My initial testing showed that:
perf bench futex hash
reported less operations/sec with private hash. After using the same
amount of buckets in the private hash as used by the global hash then
the operations/sec were about the same.
This changed once the private hash became resizable. This feature added
an RCU section and reference counting via atomic inc+dec operation into
the hot path.
The reference counting can be avoided if the private hash is made
immutable.
Extend PR_FUTEX_HASH_SET_SLOTS by a fourth argument which denotes if the
private should be made immutable. Once set (to true) the a further
resize is not allowed (same if set to global hash).
Add PR_FUTEX_HASH_GET_IMMUTABLE which returns true if the hash can not
be changed.
Update "perf bench" suite.
For comparison, results of "perf bench futex hash -s":
- Xeon CPU E5-2650, 2 NUMA nodes, total 32 CPUs:
- Before the introducing task local hash
shared Averaged 1.487.148 operations/sec (+- 0,53%), total secs = 10
private Averaged 2.192.405 operations/sec (+- 0,07%), total secs = 10
- With the series
shared Averaged 1.326.342 operations/sec (+- 0,41%), total secs = 10
-b128 Averaged 141.394 operations/sec (+- 1,15%), total secs = 10
-Ib128 Averaged 851.490 operations/sec (+- 0,67%), total secs = 10
-b8192 Averaged 131.321 operations/sec (+- 2,13%), total secs = 10
-Ib8192 Averaged 1.923.077 operations/sec (+- 0,61%), total secs = 10
128 is the default allocation of hash buckets.
8192 was the previous amount of allocated hash buckets.
- Xeon(R) CPU E7-8890 v3, 4 NUMA nodes, total 144 CPUs:
- Before the introducing task local hash
shared Averaged 1.810.936 operations/sec (+- 0,26%), total secs = 20
private Averaged 2.505.801 operations/sec (+- 0,05%), total secs = 20
- With the series
shared Averaged 1.589.002 operations/sec (+- 0,25%), total secs = 20
-b1024 Averaged 42.410 operations/sec (+- 0,20%), total secs = 20
-Ib1024 Averaged 740.638 operations/sec (+- 1,51%), total secs = 20
-b65536 Averaged 48.811 operations/sec (+- 1,35%), total secs = 20
-Ib65536 Averaged 1.963.165 operations/sec (+- 0,18%), total secs = 20
1024 is the default allocation of hash buckets.
65536 was the previous amount of allocated hash buckets.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20250416162921.513656-16-bigeasy@linutronix.de
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