MQ Patch.

[John-Mark Gurney]

Andre Oppermann wrote this message on Tue, Oct 29, 2013 at 22:25 +0100:
> b) the driver assigns the DMA rings to particular cores which by that, 
> through
>    a critnest++ can drive them lockless.  The drivers (*if_transmit) will 
>    look
>    up the core it got called on and push the traffic out on that DMA ring.
>    The problem is the actual upper stacks affinity which is not guaranteed.
>    This has to consequences: there may be reordering of packets of the same
>    flow because the protocols send function happens to be called from a
>    different core the second time.  Or the drivers (*if_transmit) has to
>    switch to the right core to complete the transmit for this flow if the
>    upper stack migrated/bounced around.  It is rather difficult to assure
>    full affinity from userspace down through the upper stack and then to
>    the driver.

I'll point you to the paper:
http://arxiv.org/abs/1106.0443

Please don't reorder packets.

Binding TX queues to cores seems not very useful, sure you can do a
lockless implementation, but is running the scheduler to change cpu's
really cheaper than paying the cost of migrating the lock?

I'll admit I haven't run benchmarks, but I doubt it.

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."