Re: question about IO-sched

[gaoqiang]

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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