Request Review: 6950075: nmethod sweeper should operate concurrently

[Vladimir Kozlov]

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
>
>