Re: [RFC PATCH net-next-2.6 0/2] Automatic XPS mapping

Hi Ben,

I've finally gotten around to looking at how XPS interacts with the HW
priority queues...

On Fri, Feb 18, 2011 at 8:13 AM, Ben Hutchings
[off-list ref] wrote:

In the same way that we maintain a mapping CPUs to RX queues for RFS
acceleration based on current IRQ affinity and the CPU topology, we can
maintain a mapping of CPUs to TX queues for queue selection in XPS.  (In
fact this may be the same mapping.)

Any more progress on this?  It seem like a good way to provide default
configuration for XPS that is usable.

Questions:
- Does this make a real difference to performance?

XPS seems to when configured correctly :-)

 (I've only barely tested this.)
- Should there be a way to disable it?
- Should the automatic mapping be made visible?

Yes, would be nice for this to be readable in the tx-<n> directory for
the queue.  Same thing for rmap in RFS acceleration.

 (This applies RFS acceleration too.)

- Should different mappings be allowed for different traffic classes,
 in case they have separate sets of TX interrupts with different
 affinity?
 (This applies to manual XPS configuration too.)

Yes.  Looking at XPS and the HW traffic class support, I realized that
they don't seem to play together at all.  If XPS is enabled, we don't
do the skb_tx_hash which is where the priority is taken into account.
It's probably worse than that, AFAICT XPS could be configured so that
packets are inadvertently sent on arbitrary priority queues.

I think the correct approach is to first choose a set of queues by
priority, and then among those queues perform XPS.  Probably requires
some new configuration to make this hierarchy visible.

Tom

Ben Hutchings (2):
 net: XPS: Allow driver to provide a default mapping of CPUs to TX
   queues
 sfc: Add CPU queue mapping for XPS

 drivers/net/sfc/efx.c     |   59 +++++++++++++++++++++++++++++++-------------
 include/linux/netdevice.h |   10 +++++--
 net/Kconfig               |   17 +++++++++----
 net/core/dev.c            |   41 +++++++++++++++++-------------
 4 files changed, 83 insertions(+), 44 deletions(-)

--
1.7.3.4


--
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.