Subject: Re: [patch] prefer TSC over PM Timer
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: dean gaudet <dean-list-linux-kernel@arctic.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <Pine.LNX.4.61.0411151531590.22091@twinlark.arctic.org>
References: <Pine.LNX.4.61.0411151531590.22091@twinlark.arctic.org>
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
Message-Id: <1100705099.420.32.camel@localhost.localdomain>
Mime-Version: 1.0
Date: Wed, 17 Nov 2004 15:25:01 +0000
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1160
Lines: 25

On Maw, 2004-11-16 at 00:23, dean gaudet wrote:
> i've heard other folks have independently run into this problem -- in fact 
> i see the most recent fc2 kernels already do this.  i'd like this to be 
> accepted into the main kernel though.

IMHO it was a mistake to make this change in FC2.

> the x86 PM Timer is an order of magnitude slower than the TSC for 
> gettimeofday calls.  i'm seeing 8%+ of the time spent doing gettimeofday 
> in someworkloads... and apparently kernel.org was seeing 80% of its time 
> go to gettimeofday during the fc3-release overload.  PM timer is also less 
> accurate than TSC.

Nobody guarantees that the TSC is clocked at the same rate per CPU and
several power management schemes break it. I see it break on my Thinkpad
600 and its one reason I have to replace the FC kernel with a 2.6-ac
kernel on that system.

Is gettimeofday supposed to return the right value or be fast ?

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/