RFR: 8336707: Contention of ForkJoinPool grows when stealing works

[Viktor Klang]

On Mon, 14 Oct 2024 20:58:43 GMT, Doug Lea <dl at openjdk.org> wrote:

> This addresses tendencies in previous update to increase fencing, scanning, and signalling that can increase contention, and slow down performance especially on ARM platforms. It also uses more ARM-friendly constructions to reduce overhead (leading to several changes that all of the same form),

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java line 305:

> 303:      *  * Slot k must be read with an acquiring read, which it must
> 304:      *    anyway to dereference and run the task if the (acquiring)
> 305:      *    CAS succeeds

Suggestion:

     *    CAS succeeds.

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java line 854:

> 852:      * usages of ForkJoinTasks ignore interrupt status when executing
> 853:      * or awaiting completion.  Otherwise, reporting task results or
> 854:      * exceptions is preferred to throwing InterruptedExeptions,

Suggestion:

     * exceptions is preferred to throwing InterruptedExceptions,

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java line 1287:

> 1285:             if (!internal)
> 1286:                 unlockPhase();
> 1287:             if ((room == 0 || U.getReference(a, pk) == null) && pool != null)

We used to look at `- 2` but now we look at `- 1`, perhaps that could account for stalls?

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java line 1913:

> 1911:                 if (!all)
> 1912:                     break;
> 1913:             }

Stylistically, it might be cleaner to do this:

Suggestion:

        do {
            WorkQueue[] qs; WorkQueue v; int sp, i;
            if ((sp = (int)c) == 0 || (qs = queues) == null ||
                qs.length <= (i = sp & SMASK) || (v = qs[i]) == null)
                break;
            if (c == (c = compareAndExchangeCtl(
                          c, ((UMASK & (c + RC_UNIT)) | (c & TC_MASK) |
                              (v.stackPred & LMASK))))) {
                v.phase = sp;
                if (v.parking != 0)
                    U.unpark(v.owner);
            } while(all);

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/21507#discussion_r1803453194
PR Review Comment: https://git.openjdk.org/jdk/pull/21507#discussion_r1803452635
PR Review Comment: https://git.openjdk.org/jdk/pull/21507#discussion_r1803455326
PR Review Comment: https://git.openjdk.org/jdk/pull/21507#discussion_r1803451989