RFR: 8300926: Several startup regressions ~6-70% in 21-b6 all platforms [v2]

[Daniel D. Daugherty]

On Fri, 17 Feb 2023 09:25:55 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hi all, please consider.
>> 
>> The original issue was when thread 1 asked to deopt nmethod set X and thread 2 asked for the same or a subset of X.
>> All method will already be marked, but the actual deoptimizing, not entrant, patching PC on stacks and patching post call nops, was not done yet. Which meant thread 2 could 'pass' thread 1.
>> Most places did deopt under Compile_lock, thus this is not an issue, but WB and clearCallSiteContext do not.
>> 
>> Since a handshakes may take long before completion and Compile_lock is used for so much more than deopts.
>> The fix in https://bugs.openjdk.org/browse/JDK-8299074 instead always emitted a handshake even when everything was already marked. (instead of adding Compile_lock to all places)
>> 
>> This turnout to be problematic in the startup, for example the number of deopt handshakes in jetty dry run startup went from 5 to 39 handshakes.
>> 
>> This fix first adds a barrier for which you do not pass until the requested deopts have happened and it coalesces the handshakes.
>> Secondly it moves handshakes part out of the Compile_lock where it is possible.
>> 
>> Which means we fix the performance bug and we reduce the contention on Compile_lock, meaning higher throughput in compiler and things such as class-loading.
>> 
>> It passes t1-t7 with flying colours! t8 still not completed and I'm redoing some testing due to last minute simplifications.
>> 
>> Thanks, Robbin
>
> Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Review fixes

src/hotspot/share/code/codeCache.cpp line 1392:

> 1390:     if (nm->is_marked_for_deoptimization() && !nm->has_been_deoptimized() && nm->can_be_deoptimized()) {
> 1391:       nm->make_not_entrant();
> 1392:       nm->make_deoptimized();

`CodeCache::make_marked_nmethods_deoptimized()` used to call
`make_nmethod_deoptimized(nm)` which made a couple of checks
before it called `nm->make_deoptimized()`. `make_deoptimized()`
does not make these checks so why are they no longer needed?

Here's the removed code:

void CodeCache::make_nmethod_deoptimized(CompiledMethod* nm) {
  if (nm->is_marked_for_deoptimization() && nm->can_be_deoptimized()) {
    nm->make_deoptimized();
  }
}

-------------

PR: https://git.openjdk.org/jdk/pull/12585