Request Review: 6950075: nmethod sweeper should operate concurrently

[Eric Caspole]

Thanks Vladimir, see rev 10:

  http://cr.openjdk.java.net/~ecaspole/concswpr/webrev.10/

Eric


On May 5, 2010, at 11:51 PM, Vladimir Kozlov wrote:

> Eric,
>
> It looks good. Just minor notes.
>
>
> Extend comment for your additional conditions:
>
> !   if ((_current == NULL && !_rescan) && !(UseCodeCacheFlushing  
> && !CompileBroker::should_compile_new_jobs())) {
>
>
> Put first call javaTimeMillis under flag:
>
> +   jlong sweep_start;
> +   if(PrintMethodFlushing)
> +     sweep_start = os::javaTimeMillis();
>
> Thanks,
> Vladimir
>
>
> On 5/5/10 6:03 PM, Eric Caspole wrote:
>> In the current JVM, the nmethod sweeper runs during each safepoint,
>> and may return early if there is no work to do. It periodically scans
>> the thread stacks to determine if not-entrant nmethods have no
>> lingering active stack frames and can be converted to zombies. It
>> then walks the code cache promoting not-entrant nmethods to zombies
>> if possible and deleting the zombies. It walks a fraction of the code
>> cache in each turn, set by NmethodSweepFraction, which is 4 by
>> default. When the whole code cache is traversed and no progress can
>> be made, the sweeper will go idle until another nmethod becomes not-
>> entrant, starting the process over again.
>>
>> In applications with a lot of compiled code, walking the code cache
>> can take many milliseconds, contributing to long safepoint pause
>> times, reducing application throughput. This change makes part of the
>> sweeper work concurrent to reduce the safepoint pause time.
>>
>> In this webrev, the stack scans continue to happen at safepoint time
>> but the code cache traversal is concurrent with the application. The
>> traversal work runs on the compiler threads where they will check to
>> sweep each time passing through the compile queue. They may take the
>> code cache and stub related locks as necessary to do the sweeping
>> safely. Only one thread will sweep at a time and it preserves the
>> existing way of traversing in chunks by NmethodSweepFraction.
>>
>> There are a few adjustments to the checks regarding the code cache
>> unloading feature to accommodate these changes. Also I added new
>> convenience methods such as CodeCache::next_nmethod() for simple
>> traversal.
>>
>> I worked out this design with Tom Rodriguez over the last month or  
>> so.
>> We'd like some other eyes on it.
>>
>> The webrev is at
>> http://cr.openjdk.java.net/~ecaspole/concswpr/webrev.09/webrev/
>>
>> Thanks,
>> Eric
>>
>>
>