Date: Mon, 6 Jul 2009 10:45:37 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Jeff Garzik <jeff@garzik.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>, andi@firstfloor.org,
       arjan@infradead.org, matthew@wil.cx, jens.axboe@oracle.com,
       linux-kernel@vger.kernel.org, douglas.w.styner@intel.com,
       chinang.ma@intel.com, terry.o.prickett@intel.com,
       matthew.r.wilcox@intel.com, Eric.Moore@lsi.com,
       DL-MPTFusionLinux@lsi.com, netdev@vger.kernel.org
Subject: Re: >10% performance degradation since 2.6.18
Message-ID: <20090706084537.GB28145@one.firstfloor.org>
References: <20090705040137.GA7747@gondor.apana.org.au> <4A5110B9.4030904@garzik.org>
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> 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.
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