On Fri, 2008-22-08 at 16:56 +1000, Herbert Xu wrote:

On Tue, Jul 22, 2008 at 01:11:41AM +0800, Herbert Xu wrote:

quoted

On Mon, Jul 21, 2008 at 10:08:21AM -0700, David Miller wrote:

I haven't had a chance to do the test yet but I've just had an
idea of how we can get the best of both worlds.

The problem with always directing traffic based on the CPU alone
is that processes move around and we don't want to introduce packet
reordering because to that.

Assuming multi-rx queues with configurable MSI or otherwise to map
to a receive processor, then in the case of routing/bridging or
otherfavoriteformofforwarding:
If you tie static filters to a specific cpu that will always work.
So no reordering there.
Local traffic i can see migration/reordering happening.

The problem with hashing based on packet headers alone is that
it doesn't take CPU affinity into account at all so we may end
up with a situation where one thread out of a thread pool (e.g.,
a web server) has n sockets which are hashed to n different
queues.

Indeed. In the forwarding case, the problem is not reordering rather
all flows will always end up in the same cpu. So if you may end up
just overloading one cpu while the other 1023 stayed idle.
My earlier statement was you could cook traffic scenarios where all
1024 are being fully utilized (the operative term is "cook");->

So here's the idea, we determine the tx queue for a flow based
on the CPU on which we saw its first packet.  Once we have decided
on a queue we store that in a dst object (see below).  This
ensures that all subsequent packets of that flow ends up in
the same queue so there is no reordering.  It also avoids the
problem where traffic genreated by one CPU gets scattered across
queues.

Wont work with static multi-rx nic; iirc, changing those filters is
_expensive_. so you want it to stay static.

cheers,
jamal

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