RFR(S): 8027593: performance drop with constrained codecache starting with hs25 b111

Chris Plummer chris.plummer at oracle.com
Thu Nov 7 10:12:42 PST 2013

On 11/7/13 3:24 AM, Albert Noll wrote:
> Hi Chris,
> On 11/06/2013 03:18 AM, Chris Plummer wrote:
>> BTW, one thing I forgot to mention is I now see a lot of messages for 
>> the codecache filling up. For example:
>> Java HotSpot(TM) Client VM warning: CodeCache is full. Compiler has 
>> been disabled.
>> Java HotSpot(TM) Client VM warning: Try increasing the code cache 
>> size using -XX:ReservedCodeCacheSize=
>> CodeCache: size=2700Kb used=2196Kb max_used=2196Kb free=503Kb
>> With b111, I was only seeing one message. I suspect with b111, once 
>> this message appeared compilation was never re-enabled so the message 
>> never appeared again. In that case seeing in many times now is 
>> actually a good indicator. However, it appears even when not using 
>> -XX:+PrintCodeCache, and I can see this output being a distraction 
>> for programs whose normal operation may involve constraining the 
>> codecache and having it constantly filling up. Perhaps this message 
>> should be off by default, or possibly only appear once.
> You are right. The previous version just never re-enabled compilation. 
> I also agree that the
> output is distracting. There are multiple ways to solve this issue. I 
> would go for a product -XX flag
> which allows to turn this warning on/off. Would that be ok or do you 
> have a different solution in mind?
I think a product flag would be fine. You might want to make it default 
to off when ReservedCodeCacheSize is set lower than the default, and 
have it on otherwise.


> Best,
> Albert
>> cheers,
>> Chris
>> On 11/5/13 5:59 PM, Chris Plummer wrote:
>>> Hi Albert,
>>> I applied your patch and got some new numbers. Performance is now 
>>> even better than it was with b110. See the chart I added to the bug.
>>> Nice work!
>>> Chris
>>> On 11/5/13 6:44 AM, Albert Noll wrote:
>>>> Hi,
>>>> could I get reviews for this small patch?
>>>> bug: https://bugs.openjdk.java.net/browse/JDK-8027593
>>>> webrev: http://cr.openjdk.java.net/~anoll/8027593/webrev.00/
>>>> Problem: The implementation of the sweeper (8020151) causes a 
>>>> performance regression for small code cache sizes. There are two 
>>>> issues that cause this regression:
>>>>   1) NmethodSweepFraction is only adjusted according to the 
>>>> ReservedCodecacheSize if TieredCompilation is enabled. As a result, 
>>>> NmethodSweepFraction remains 16 (if TieredCompilation is not used). 
>>>> This is way too large for small code cache sizes (e.g., <5m).
>>>>  2) _request_mark_phase (sweeper.cpp) is initialized to false. As a 
>>>> result, mark_active_nmethods() did not set _invocations and 
>>>> _current, which results in not invoking the sweeper (calling 
>>>> sweep_code_cache()) at all. When TieredCompilation is enabled this 
>>>> was not an issue, since NmethodSweeper::notify() (which sets 
>>>> _request_mark_phase) is called much more frequently.
>>>> Solution: 1) Move setting of NmethodSweepFraction so that it is 
>>>> always executed.
>>>> Solution: 2) Remove need_marking_phase(), 
>>>> request_nmethod_marking(), and reset_nmetod_marking().
>>>>                    I think that these checks are not needed since 
>>>> 8020151, since we do stack scanning of
>>>>                    active nmethods irrespective of the value of 
>>>> what need_marking_phase() returns. Since
>>>>                    the patch removes need_marking_phase() printing 
>>>> out the warning (line 327 in
>>>>                    sweeper.cpp) is incorrect, i.e., we continue to 
>>>> invoke the sweeper. I removed the warning
>>>>                    and the associated code.
>>>> Also, I think that we can either remove -XX:MethodFlushing or 
>>>> -XX:UseCodeCacheFlushing. Since 8020151, one of them is redundant 
>>>> and can be removed. I am not quite sure if we should do that now so 
>>>> it is not included in the patch.
>>>> Testing
>>>> bug: https://bugs.openjdk.java.net/browse/JDK-8027593 also shows a 
>>>> performance evaluation.
>>>> Many thanks for looking at the patch.
>>>> Best,
>>>> Albert

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