Re: xfs-trace-ilock-more

[Jeff Mahoney <jeffm@suse.com>]

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On 12/14/2011 03:32 PM, Dave Chinner wrote:
> On Wed, Dec 14, 2011 at 02:24:32PM -0500, Jeff Mahoney wrote:
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>> 
>> On 12/14/2011 01:27 PM, Mark Fasheh wrote:
>>> Hey Christoph,
>>> 
>>> On Tue, Dec 13, 2011 at 09:40:40PM -0500, Christoph Hellwig
>>> wrote:
>>>> Can you explain the story behid this patch in SLES11SP1?
>>> 
>>> We were looking at some performance issues and needed a bit
>>> more information on the amount of time spent in ilock. I can
>>> give you more specifics if you want, I just have to dig up the
>>> e-mails (it's been a while).
>> 
>> That's pretty much the explanation. With heavy reader load,
>> buffered writes were stalling for 80 ms and sometimes longer. I
>> suspected it was contention on the ilock and the tracing with
>> that patch demonstrated a delay there. Since we were chasing a
>> similar issue at another site, it seemed worthwhile to just keep
>> it around. We're still tracking down the cause. I'm not sure if
>> more recent kernels have the same issue as there's been quite a
>> lot of churn.
> 
> I'm not surprised - there's nothing really guaranteeing bound
> shared vs exclusive access to the ilock. It's all down to the
> read/write bias of the rwsem - readers will hold off the writer for
> some time. Still, it would be nice to see a trace from such a
> holdoff to confirm this is actually the case...

Sure, let me dig it up. We do actually have real results from this
workload with the trace running.

> FWIW, if you have an app that requires concurrent, low latency
> reads and writes to the same file, that's what the XFS Direct IO
> was designed for - in most cases the iolock is taken in shared mode
> for both read and write, and so such hold-offs don't generally
> happen...

You don't need to convince me. I agree with you. :)

- -Jeff

- -- 
Jeff Mahoney
SUSE Labs
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