Re: Performance problem - reads slower than writes

Raghavendra D Prabhu
2012-02-01T00:51:08Z

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Hi,

* On Tue, Jan 31, 2012 at 09:52:10PM +0000, Brian Candler <brian@soundmouse.com> wrote:
>On Tue, Jan 31, 2012 at 09:52:05AM -0500, Christoph Hellwig wrote:
>> You don't just read a single file at a time but multiple ones, don't
>> you?
>
>It's sequential at the moment, although I'll do further tests with the -c
>(concurrency) option to bonnie++
>
>> Try playing with the following tweaks to get larger I/O to the disk:
>>
>> a) make sure you use the noop or deadline elevators
>> b) increase /sys/block/sdX/queue/max_sectors_kb from its low default
>> c) dramatically increase /sys/devices/virtual/bdi/<major>:<minor>/read_ahead_kb
>
>Thank you very much: I will do further tests with these.
>
>Is the read_ahead_kb knob aware of file boundaries? That is, is there any
>risk that if I set it too large it would read useless blocks past the end of
>the file?

The read_ahead_kb knob is used the by memory subsystem
readahead code to set the initial readahead to scale from (it
uses a dynamic scaling window). It is set by default based on
device readahead value (probably obtained in a way similar to
hdparm -I).

Setting it higher will be beneficial for sequential workloads
and the risk you mentioned is not there since it file
boundary aware -- check
http://lxr.linux.no/linux+*/mm/readahead.c#L151 for more
details.
>
>Regards,
>
>Brian.
>
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Regards,
--
Raghavendra Prabhu
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www: wnohang.net

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