netdev - Re: [PATCH] xps-mq: Transmit Packet Steering for multiqueue

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Date:	Mon, 23 Aug 2010 18:50:55 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH] xps-mq: Transmit Packet Steering for multiqueue

On Mon, 2010-08-23 at 10:30 -0700, Tom Herbert wrote:
> 
> 
> On Mon, Aug 23, 2010 at 10:09 AM, Ben Hutchings
> <bhutchings@...arflare.com> wrote:
>         On Sun, 2010-08-22 at 22:39 -0700, Tom Herbert wrote:
>         [...]
>         > Each transmit queue can be associated with a number of CPUs
>         which will
>         > used the queue to send packets.  This is configured as a CPU
>         mask on a
>         > per queue basis in:
>         >
>         > /sys/class/net/eth<n>/queues/tx-<n>/xps_cpus
>         >
>         > The mappings are stored per device in an inverted data
>         structure that
>         > maps CPUs to queues.  In the netdevice structure this is an
>         array of
>         > num_possible_cpu structures where each array entry contains
>         a bit map
>         > of queues which that CPU can use.
>         
>         [...]
>         
>         The mapping of TX queue to CPU should match the affinity of
>         the
>         completion IRQ for that queue.  It should not be a separate
>         setting.
>         
>         
>         
> That implies one possible configuration, but there are others.  For
> instance, there may be fewer queues than CPUs in which case a TX queue
> would be used by more than just the CPU handling the IRQ.

The affinity of IRQs is not restricted to a single CPU either, but I
take your point.

The IRQ affinity and the mapping of sender to queue do at least need to
be coordinated, and I think that continuing to add independent knobs for
CPU affinity of closely-related objects makes it too hard for
administrators to get this right.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
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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