Linus Torvalds wrote:
On Tue, 26 Jun 2007, Nick Piggin wrote:
Hmm, not that I have a strong opinion one way or the other, but I
don't know that they would encourage bad code. They are not going to
reduce latency under a locked section, but will improve determinism
in the contended case.
xadd really generally *is* slower than an add. One is often microcoded,
the other is not.
Oh. I found xadd to be not hugely slower on my P4, but it was a little
bit.
But the real problem is that your "unlock" sequence is now about two
orders of magnitude slower than it used to be. So it used to be that a
spinlocked sequence only had a single synchronization point, now it has
two. *That* is really bad, and I guarantee that it makes your spinlocks
effectively twice as slow for the non-contended parts.
I don't know why my unlock sequence should be that much slower? Unlocked
mov vs unlocked add? Definitely in dumb micro-benchmark testing it wasn't
twice as slow (IIRC).
But your xadd thing might be worth looking at, just to see how expensive
it is. As an _alternative_ to spinlocks, it's certainly viable.
(Side note: why make it a word? Word operations are slower on many x86
implementations, because they add yet another prefix. You only need a
byte)
No real reason I guess. I'll change it.
--
SUSE Labs, Novell Inc.
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