Re: >10% performance degradation since 2.6.18

Andi Kleen
2009-07-06T08:45:45Z

> That seems to presume it is impossible to reprogram the NIC RX queue
> selection rules?
>
> If you can add a new 'flow' to a NIC hardware RX queue, surely you can
> imagine a remove + add operation for a migrated 'flow'... and thus, at
> least on the NIC hardware level, flows can follow processes.

The standard on modern NIC hardware is stateless hashing: as in you
don't program in flows, but the hardware just uses a fixed hash
to map the header to a rx queue. You can't really significantly influence
the hash on a per flow basis there (in theory you could chose
specific local port numbers, but you can't do that for the remote
ports or for well known sockets)

There are a few highend NICs that optionally support arbitary flow matching,
but it typically only supports a very limited number of flows
(so you need some fallback anyways) and of course it would
be costly to reprogram the NIC on every socket state change.

-Andi
--
ak@linux.intel.com -- Speaking for myself only.