmap_page_range
>>      1.28%  [kernel.kallsyms]                       [k]
>>      filemap_map_pages
>>      1.26%  libLLVMSupport.so.6.0.0svn              [.] sstep
>> % grep -v '^#' perf-sysv/perf.report-by-dso-sym | head
>>     42.18%  ld-2.24.so                              [.] do_lookup_x
>>     17.73%  ld-2.24.so                              [.] check_match
>>     14.41%  ld-2.24.so                              [.] strcmp
>>      1.22%  ld-2.24.so                              [.]
>> _dl_relocate_object
>>      1.13%  ld-2.24.so                              [.]
>> _dl_lookup_symbol_x
>>      0.91%  ld-2.24.so                              [.] _dl_name_match_p
>>      0.67%  ld-2.24.so                              [.] _dl_map_object
>>      0.65%  [kernel.kallsyms]                       [k] unmap_page_range
>>      0.63%  [kernel.kallsyms]                       [k] copy_page
>>      0.59%  libLLVMSupport.so.6.0.0svn              [.] sstep
>>
>> So the gnu hash table helps a lot, but BUILD_SHARED_LIBS is still crazy
>> inefficient.
>
>
> What is "100%" in these numbers? If 100% means all execution time,
> ld-2.24.so takes more than 70% of execution time. Is this real?

I think so, BUILD_SHARED_LIBS is very slow.

On another machine this time (amazon c5.9x) I just checked the time that
lit reports in "ninja check-llvm":

regular build:     Testing Time: 23.69s
BUILD_SHARED_LIBS: Testing Time: 57.60s

It is a lot of libraries where almost all the symbols have default
visibility.

Cheers,
Rafael