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Hello GC and Compiler teams!

One of our services that runs with several thousand threads recently noticed an increase

in safepoint stop times, but not gc times, upon transitioning to JDK 8.

Further investigation revealed that most of the delta was related to the so-called

pre-gc/vmop "cleanup" phase when various book-keeping activities are performed,

and more specifically in the portion that walks java thread stacks single-threaded (!)

and updates the hotness counters for the active nmethods. This code appears to

be new to JDK 8 (in jdk 7 one would walk the stacks only during code cache sweeps).

I have two questions:

(1) has anyone else (typically, I'd expect applications with many hundreds or thousands of threads)

noticed this regression?

(2) Can we do better, for example, by:

(a) doing these updates by walking thread stacks in multiple worker threads in parallel, or best of all:

(b) doing these updates when we walk the thread stacks during GC, and skipping this phase entirely

for non-GC safepoints (with attendant loss in frequency of this update in low GC frequency

scenarios).

It seems kind of silly to do GC's with many multiple worker threads, but do these thread stack

walks single-threaded when it is embarrasingly parallel (one could predicate the parallelization

based on the measured stack sizes and thread population, if there was concern on the ovrhead of

activating and deactivating the thread gangs for the work).

A followup question: Any guesses as to how code cache sweep/eviction quality might be compromised if one

were to dispense with these hotness updates entirely (or at a much reduced frequency), as a temporary

workaround to the performance problem?

Thoughts/Comments? In particular, has this issue been addressed perhaps in newer JVMs?

Thanks for any comments, feedback, pointers!

-- ramki

PS: for comparison, here's data with +TraceSafepointCleanup from JDK 7 (first, where this isn't done)

vs JDK 8 (where this is done) with a program that has a few thousands of threads:

JDK 7:

..

2827.308: \[sweeping nmethods, 0.0000020 secs\]

2828.679: \[sweeping nmethods, 0.0000030 secs\]

2829.984: \[sweeping nmethods, 0.0000030 secs\]

2830.956: \[sweeping nmethods, 0.0000030 secs\]

..

JDK 8:

..

7368.634: \[mark nmethods, 0.0177030 secs\]

7369.587: \[mark nmethods, 0.0178305 secs\]

7370.479: \[mark nmethods, 0.0180260 secs\]

7371.503: \[mark nmethods, 0.0186494 secs\]

..