RFR: 8251462: Remove legacy compilation policy [v2]

[Igor Veresov]

On Thu, 7 Jan 2021 20:50:50 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Igor Veresov has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix s390 build
>
> test/hotspot/jtreg/vmTestbase/nsk/jvmti/scenarios/hotswap/HS203/hs203t004/hs203t004.java line 35:
> 
>> 33:  *     While running the thread, it calls a method (doTask2() ) for number of times (10000).
>> 34:  *     That would cause this method to be compiled, which causes a jvmti callback for compiled method load.
>> 35:  *     (Hint : to force method compilation -XX:CompileThreshold=900 is used).
> 
> The test still uses (and the comments state it uses) -XX:CompileThreshold=900 to force compilation, yet it looks like you needed to bump the number of times the method is called from 1000 to 10000 to keep this test working. Why doesn't 1000 still work?

Tiered policy (which the -TieredCompilation emulation mode piggybacks on now) functions by doing periodic callbacks into the runtime to make policy decisions. The callback intervals are a power-of-2. So, because of the resulting rounding I cannot guarantee that the compilation will be triggered exactly at the specified threshold. Plus there are various adaptive mechanisms that I kept on even in the emulation mode that may also result in a method being compiled later than expected. To summarize, in the new world, the advisable way of triggering a compilation is to use the WB API. If that's too much effort, make sure that the method that you'd like to compile is invoked more than 2^(log2(CompileThreshold) + 1) times, plus a bit more. Very few tests actually depend on the legacy behavior, this just happens to be one of those.

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

PR: https://git.openjdk.java.net/jdk/pull/1985