RFR 8246050: Improve scalability of MemoryScope

[Martin Buchholz]

StampedLock iis a sharp knife, hard to use well, but an excellent tool
for low-level performance work.

On Thu, May 28, 2020 at 10:56 AM Paul Sandoz <paul.sandoz at oracle.com> wrote:
>
> +1 (previously reviewed on the panama-dev list)
>
> It’s very pleasing to see this get simplified through some good collaboration. StampedLock is quite powerful, and likely an under utilized resource.
>
> Paul.
>
> > On May 28, 2020, at 4:20 AM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
> >
> > Hi,
> > during the review of [1] it emerged that the implementation of the memory scope abstraction (which is used to keep track of temporal scope of a memory segment) does not scale well in situations where there is a lot of contention on the acquire() method due to many threads working simultaneously on different chunks of the segment.
> >
> > Peter has proposed an alternate implementation [2] which, instead of using CAS, it cleverly uses LongAdders.
> >
> > While that implementation worked correctly, we managed to simplify it further, by realizing that what we needed here was an instance of a read-write lock: a thread that acquires a segment does a "read", while a thread closing a segment does a "write". By using optimistic reads with a StampedLock we were able to gain back scalability and maintain the code relatively readable.
> >
> > Webrev:
> >
> > http://cr.openjdk.java.net/~mcimadamore/8246050/webrev/
> >
> > Cheers
> > Maurizio
> >
> > [1] - https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-April/066136.html
> > [2] - https://git.openjdk.java.net/panama-foreign/pull/142
> >
>