[concurrency-interest] Low-latency pause in JDK

Francesco Nigro nigro.fra at gmail.com
Sat Oct 26 06:23:30 EDT 2019


I mean RDTSC, missed the final C :P

Il sab 26 ott 2019, 12:07 Francesco Nigro <nigro.fra at gmail.com> ha scritto:

> I was hoping that for parking times <  granularity offered by the OS would
> make sense a mm_pause (possibility not broken) spin loop while using rdts
> to measure the elapsed time (+ lfence, if needed, but maybe with the pause
> isn't necessary). And no safepoint polls in the spin wait loop.
> I know that on oversubscribed systems (more active threads then core)
> isn't a great solution but would be nice to have some way to perform a low
> latency sleep.
> Implementing by myself saving the safepoint poll to be injected in an
> uncounted loop is not that trivial...
>
> Il sab 26 ott 2019, 11:23 Andrew Haley via Concurrency-interest <
> concurrency-interest at cs.oswego.edu> ha scritto:
>
>> On 10/25/19 11:11 AM, Viktor Klang via Concurrency-interest wrote:
>> >
>> > Is there any jdk-builtin Java8+ method which tries to be clever
>> > about low-nanos/micros parking?
>> >
>> > I'm currently considering LockSupport.parkNanos but want to avoid
>> > having the Thread parked when parking + wake-up latency is more
>> > likely to be much greater than the requested time.
>> >
>> > I.e. some combination of onSpinWait + some non-cache-polluting
>> > computation + yielding + actual parking. I'd like to avoid having to
>> > custom-roll it, hence the question for prior art ;)
>>
>> As I understand it, the common wisdom is to wait for about half the
>> round-trip time for a system call and then park. It doesn't sound
>> terribly hard to write something to do that.
>>
>> Please forgive me for digressing, but:
>>
>> Arm has a mechanism to do this, WFE. When a core fails to obtain a
>> lock it executes a WFE instruction which waits on the cache line
>> containing the lock. When that cache line is written to by the core
>> releasing the lock it awakens the waiting core.
>>
>> I'd like to find some way to expose this in a high-level language but
>> it's not at all easy to do.
>>
>> I believe that Intel has MWAIT which is similar, but it's a privileged
>> instruction so no use to us.
>>
>> --
>> Andrew Haley  (he/him)
>> Java Platform Lead Engineer
>> Red Hat UK Ltd. <https://www.redhat.com>
>> https://keybase.io/andrewhaley
>> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
>>
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>
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