linux-kernel - Re: [PATCH RESEND v3 5/6] AHCI: Optimize single IRQ interrupt processing

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Message-ID: <20141001153114.GE8971@agordeev.usersys.redhat.com>
Date:	Wed, 1 Oct 2014 16:31:15 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: [PATCH RESEND v3 5/6] AHCI: Optimize single IRQ interrupt
 processing

On Wed, Sep 24, 2014 at 10:39:13AM -0400, Tejun Heo wrote:
> Hello, Alexander.
> 
> On Wed, Sep 24, 2014 at 03:08:44PM +0100, Alexander Gordeev wrote:
> > > Hmmm, how would the whole system benefit from it if there's only
> > > single device?  Each individual servicing of the interrupt does more
> > > now which includes scheduling which may end up adding to completion
> > > latency.
> > 
> > As Chuck noticed, non-AHCI hardware context handlers will benefit.
> 
> Maybe I'm off but I'm kinda skeptical that we'd be gaining back the
> overhead we pay by punting to a thread.

Hi Tejun,

As odd as it sounds, I did not mention there is *no* change in IO
performance at all (in my system): neither with one drive nor two.
The change is only about how the interrupt handlers co-exist with
other devices.

I am attaching excerpts from some new perf tests I have done (this
time in legacy interrupt mode). As you can notice, ahci_interrupt()
CPU time drops from 4% to none.

As of your concern wrt threaded handler invocation overhead - I am
not quite sure here, but if SCHED_FIFO policy (the handler runs with)
makes the difference? Anyway, as said above the overall IO does not
suffer.

> -- 
> tejun

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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