Subject: Re: [RFC] [PATCH 0/3] ioat: DMA engine support

From: Benjamin LaHaise <[email-masked]>
Date: Wed, 23 Nov 2005 19:17:01 -0500

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
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