RFR 8080623 CPU overhead in FJ due to spinning in awaitWork

[Paul Sandoz]

On May 20, 2015, at 11:46 AM, David Holmes <david.holmes at oracle.com> wrote:

> On 20/05/2015 7:28 PM, Paul Sandoz wrote:
>> 
>> On May 20, 2015, at 9:07 AM, David Holmes <david.holmes at oracle.com> wrote:
>> 
>>> On 20/05/2015 3:55 AM, Paul Sandoz wrote:
>>>> Hi,
>>>> 
>>>> https://bugs.openjdk.java.net/browse/JDK-8080623
>>>> 
>>>> diff -r ea3ca5cfc3c6 src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java
>>>> --- a/src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java	Tue May 19 20:04:29 2015 +0300
>>>> +++ b/src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java	Tue May 19 19:54:00 2015 +0200
>>>> @@ -1328,13 +1328,9 @@
>>>>      /**
>>>>       * Number of times to spin-wait before blocking. The spins (in
>>>>       * awaitRunStateLock and awaitWork) currently use randomized
>>>> -     * spins. If/when MWAIT-like intrinsics becomes available, they
>>>> -     * may allow quieter spinning. The value of SPINS must be a power
>>>> -     * of two, at least 4. The current value causes spinning for a
>>>> -     * small fraction of typical context-switch times, well worthwhile
>>>> -     * given the typical likelihoods that blocking is not necessary.
>>>> +     * spins. Currently set to zero to reduce CPU usage.
>>> 
>>> I'd keep the commentary even if disabling spinning at this time.
>>> 
>> 
>> Some of removed commentary makes less sense given the motivation to set the value to zero. How about the following tweak:
>> 
>>      /**
>>       * Number of times to spin-wait before blocking. The spins (in
>>       * awaitRunStateLock and awaitWork) currently use randomized
>> -     * spins. If/when MWAIT-like intrinsics becomes available, they
>> -     * may allow quieter spinning. The value of SPINS must be a power
>> -     * of two, at least 4. The current value causes spinning for a
>> -     * small fraction of typical context-switch times, well worthwhile
>> -     * given the typical likelihoods that blocking is not necessary.
>> +     * spins. Currently set to zero to reduce CPU usage.
> 
> I've lost the context for what "spin" is getting randomized ??
> 

Not sure there was much context to begin with :-) best to look at the implementation: a spin value, initialized to SPINS, is (if > 0) decremented if the next value from a PRNG (Marsaglia-like) is non-negative.



>> +     *
>> +     * If greater than zero the value of SPINS must be a power
>> +     * of two, at least 4.  A value of 2048 causes spinning for a
>> +     * small fraction of typical context-switch times.
>> +     *
>> +     * If/when MWAIT-like intrinsics becomes available, they
>> +     * may allow quieter spinning.
> 
> Yep that's fine - thanks.
> 

Thanks,
Paul.