Messages in this thread | | | Date | Wed, 08 Dec 2010 22:26:59 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [cpuops cmpxchg V1 2/4] x86: this_cpu_cmpxchg and this_cpu_xchg operations |
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On 12/08/2010 10:17 AM, Mathieu Desnoyers wrote: > > Hi Christoph, > > Can you show if this provides savings in terms of: > > - instruction cache footprint > - cycles required to run > - large-scale impact on the branch prediction buffers > > Given that this targets per-cpu data only, the additional impact on cache-line > exchange traffic of using cmpxchg over xchg (cache-line not grabbed as exclusive > by the initial read) should not really matter. > > I'm CCing Arjan and HPA, because they might have some interesting insight into > the performance impact of lock-prefixed xchg vs using local cmpxchg in a loop. >
XCHG is always locked; it doesn't need the prefix. Unfortunately, unlike on the 8086 on modern processors locks have a real cost.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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