Date:	Fri, 15 Apr 2011 21:59:38 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: net: Automatic IRQ siloing for network devices

On Fri, Apr 15, 2011 at 11:54:29PM +0100, Ben Hutchings wrote:
> On Fri, 2011-04-15 at 16:17 -0400, Neil Horman wrote:
> > Automatic IRQ siloing for network devices
> > 
> > At last years netconf:
> > http://vger.kernel.org/netconf2010.html
> > 
> > Tom Herbert gave a talk in which he outlined some of the things we can do to
> > improve scalability and througput in our network stack
> > 
> > One of the big items on the slides was the notion of siloing irqs, which is the
> > practice of setting irq affinity to a cpu or cpu set that was 'close' to the
> > process that would be consuming data.  The idea was to ensure that a hard irq
> > for a nic (and its subsequent softirq) would execute on the same cpu as the
> > process consuming the data, increasing cache hit rates and speeding up overall
> > throughput.
> > 
> > I had taken an idea away from that talk, and have finally gotten around to
> > implementing it.  One of the problems with the above approach is that its all
> > quite manual.  I.e. to properly enact this siloiong, you have to do a few things
> > by hand:
> > 
> > 1) decide which process is the heaviest user of a given rx queue 
> > 2) restrict the cpus which that task will run on
> > 3) identify the irq which the rx queue in (1) maps to
> > 4) manually set the affinity for the irq in (3) to cpus which match the cpus in
> > (2)
> [...]
> 
> This presumably works well with small numbers of flows and/or large
> numbers of queues.  You could scale it up somewhat by manipulating the
> device's flow hash indirection table, but that usually only has 128
> entries.  (Changing the indirection table is currently quite expensive,
> though that could be changed.)
> 
> I see RFS and accelerated RFS as the only reasonable way to scale to
> large numbers of flows.  And as part of accelerated RFS, I already did
> the work for mapping CPUs to IRQs (note, not the other way round).  If
> IRQ affinity keeps changing then it will significantly undermine the
> usefulness of hardware flow steering.
> 
> Now I'm not saying that your approach is useless.  There is more
> hardware out there with flow hashing than with flow steering, and there
> are presumably many systems with small numbers of active flows.  But I
> think we need to avoid having two features that conflict and a
> requirement for administrators to make a careful selection between them.
> 
> Ben.
> 
I hear what your saying and I agree, theres no point in having features work
against each other.  That said, I'm not sure I agree that these features have to
work against one another, nor does a sysadmin need to make a choice between the
two.  Note the third patch in this series.  Making this work requires that
network drivers wanting to participate in this affinity algorithm opt in by
using the request_net_irq macro to attach the interrupt to the rfs affinity code
that I added.  Theres no reason that a driver which supports hardware that still
uses flow steering can't opt out of this algorithm, and as a result irqbalance
will still treat those interrupts as it normally does.  And for those drivers
which do opt in, irqbalance can take care of affinity assignment, using the
provided hint.  No need for sysadmin intervention.

I'm sure there can be improvements made to this code, but I think theres less
conflict between the work you've done and this code than there appears to be at
first blush.

Best
Neil

> -- 
> Ben Hutchings, Senior Software Engineer, Solarflare
> Not speaking for my employer; that's the marketing department's job.
> They asked us to note that Solarflare product names are trademarked.
> 
> 
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