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Date: Fri, 25 Jan 2002 17:20:04 -0800 (PST)
From: Linus Torvalds <torvalds@transmeta.com>
To: Andrew Morton <akpm@zip.com.au>
cc: David Howells <dhowells@redhat.com>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] syscall latency improvement #1
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On Fri, 25 Jan 2002, Andrew Morton wrote:
> >
> > Looking at the code, I suspect that 99.9% of this "improvement" comes from
> > one thing, and one thing only: you removed the "cli" in the system call
> > return path.
>
> Before the cli was in the stock kernel, I had added it in the
> low-latency patch.  Careful testing showed that it added
> 13 machine cycles to a system call on a P3.

That sounds about right. The empty system call path is basically dominated
by the trap/iret costs, and is on the order of 200 cycles or so on most
CPU's. So 13 cycles would account for the roughly 5% improvement.

		Linus

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