RFR 8080623 CPU overhead in FJ due to spinning in awaitWork

[David Holmes]

On 20/05/2015 8:24 PM, Paul Sandoz wrote:
>
> 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.

Not sure I grok why we would randomize the spin but ...

Thanks,
David

>
>
>>> +     *
>>> +     * 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.
>