Spin Loop Hint support: Draft JEP proposal (original) (raw)

Peter Levart peter.levart at gmail.com
Mon Oct 5 14:59:12 UTC 2015

On 10/05/2015 11:41 AM, Andrew Haley wrote:

Hi Gil,

On 04/10/15 17:22, Gil Tene wrote:

Summary

Add an API that would allow Java code to hint that a spin loop is being executed. I don't think this will work for ARM, which has a rather different spinlock mechanism. Instead of PAUSE, we wait on a lock word with WFE. WFE puts a core into a lightweight sleep state waiting on a particular address (the lock word) and a write to the lock word wakes it up. This is very useful and somewhat analogous to 86's MONITOR/MWAIT. I can't immediately see how to generalize your proposal to ARM, which is a shame. Andrew.

Just a thought...

ARM WaitForEvent/SendEvent instructions sound like a kind of park/unpark, but on a CPU-core-level (WFE) and global system-level (SendEvent). I wonder whether it would be possible to use them to optimize the latency of the implementation of park/unpark. The same goes for Spin Loop Hint. Would it be possible to incorporate spin-looping in the park/unpark implementation for x86 itself? Higher-level synchronization constructs (like locks, synchronizers, etc..) would then just use park/unpark and not bother with spin-looping themselves.

Regards, Peter

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