Date: Sat, 25 Nov 2006 11:23:21 -0800
From: Andrew Morton <akpm@osdl.org>
To: "Martin A. Fink" <fink@mpe.mpg.de>
Cc: linux-kernel@vger.kernel.org
Subject: Re: SATA Performance with Intel ICH6
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On Fri, 24 Nov 2006 15:10:11 +0100
"Martin A. Fink" <fink@mpe.mpg.de> wrote:

> Here some measurement results:
> 
> time dd if=/dev/zero of=test.zero bs=1M count=1000
> results in
> 
> real 0m52.561s
> user 0m0.003s
> sys  0m7.407s
> 
> and strace dd... gives among other information
> 6.84s 1004calls syscall: write
> 
> So I spend 45s of 52s within the kernel. Why so long?

Profiling the kernel will tell.  Preferably with oprofile, but if for some
reason that doesn't work, with the old profiler.

I use this:

#!/bin/sh
sudo opcontrol --stop
sudo opcontrol --shutdown
sudo rm -rf /var/lib/oprofile
sudo opcontrol --vmlinux=/boot/vmlinux-$(uname -r)
sudo opcontrol --start-daemon
sudo opcontrol --start
time $@
sudo opcontrol --stop
sudo opcontrol --shutdown
sudo opreport -l /boot/vmlinux-$(uname -r) | head -50

I have seen PIO and MMIO operations on PCI busses take tremendous amounts
of time when that bus is saturated with DMA transfers.  But that was back
in the old days.
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