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Date: Wed, 15 Nov 2006 20:06:06 +0100 From: Ingo Molnar <mingo@...e.hu> To: Jeremy Fitzhardinge <jeremy@...p.org> Cc: Arjan van de Ven <arjan@...radead.org>, Andi Kleen <ak@...e.de>, Eric Dumazet <dada1@...mosbay.com>, akpm@...l.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] i386-pda UP optimization * Jeremy Fitzhardinge <jeremy@...p.org> wrote: > Arjan van de Ven wrote: > > segment register accesses really are not cheap. > > Also really it'll be better to use the register userspace is not using, > > but we had that discussion before; could you remind me why you picked > > %gs in the first place? > > > > To leave open the possibility of using the compiler's TLS support in > the kernel for percpu. I also measured the cost of reloading %gs vs > %fs, and found no difference between reloading a null selector vs a > non-null selector. what point would there be in using it? It's not like the kernel could make use of the thread keyword anytime soon (it would need /all/ architectures to support it) ... and the kernel doesnt mind how the current per_cpu() primitives are implemented, via assembly or via C. In any case, it very much matters to see the precise cost of having the pda selector value in %gs versus %fs. Ingo - 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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