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SubjectRe: [BUG] long freezes on thinkpad t60
Linus Torvalds wrote:
>
> On Wed, 27 Jun 2007, Nick Piggin wrote:
>
>>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).
>
>
> Oh, that releasing "add" can be unlocked, and only the holder of the lock
> ever touches that field?

Right.


> I must not have looked closely enough. In that case, I withdraw that
> objection, and the sequence-number-based spinlock sounds like a perfectly
> fine one.
>
> Yes, the add will be slightly slower than the plain byte move, and the
> locked xadd will be slightly slower than a regular locked add, but
> compared to the serialization cost, that should be small. For some reason
> I thought you needed a locked instruction for the unlock too.
>
> So try it with just a byte counter, and test some stupid micro-benchmark
> on both a P4 and a Core 2 Duo, and if it's in the noise, maybe we can make
> it the normal spinlock sequence just because it isn't noticeably slower.
>
> In fact, I think a "incb <mem>" instruction is even a byte shorter than
> "movb $1,mem", and with "unlock" being inlined, that could actually be a
> slight _win_.

OK, I'll try running some tests and get back to you on it.

--
SUSE Labs, Novell Inc.
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