On 03/20/2014 08:49 AM, Aleksey Shipilev wrote:
On 03/20/2014 11:06 AM, Peter Levart wrote:
I was thinking about last night, for question: "Why is this
double-checked non-volatile-then-volatile trick not any faster than pure
volatile variant even on ARM platform where volatile read should have
some penalty compared to normal read?", might be in the fact that
Raspberry Pi is a single-core/single-thread "machine". Would anyone with
JVM JIT compiler expertise care to share some insight? I suspect that on
such platform, the compiler optimizes volatile accesses so that they are
performed without otherwise necessary memory fences...
Yes, at least C2 is known to not emit memory fences on uniprocessor
machines. You need to have a multicore ARM. If you are still interested,
contact me privately and I can arrange the access to my personal
quad-core Cortex-A9.
-Aleksey.
Hi,
Thanks to Aleksey for re-establishing the access, I bring you results of
the microbenchmark from his quad-core Cortex-A9:
JDK 8 options: -client, org.openjdk.jmh.Main parameters: ".*" -i 10 -r 5 -wi 5
-w 1 -f 1 [-t 1|max]
--- Baseline, 1-thread ---
Benchmark Mode Samples Mean Mean error
Units
o.t.Bench6375303.testFirstToString avgt 10 69292.305 299.516
ns/op
o.t.Bench6375303.testToString avgt 10*20.003* 0.433
ns/op
--- Baseline, 4-threads ---
Benchmark Mode Samples Mean Mean error
Units
o.t.Bench6375303.testFirstToString avgt 10 100390.024 2158.132
ns/op
o.t.Bench6375303.testToString avgt 10*20.151* 0.677
ns/op
--- double-checked nonvolatile-then-volatile-read+CAS, 1-thread ---
Benchmark Mode Samples Mean Mean error
Units
o.t.Bench6375303.testFirstToString avgt 10 69951.406 221.516
ns/op
o.t.Bench6375303.testToString avgt 10*19.681* 0.025
ns/op
--- double-checked nonvolatile-then-volatile-read+CAS, 4-threads ---
Benchmark Mode Samples Mean Mean error
Units
o.t.Bench6375303.testFirstToString avgt 10 104231.335 3842.095
ns/op
o.t.Bench6375303.testToString avgt 10*20.030* 0.595
ns/op
--- classic volatile read+CAS, 1-thread ---
Benchmark Mode Samples Mean Mean error
Units
o.t.Bench6375303.testFirstToString avgt 10 69753.542 180.110
ns/op
o.t.Bench6375303.testToString avgt 10*23.285* 0.267
ns/op
--- classic volatile read+CAS, 4-threads ---
Benchmark Mode Samples Mean Mean error
Units
o.t.Bench6375303.testFirstToString avgt 10 99664.256 1814.090
ns/op
o.t.Bench6375303.testToString avgt 10*23.491* 0.606
ns/op
...as can be seen, the double-checked read-then-volatile-read+CAS trick
is about 15% faster than classic volatile-read+CAS in this case.
Regards, Peter
