On Fri, 24 Nov 2006 15:10:11 +0100
"Martin A. Fink" <[email protected]> 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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