On Fri, Apr 11, 2008 at 09:52:45PM +0300, Pasi Kärkkäinen wrote:
>
> So.. it seems with the read-ahead of 4096 (2 MB) the CPU on the
> target maxes out.. with most of the time spent on iowait.
>
> The question is why this happens?
>
> Some settings I should tune on the target?
>
And now I tried with nullio target..
read-ahead of 4096 on the initiator:
iostat on initiator:
avg-cpu: %user %nice %system %iowait %steal %idle
1.52 0.00 17.68 30.81 0.00 50.00
iostat on target:
avg-cpu: %user %nice %system %iowait %steal %idle
0.00 0.00 27.00 0.00 0.00 73.00
# dd if=/dev/sdb of=/dev/null bs=4k count=524288
524288+0 records in
524288+0 records out
2147483648 bytes (2.1 GB) copied, 18.5658 seconds, 116 MB/s
read-ahead of 1024 on the initiator:
iostat on initiator:
avg-cpu: %user %nice %system %iowait %steal %idle
1.01 0.00 20.71 29.80 0.00 48.48
iostat on target:
avg-cpu: %user %nice %system %iowait %steal %idle
0.00 0.00 14.00 0.00 0.00 86.00
# dd if=/dev/sdb of=/dev/null bs=4k count=524288
524288+0 records in
524288+0 records out
2147483648 bytes (2.1 GB) copied, 18.5787 seconds, 116 MB/s
read-ahead of 256 on the initiator:
iostat on initiator:
avg-cpu: %user %nice %system %iowait %steal %idle
0.50 0.00 22.61 26.63 0.00 50.25
iostat on target:
avg-cpu: %user %nice %system %iowait %steal %idle
0.00 0.00 14.50 0.00 0.00 85.50
# dd if=/dev/sdb of=/dev/null bs=4k count=524288
524288+0 records in
524288+0 records out
2147483648 bytes (2.1 GB) copied, 23.1832 seconds, 92.6 MB/s
So.. the effect of bigger read-ahead on the initiator causing
more CPU usage on the target can be seen here too..
-- Pasi
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