many thanks. but why the sys_read operation hangs on sync_page ? there are
still
many free memory.I mean ,the actually free memory,excluding the various
kinds of
caches or buffers.
在 Fri, 13 Jul 2012 22:15:31 +0800，Corrado Zoccolo <czoccolo@xxxxxxxxx>
写道:
Hi,
the catch is that writes are "fire and forget", so they keep accumulating
in the I/O sched, and there is always plenty of them to schedule (unless
you explicitly make sync writes).
The reader, instead, waits for the result of each read operation before
scheduling a new read, so there is at most one outstanding read, and some
time nothing.
The deadline scheduler is work conserving, meaning that it never leaves
the
disk idle when there is work queued, and most of the time after an
operation completes, there is only write work queued, so you see much
more
writes being sent to the device.
Only schedulers that delay writes waiting for reads (as Anticipatory in
old
kernels, and now CFQ) can achieve higher read to write ratios.
Cheers
Corrado
On Thu, Jul 12, 2012 at 11:01 AM, gaoqiang <gaoqiangscut@xxxxxxxxx>
wrote:
Hi,all
I have long known that deadline is read-prefered. but a simple
test gives the opposite result.
with two processes running at the same time,one for read and one
for write.actually,they did nothing bug IO operation.
while(true)
{
read();
}
the other:
while(true)
{
write();
}
with deadline IO-sched and ext4 filesystem.as a result, read
ratio was about below 3M/s.and write about 100M/s. I have tested both
kernel-2.6.18 and kernel-2.6.32,getting the same result.
I add some debug information in the kernel and recompile,found
that,it has little to do with IO-sched layer because read request
dropped
into deadline was 5% of write request .from /proc/<pid>/stack,the read
process hands on sync_page most of the time.
what is the matter ? anyone help me ?
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