From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin LaHaise Subject: Re: [RFC] [PATCH 0/3] ioat: DMA engine support Date: Wed, 23 Nov 2005 19:17:01 -0500 Message-ID: <20051124001700.GC14246@kvack.org> References: <4384E7F2.2030508@pobox.com> <20051123223007.GA5921@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jeff Garzik , Andrew Grover , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, john.ronciak@intel.com, christopher.leech@intel.com Return-path: To: Andi Kleen Content-Disposition: inline In-Reply-To: <20051123223007.GA5921@wotan.suse.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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 Don't Email: .