Cost of single-threaded nmethod hotness updates at each safepoint (in JDK 8)

Srinivas Ramakrishna ysr1729 at gmail.com
Fri Jul 31 19:07:18 UTC 2015


Hi Vladimir --


Here's a snapshot of the counters:

sun.ci.codeCacheCapacity=251658240

sun.ci.codeCacheMaxCapacity=251658240

sun.ci.codeCacheMethodsReclaimedNum=3450

sun.ci.codeCacheSweepsTotalNum=58

sun.ci.codeCacheSweepsTotalTimeMillis=1111

sun.ci.codeCacheUsed=35888704


Notice that the code cache usage is less that 35 MB, for the 240 MB
capacity, yet it seems we have had 58 sweeps already, and safepoint cleanup
says:

[mark nmethods, 0.0165062 secs]

Even if the two closures do little or no work, the single-threaded walk
over deep stacks of a thousand threads will cost time for applications with
many threads, and this is now done at each safepoint irrespective of the
sweeper activity as far as I can tell. It seems as if this work should be
somehow rolled up (via a suitable injection) into GC's thread walks that
are done in parallel, rather than doing this in a pre-GC phase (unless I am
mssing some reason that the sequencing is necessary, which it doesn't seem
to be here).

-- ramki

On Fri, Jul 31, 2015 at 11:48 AM, Srinivas Ramakrishna <ysr1729 at gmail.com>
wrote:

> Hi Vladimir --
>
> I noticed the increase even with Initial and Reserved set to the default
> of 240 MB, but actual usage much lower (less than a quarter).
>
> Look at this code path. Note that this is invoked at every safepoint
> (although it says "periodically" in the comment).
> In the mark_active_nmethods() method, there's a thread iteration in both
> branches of the if. I haven't checked to
> see which of the two was the culprit here, yet (if either).
>
> // Various cleaning tasks that should be done periodically at safepoints
>
> void SafepointSynchronize::do_cleanup_tasks() {
>
> ....
>
>   {
>
>     TraceTime t4("mark nmethods", TraceSafepointCleanupTime);
>
>     NMethodSweeper::mark_active_nmethods();
>
>   }
>
> ..
>
> }
>
>
> void NMethodSweeper::mark_active_nmethods() {
>
>  ...
>
>   if (!sweep_in_progress()) {
>
>     _seen = 0;
>
>     _sweep_fractions_left = NmethodSweepFraction;
>
>     _current = CodeCache::first_nmethod();
>
>     _traversals += 1;
>
>     _total_time_this_sweep = Tickspan();
>
>
>     if (PrintMethodFlushing) {
>
>       tty->print_cr("### Sweep: stack traversal %d", _traversals);
>
>     }
>
>     Threads::nmethods_do(&mark_activation_closure);
>
>
>   } else {
>
>     // Only set hotness counter
>
>     Threads::nmethods_do(&set_hotness_closure);
>
>   }
>
>
>   OrderAccess::storestore();
>
> }
>
> On Fri, Jul 31, 2015 at 11:43 AM, Vladimir Kozlov <
> vladimir.kozlov at oracle.com> wrote:
>
>> Hi Ramki,
>>
>> Did you fill up CodeCache? It start scanning aggressive only with full
>> CodeCache:
>>
>>   // Force stack scanning if there is only 10% free space in the code
>> cache.
>>   // We force stack scanning only non-profiled code heap gets full, since
>> critical
>>   // allocation go to the non-profiled heap and we must be make sure that
>> there is
>>   // enough space.
>>   double free_percent = 1 /
>> CodeCache::reverse_free_ratio(CodeBlobType::MethodNonProfiled) * 100;
>>   if (free_percent <= StartAggressiveSweepingAt) {
>>     do_stack_scanning();
>>   }
>>
>> Vladimir
>>
>> On 7/31/15 11:33 AM, Srinivas Ramakrishna wrote:
>>
>>>
>>> Yes.
>>>
>>>
>>> On Fri, Jul 31, 2015 at 11:31 AM, Vitaly Davidovich <vitalyd at gmail.com
>>> <mailto:vitalyd at gmail.com>> wrote:
>>>
>>>     Ramki, are you running tiered compilation?
>>>
>>>     sent from my phone
>>>
>>>     On Jul 31, 2015 2:19 PM, "Srinivas Ramakrishna" <ysr1729 at gmail.com
>>>     <mailto:ysr1729 at gmail.com>> wrote:
>>>
>>>
>>>         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]
>>>         ..
>>>
>>>
>>>
>
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