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

[Alexander Gordeev]

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@xxxxxxxxxx
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