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

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Date:	Wed, 24 Sep 2014 08:27:12 -0500
From:	Chuck Ebbert <cebbert.lkml@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Alexander Gordeev <agordeev@...hat.com>,
	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, 24 Sep 2014 09:04:44 -0400
Tejun Heo <tj@...nel.org> wrote:

> Hello, Alexander.
> 
> On Wed, Sep 24, 2014 at 11:42:15AM +0100, Alexander Gordeev wrote:
> > On Tue, Sep 23, 2014 at 04:57:10PM -0400, Tejun Heo wrote:
> > > Hmmm... how does it affect single device operation tho?  It does make
> > > individual interrupt handling heavier, no?
> > 
> > I think it is difficult to assess "individual interrupt handling", since
> > it depends from both the hardware and device access pattern. On the system
> > I use the results are rather counter-intuitive: ahci_thread_fn() does not
> > show up in perf report at all, nor ahci_single_irq_intr(). While before
> > the change ahci_single_irq_intr() reported 0.00%.
> > 
> > But since the handling is split in two parts it is rather incorrect to
> > apply the same metric to the threaded context. Obviously, the threaded
> > handler is expected slowed down by other interrupts handlers, but the
> > whole system should benefit from it, which is exactly the aim of this
> > change.
> 
> 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.
> 

I think he meant other, non-AHCI, interrupt handlers would benefit. A
good test of this patch might be to stream 10Gb ethernet while also
streaming writes to an AHCI device.
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