linux-kernel - Re: [PATCH 0/2]: Remote softirq invocation infrastructure.

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Date:	Sat, 20 Sep 2008 10:40:04 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, jens.axboe@...cle.com,
	steffen.klassert@...unet.com
Subject: Re: [PATCH 0/2]: Remote softirq invocation infrastructure.

On Sat, 2008-09-20 at 09:19 -0700, Arjan van de Ven wrote:
> On Sat, 20 Sep 2008 09:02:09 -0700
> Daniel Walker <dwalker@...sta.com> wrote:
> 
> > On Sat, 2008-09-20 at 08:45 -0700, Arjan van de Ven wrote:
> > > On Sat, 20 Sep 2008 08:29:21 -0700
> > > > 
> > > > > Jen's, as stated, has block layer uses for this.  I intend to
> > > > > use this for receive side flow seperation on non-multiqueue
> > > > > network cards.  And Steffen Klassert has a set of IPSEC
> > > > > parallelization changes that can very likely make use of this.
> > > > 
> > > > What's the benefit that you (or Jens) sees from migrating softirqs
> > > > from specific cpu's to others?
> > > 
> > > it means you do all the processing on the CPU that submitted the IO
> > > in the first place, and likely still has the various metadata
> > > pieces in its CPU cache (or at least you know you won't need to
> > > bounce them over)
> > 
> > 
> > In the case of networking and block I would think a lot of the softirq
> > activity is asserted from userspace.. Maybe the scheduler shouldn't be
> > migrating these tasks, or could take this softirq activity into
> > account ..
> 
> well a lot of it comes from completion interrupts.

Yeah, partly I would think.

> and moving userspace isn't a good option; think of the case of 1 nic
> but 4 apache processes doing the work...
> 

One nic, so one interrupt ? I guess we're talking about an SMP machine? 
It seems case dependent .. If you send a lot, or receive a lot.. BUT
it's all speculation on my part..

Dave didn't supply the users of his code, or what kind of improvement
was seen, or the case in which it would be needed. I think Dave knowns
his subsystem, but the code on the surface looks like an end run around
some other problem area..

Daniel

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