On Fri, 2011-12-09 at 16:01 +1030, Rusty Russell wrote: > On Wed, 7 Dec 2011 17:02:04 +0000, Ben Hutchings <bhutchings@xxxxxxxxxxxxxx> wrote: > > Solarflare controllers (sfc driver) have 8192 perfect filters for > > TCP/IPv4 and UDP/IPv4 which can be used for flow steering. (The filters > > are organised as a hash table, but matched based on 5-tuples.) I > > implemented the 'accelerated RFS' interface in this driver. > > > > I believe the Intel 82599 controllers (ixgbe driver) have both > > hash-based and perfect filter modes and the driver can be configured to > > use one or the other. The driver has its own independent mechanism for > > steering RX and TX flows which predates RFS; I don't know whether it > > uses hash-based or perfect filters. > > Thanks for this summary (and Jason, too). I've fallen a long way behind > NIC state-of-the-art. > > > Most multi-queue controllers could support a kind of hash-based > > filtering for TCP/IP by adjusting the RSS indirection table. However, > > this table is usually quite small (64-256 entries). This means that > > hash collisions will be quite common and this can result in reordering. > > The same applies to the small table Jason has proposed for virtio-net. > > But this happens on real hardware today. Better that real hardware is > nice, but is it overkill? What do you mean, it happens on real hardware today? So far as I know, the only cases where we have dynamic adjustment of flow steering are in ixgbe (big table of hash filters, I think) and sfc (perfect filters). I don't think that anyone's currently doing flow steering with the RSS indirection table. (At least, not on Linux. I think that Microsoft was intending to do so on Windows, but I don't know whether they ever did.) > And can't you reorder even with perfect matching, since prior packets > will be on the old queue and more recent ones on the new queue? Does it > discard or requeue old ones? Or am I missing a trick? Yes, that is possible. RFS is careful to avoid such reordering by only changing the steering of a flow when none of its packets can be in a software receive queue. It is not generally possible to do the same for hardware receive queues. However, when the first condition is met it is likely that there won't be a whole lot of packets for that flow in the hardware receive queue either. (But if there are, then I think as a side-effect of commit 09994d1 RFS will repeatedly ask the driver to steer the flow. Which isn't ideal.) Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
