lkml.org 
[lkml]   [2013]   [Sep]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates
From
On Fri, Sep 20, 2013 at 10:45 AM, Will Deacon <will.deacon@arm.com> wrote:
>
> Right, turns out I can get some interesting numbers from your simple t.c
> program on my dual-cluster, 5 CPU ARMv7 machine. The new cmpxchg-based lockref
> code gives ~50% improvement, but the fun part is that implementing cmpxchg64
> without memory barriers doubles this win to ~100% over current mainline.

Ok, that's certainly noticeable.

> If we can guarantee that the CODE just messes around with the lockref, those
> barriers probably aren't needed...

Yes. I've been thyinking about the barrier issue, and as far as I can
see, as long as the lockref code only ever messes with the reference
count, a totally unordered cmpxchg is fine.

And at least right now we indeed only ever mess with the reference count.

I have been idly toying with the concept of using the cmpxchg also for
possibly taking the lock (for the "xyz_or_lock" versions), but every
time I look at it it seems unlikely to help, and it would require
memory ordering and various architecture-dependent issues, so I
suspect it's never going to make much sense. So yes, an unordered
cmpxchg64 should be perfectly fine.

> As for AIM7/re-aim, I'm having a hard time getting repeatable numbers out of
> it to establish a baseline, so it's not proving to be especially helpful.

That's fine, and yeah, I doubt the t.c improvement really shows
anywhere else (it's kind of extreme), but your numbers are certainly
already sufficient to say "ok, it makes sense even on 32-bit
machines".

Linus


\
 
 \ /
  Last update: 2013-09-20 18:21    [W:0.190 / U:0.064 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site