Re: question about IO-sched

[Corrado Zoccolo]

On Sun, Jul 15, 2012 at 9:08 AM, gaoqiang <gaoqiangscut@xxxxxxxxx> wrote:
>
> 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.
http://kerneltrap.org/node/4941 explains sync_page:
>
> ->sync_page() is an awful misnomer. Usually, when page IO operation is
> requested by calling ->writepage() or ->readpage(), file-system queues
> IO request (e.g., disk-based file system may do this my calling
> submit_bio()), but underlying device driver does not proceed with this
> IO immediately, because IO scheduling is more efficient when there are
> multiple requests in the queue.
> Only when something really wants to wait for IO completion
> (wait_on_page_{locked,writeback}() are used to wait for read and write
> completion respectively) IO queue is processed. To do this
> wait_on_page_bit() calls ->sync_page() (see block_sync_page()---standard
> implementation of ->sync_page() for disk-based file systems).
> So, semantics of ->sync_page() are roughly "kick underlying storage
> driver to actually perform all IO queued for this page, and, maybe, for
> other pages on this device too".

It is expected that sys_read will wait until the data is available for
the process.
If you don't want to wait (because you can do other stuff in the mean
time, including queuing other I/O operations), you can use aio_read.
The kernel will notify your process when the operation completes and
the data is available in memory.

Thanks,
Corrado

>
>
> å Fri, 13 Jul 2012 22:15:31 +0800ïCorrado Zoccolo <czoccolo@xxxxxxxxx> åé:
>
>> 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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>>
>>
>>
>
>
> --
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dott. Corrado Zoccolo                          mailto:czoccolo@xxxxxxxxx
PhD - Department of Computer Science - University of Pisa, Italy
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