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Re: Performance problem - reads slower than writes

To: Brian Candler <brian@xxxxxxxxxxxxxx>
Subject: Re: Performance problem - reads slower than writes
From: Raghavendra D Prabhu <raghu.prabhu13@xxxxxxxxx>
Date: Wed, 1 Feb 2012 06:20:57 +0530
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
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Hi,


* On Tue, Jan 31, 2012 at 09:52:10PM +0000, Brian Candler 
<brian@xxxxxxxxxxxxxx> 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,
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Raghavendra Prabhu
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