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SubjectRe: [PATCH] Yielding processor resources during lock contention
On Sat, 11 Sep 2004, Linus Torvalds wrote:

> I'd seriously suggest you ask Intel for an official opinion on this. Last
> I heard (and that was, I believe, before monitor/mwait had been officially
> announced, so it's certainly a bit dated now) it wasn't architecturally
> clear that it's a good idea using it for things like spinlocks.

Indeed last i heard, it was unuseable for low latency locking, for this
case (lock acquisition after relinquish on remote processor) we may be
able to put up with the higher latency.

> In particular, if the CPU idly waits for a cacheline to be dirtied, it is
> entirely possible that the other CPU that owns the lock and releases it
> won't actually _tell_ the world that the lock has been released for quite
> some time. After all, why should it - if it is the exclusive owner, and it
> sees no memory traffic on the bus, it may have no reason to push out the
> fact that it just released the lock. Just keep it dirty in its caches.
> In other words: monitor/mwait on purpose obviously causes fewer bus
> cycles. But that very fact may well mean (at least in theory) that you get
> very high latencies. It could make spinlock contention very very unfair
> (the original CPU keeps getting the lock over and over again, while the
> monitor/mwait one never gets to play), and it might also make ping-pong
> locking latency be extremely high.

Good point, i can see this scenario occuring on current processors.

> Also, it's entirely possible that monitor/mwait ends up shutting down the
> CPU to the point that getting out of a lower-power mode might have a
> latency of microseconds or even milliseconds. External (or even internal)
> voltage regulators and frequency changes are not instantaneous by any
> means..

Yes i noted too, currently it doesn't support any additional options
to affect how the processor halts during mwait, intel could indeed pull a
number and make the default ultra high wakeup latency mode. I truly
hope they don't as it really would make the whole thing useless.

> In other words, I would strongly suggest you _not_ really consider this a
> serious thing (feel free to play around with it and try to get some
> numbers) with this without getting an answer from Intel about what the
> _architected_ behaviour of monitor/mwait is, from a latency standpoint.

Well the i386 and x86_64 versions were purely for testing purposes on my
part, i'm content with dropping them. The main user was going to be PPC64,
but i felt compelled to throw in an architecture implementation.

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