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Stuart Monteith 3f26a5b692 8371260: Improve scaling of downcalls using MemorySegments allocated with shared arenas

MemorySegments allocated from shared Arena from
java.lang.foreign.Arena.ofShared() have their lifecycle controlled by
jdk.internal.foreign.SharedSession. This class ensures that the
MemorySegments can't be freed until after a thread has called
Arena.close(). This is implemented using a counter that is atomically
incremented when used, and decremented when not used, on every invocation
of a downcall. While shared Arenas allow any thread to use it and to
close it, this tracking has a cost when multiple threads are contended
on it. This patch changes the implementation to use multiple counters to
reduce contention. sun.nio.ch.IOUtil, java.nio.Buffer and
sun.nio.ch.SimpleAsynchronousFileChannelImpl are modified as they have
threads releasing the scope different from the ones that allocated them,
so a ticket that tracks the counter has to be passed over.

The microbenchmark org.openjdk.bench.java.lang.foreign.
CallOverheadConstant.panama_identity_memory_address_shared_3
was used to generate the following results. The scalability was checked
on a number of platforms with the JMH parameter "-t" specifying the
number of threads. Measurements are in ns/op .

The hardware are the Neoverse-N1, N2, V1 and V2, Intel Xeon 8375c and the
AMD Epyc 9654.

Threads    N1         N2            V1         V2       Xeon       Epyc
    1      30.88      32.15      33.54      32.82      27.46       8.45
    2     142.56     134.48     132.01     131.50     116.68      46.53
    4     310.18     282.75     287.59     271.82     251.88      86.11
    8     702.02     710.29     736.72     670.63     533.46     194.60
   16   1,436.17   1,684.80   1,833.69   1,782.78   1,100.15     827.28
   24   2,185.55   2,508.86   2,732.22   2,815.26   1,646.09   1,530.28
   32   2,942.48   3,432.84   3,643.64   3,782.23   2,236.81   2,278.52
   48   4,466.56   5,174.72   5,401.95   5,621.41   4,926.30   3,026.58

After
Threads    N1         N2            V1       V2       Xeon       Epyc
    1      32.41      32.11      34.43    31.32      27.94       9.82
    2      32.64      33.72      35.11    31.30      28.02       9.81
    4      32.71      36.84      34.67    31.35      28.12      10.49
    8      58.22      31.60      36.87    31.72      47.09      16.52
   16      70.15      47.76      52.37    47.26      70.91      14.53
   24      77.38      78.14      81.67    71.98      87.20      21.70
   32      87.54      98.01      84.73    86.79     109.25      22.65
   48     121.54     128.14     120.51   104.35     175.08      26.85

2025-12-01 11:58:13 +00:00

.github

8369226: GHA: Switch to MacOS 15

2025-10-07 16:54:36 +00:00

.jcheck

8355746: Start of release updates for JDK 26

2025-06-05 16:01:32 +00:00

bin

8367259: Clean up make/scripts and bin directory

2025-09-10 09:57:44 +00:00

doc

8247690: RunTest does not support running of JTREG manual tests