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Date: Wed, 15 Dec 2010 09:31:14 -0800 From: "H. Peter Anvin" <hpa@...or.com> To: Peter Zijlstra <a.p.zijlstra@...llo.nl> CC: Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>, linux-kernel@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Subject: Re: [cpuops cmpxchg V2 3/5] irq_work: Use per cpu atomics instead of regular atomics On 12/15/2010 09:18 AM, Peter Zijlstra wrote: > On Wed, 2010-12-15 at 11:04 -0600, Christoph Lameter wrote: > >> Prefixes are faster than explicit address calculations. A prefix allows >> you to integrate the per cpu address calculation into an arithmetic >> operation. > > Well, depends on how often you need that address I'd think. If you'd > have a per-cpu struct and need to frob lots of variables in that struct > it might be cheaper to simply compute the struct address once and then > use relative addresses than to prefix everything with %fs. > Let's just make it clear -- current x86 CPUs generally do not have a penalty for prefixes (it might be that under very unusual pipeline conditions they do, I am not 100% sure.) In fact, we changed patching LOCK prefixes from NOP to %ds: because it made the code faster. Some older CPUs do, but those are no longer relevant for performance decisions. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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