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SubjectRE: CCISS performance drop in buffered disk reads in newer kernels


> -----Original Message-----
> From: Jens Axboe [mailto:jens.axboe@oracle.com]
> Sent: Monday, December 07, 2009 12:40 PM
> To: Ozan Ça??layan
> Cc: Miller, Mike (OS Dev); linux-kernel; scameron@beardog.cce.hp.com
> Subject: Re: CCISS performance drop in buffered disk reads in
> newer kernels
>
> On Mon, Dec 07 2009, Ozan Ça??layan wrote:
> > Miller, Mike (OS Dev) wrote:
> > > Ozan,
> > > I'm aware of the performance drop. Please see:
> http://bugzilla.kernel.org/show_bug.cgi?id=13127. I removed
> the huge read ahead value of 1024 that we used because users
> were complaining about small writes being starved. That was
> back around the 2.6.25 timeframe. Since that timeframe there
> have no changes in the main i/o path. I'll get back on this
> as time allows.
> > >
> > > Meanwhile, you can tweak some of the block layer tunables as such.
> > >
> > > echo 64 > /sys/block/cciss\!c0d1/queue/read_ahead_kb
> > > OR
> > > blockdev --setra 128 /dev/cciss/c0d1
> > >
> > > These are just example values. There is also
> max_hw_sectors_kb and max_sectors_kb that be adjusted.
> > >
> >
> > Hi,
> >
> > Actually the "#define READ_AHEAD 1024" was removed on March
> 2008 which
> > was included in the 2.6.25.y tree so 2.6.25.20 has 128kB read_ahead
> > value too.
> >
> > *But* setting read_ahead to 2048 increases buffered disk
> read average
> > from 60~MB/s to 190~MB/s hence the kernel compile time
> drops to 2 minutes.
> >
> > So maybe the regression/change is in another place?
> >
> > The server is just a compile-farm so it's triggered by
> hand, compiles
> > distribution's packages and stays idle until the next
> compilation queue.
> > Is it safe/OK to use that 2048kB read_ahead value for such workload?
>
> Yes, it's definitely safe.

I agree.

>
> > (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?

-- mikem

>
> --
> Jens Axboe
>
> --
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