Re: big picture UDP/IP performance question re 2.6.18 -> 2.6.32

From: starlight
Date: Fri Oct 07 2011 - 14:48:01 EST

At 02:09 PM 10/7/2011 -0400, chetan loke wrote:
>I'm a little confused. Seems like there are
>conflicting goals. If you want to bypass the
>kernel-protocol-stack then you have the following
>options: a) kernel af_packet. This is where we
>would get a chance to test all the kernel features

Perhaps I haven't been sufficiently clear.
The "packet socket" mode I refer to in the
earlier post was using AF/PF_PACKET mode sockets
as in

socket(PF_PACKET, SOCK_RAW, eth_p_all);

Have run it in both normal and memory mapped
modes. MMAP mode is a slight bit more expensive
due to the cache pressure from the additional
copy. On the 6174 MMAP seems to be a smidgen
better in certain tests, but in the end both
read() and mapped approaches are effectively
identical on performance--and generally match
the cost of UDP sockets almost exactly.

b) Use non-commodity(?) NICs(from vendors
>you mentioned): where it might have some on-board
>memory(cushion) and so it can absorb the spikes
>and can also smoothen out too many
>PCI-transactions for bursty (and small payload -
>as in 64 byte traffic). But wait, when you use the
>libs provided by these vendors, then their
>driver(especially the Rx path) is not so much
>working in inline mode as NIC drivers in case a)
>above. This driver with a special Rx-path purely
>exists for managing your mmap'd queues.So
>of-course it's going to be faster that the
>traditional inline drivers. In this partial-inline
>mode, the adapter might i) batch the packets and
>ii) send a single notification to the
>host-side. With that single event you are now
>processing 1+ packets.

Kernel bypass is probably the best answer for
what we do. Problem has been lack of maturity
in their driver software. Looks like it's reaching
a point where they cover our use case. As mentioned
earlier, Solarflare could not match the Intel
82599 + ixgbe for this app last year. Was a
disaster. Myricom is focused on UDP (better
for us), but only just added multi-core IRQ
doorbell wakeups in recent months. Previously
one had to accept all IRQs on a single core or
poll, neither of which works for us.

>You got it. In case of tilera there are two modes:
>tile-cpu in device mode: beats most of the
>non-COTS NICs. It runs linux on the adapter
>side. Imagine having the flexibility/power to
>program the ASIC using your favorite OS. Its
>orgasmic. So go for it! tile-cpu in host-mode:
>Yes, it could be a game changer.

We almost went for the 1st gen Tile64 outboard
NIC approach, but were concerned about whether
they would survive--still are. Intel has
crushed more than a few competitors along
the way. If Google or Facebook buys into the
Tile-Gx it becomes a safe choice overnight.

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