|Subject:||Re: Zero copy transmit|
|From:||Steve Modica <modica@xxxxxxx>|
|Date:||Tue, 29 Apr 2003 14:41:27 -0500|
|User-agent:||Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4b) Gecko/20030425|
Andi Kleen wrote:
On Tue, Apr 29, 2003 at 01:44:15PM -0500, Steve Modica wrote:We are doing some experiementing with Altix systems (Itanium II with NUMA) and we're taking a big hit from __copy_user traffic. We would like to modify the write, writev, send and sendto interfaces such that we can avoid the __copy_user call by marking pages copy-on-write (COW)and handing them off to be transmitted. Since this requires TLB updates, we would only implement this code on platforms that defined themselves as capable of fast TLB updates.A much better way would be to use the POSIX aio interfaces. They support zero copy transmit, but don't require COW. Instead they just tell the user process when it is safe to touch the buffer again. There was already some code to do aio TCP sending, but it didn't do zero copy and was not merged for some reason.Also you can already do zero copy transmit using sendfile()Linux basically has all the infrastructure you need for this already; just the high level interface to the AIO system calls is still missing. -Andi
Hi Andi,We are aware of sendfile() and used it for the purposes of proving that zero copy would make a big difference for us.
At issue is really application capture and customer adoption. There aretons of apps and lots of engineers that know socket operations and write/writev. Asking all ISVs to recode for linux would leave them with two separate APIs to deal with. They would have send/sendto or write/writev on Solaris, HPUX and whatever else, and linux would have sendfile.
We really want to do this in such a way that it doesn't create a huge footprint (and we think we can) and we want to make sure we don't impact systems that can't take advantage of fast TLB updates.
Steve -- Steve Modica work: 651-683-3224 mobile: 651-261-3201 Manager - Networking Drivers Group "Give a man a fish, and he will eat for a day, hit him with a fish and he leaves you alone" - me
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