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SubjectRe: [RFC] [PATCH 0/3] ioat: DMA engine support
On Wed, Nov 23, 2005 at 11:30:08PM +0100, Andi Kleen wrote:
> The main problem I see is that it'll likely only pay off when you can keep
> the queue of copies long (to amortize the cost of
> talking to an external chip). At least for the standard recvmsg
> skb->user space, user space-> skb cases these queues are
> likely short in most cases. That's because most applications
> do relatively small recvmsg or sendmsgs.

Don't forget that there are benefits of not polluting the cache with the
traffic for the incoming skbs.

> Longer term the right way to handle this would be likely to use
> POSIX AIO on sockets. With that interface it would be easier
> to keep long queues of data in flight, which would be best for
> the DMA engine.

Yes, that's something I'd like to try soon.

> But it's not clear it's a good idea: a lot of these applications prefer to
> have the target in cache. And IOAT will force it out of cache.

In the I/O AT case it might make sense to do a few prefetch()es of the
userland data on the return-to-userspace code path. Similarly, we should
make sure that network drivers prefetch the header at the earliest possible
time, too.

> I remember the registers in the Amiga Blitter for this and I'm
> still scared... Maybe it's better to keep it simple.

*grin* but you could use it for such cool tasks as MFM en/decoding! =-)

-ben
--
"Time is what keeps everything from happening all at once." -- John Wheeler
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