On 10/05/2015 05:19 PM, Andrew Haley wrote:
On 10/05/2015 03:59 PM, Peter Levart wrote:
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.
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).
Pretty much, although the wakeup is just a store.
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.
I spent a while thinking about that, and I'm not sure it's possible.
WFE can wait for a very long time, and there is a general presumption
that if a thread blocks it really should be descheduled rather than
hog a core with a WFE. WFE is excellent if you have many cores (more
cores than threads) and don't mind dedicating them to particular
tasks: it's great for fork/join thread pools, etc, because it doesn't
have the overhead of blocking and unblocking. Maybe the best thing
would be a different version of park/unpark.
Andrew.
Yes, perhaps park/unpark is already to high-level as it requires Thread
to be passed to unpark and therefore requires a thread to publish itself
before doing park(), etc. Spin-looping is usually used without thread
bookkeeping.
I wonder how Gil's proposed PerformanceHints.spinLoopHint() is to be
called. Just before entering the spin-loop or at each iteration of the
loop? Is this some kind of CPU.yield() (by analogy to Thread.yield())?
Regards, Peter
