RFR 8080623 CPU overhead in FJ due to spinning in awaitWork

[Paul Sandoz]

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.
+     *
+     * 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.
      */
-    private static final int SPINS  = 1 << 11;
+    private static final int SPINS  = 0;

Paul.