From: Jens Axboe <jens.axboe@oracle.com>
To: "Miller, Mike (OS Dev)" <Mike.Miller@hp.com>
Cc: "Ozan Ça??layan" <ozan@pardus.org.tr>,
linux-kernel <linux-kernel@vger.kernel.org>,
"scameron@beardog.cce.hp.com" <scameron@beardog.cce.hp.com>
Subject: Re: CCISS performance drop in buffered disk reads in newer kernels
Date: Mon, 7 Dec 2009 20:32:54 +0100 [thread overview]
Message-ID: <20091207193254.GF8742@kernel.dk> (raw)
In-Reply-To: <0F5B06BAB751E047AB5C87D1F77A77886994120A79@GVW0547EXC.americas.hpqcorp.net>
On Mon, Dec 07 2009, Miller, Mike (OS Dev) wrote:
> > > (max_hw_sectors_kb is 512 on my 2.6.25.20 setup and 1024 on
> > 2.6.30.9
> > > but it seems that it's read-only)
> >
> > The *_hw_* values are the driver exported hardware limits, so
> > they are always read-only.
>
> Ahhh, I didn't know that. There is also an nr_requests attribute which
> to me implies limiting requests somewhere. The value of nr_request is
> 128 but the max commands to the cciss controllers exceed that value.
> What is nr_request supposed to do?
It controls what the block layer queue depth may be. As a rule of thumb,
it should be twice the hardware queue depth. A value of 128 means you
can have at most 128 reads and 128 writes queued in the IO scheduler. In
practice it's a bit more due to request allocation batching.
--
Jens Axboe
prev parent reply other threads:[~2009-12-07 19:32 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-12-07 10:45 CCISS performance drop in buffered disk reads in newer kernels Ozan Çağlayan
2009-12-07 16:32 ` Miller, Mike (OS Dev)
2009-12-07 18:21 ` Ozan Çağlayan
2009-12-07 18:39 ` Jens Axboe
2009-12-07 19:30 ` Miller, Mike (OS Dev)
2009-12-07 19:32 ` Jens Axboe [this message]
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