Hi,
* Martin Bligh ([email protected]) wrote:
> Christoph Lameter wrote:
> >On Mon, 9 Jul 2007, Martin Bligh wrote:
> >
> >>Those numbers came from Mathieu Desnoyers (LTTng) if you
> >>want more details.
> >
> >Okay the source for these numbers is in his paper for the OLS 2006: Volume
> >1 page 208-209? I do not see the exact number that you referred to there.
>
Hrm, the reference page number is wrong: it is in OLS 2006, Vol. 1 page
216 (section 4.5.2 Scalability). I originally pulled out the page number
from my local paper copy. oops.
> Nope, he was a direct co-author on the paper, was
> working here, and measured it.
>
> >He seems to be comparing spinlock acquire / release vs. cmpxchg. So I
> >guess you got your material from somewhere else?
> >
I ran a test specifically for this paper where I got this result
comparing the local irq enable/disable to local cmpxchg.
> >Also the cmpxchg used there is the lockless variant. cmpxchg 29 cycles w/o
> >lock prefix and 112 with lock prefix.
Yep, I volountarily used the variant without lock prefix because the
data is per cpu and I disable preemption.
> >
> >I see you reference another paper by Desnoyers:
> >http://tree.celinuxforum.org/CelfPubWiki/ELC2006Presentations?action=AttachFile&do=get&target=celf2006-desnoyers.pdf
> >
> >I do not see anything relevant there. Where did those numbers come from?
> >
> >The lockless cmpxchg is certainly an interesting idea. Certain for some
> >platforms I could disable preempt and then do a lockless cmpxchg.
>
Yes, preempt disabling or, eventually, the new thread migration
disabling I just proposed as an RFC on LKML. (that would make -rt people
happier)
> Mathieu, can you give some more details? Obviously the exact numbers
> will vary by archicture, machine size, etc, but it's a good point
> for discussion.
>
Sure, also note that the UP cmpxchg (see asm-$ARCH/local.h in 2.6.22) is
faster on architectures like powerpc and MIPS where it is possible to
remove some memory barriers.
See 2.6.22 Documentation/local_ops.txt for a thorough discussion. Don't
hesitate ping me if you have more questions.
Regards,
Mathieu
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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