From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nivedita Singhvi Subject: Re: networking checksum errors again Date: Sat, 01 Apr 2006 13:44:01 -0800 Message-ID: <442EF421.2080708@comcast.net> References: <20060331074641.E26981@web.ray.net> <20060331083153.F26981@web.ray.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Jason Cc: Florian Kirstein , xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org Jason wrote: > w00t! That fixed it. I read your email on the logic for not doing the > tcp checksums and, while I > am by no means in possesion of the brains the developers have, I am left > wondering why that > decision was made. Would anyone care to comment? > I think I missed a post somewhere along this thread... Which decision are you referring to? Avoiding the TCP checksum between domains? The rationale for not doing the checksum is that it is a significant savings in performance. Even for traffic that goes out to a public net and must contain a checksum, deferring it to dom0 when the OS can offload it to the physical NIC (most NICs these days are capable of computing the checksums in hardware) is a significant saving. The native Linux stack offloads the calculation of the checksum to the NIC, and not doing so in the virtual environment increases the gap when comparing Xen to bare metal Linux. There are a lot of other issues here to resolve, of course, and fixing some of the current issues is being worked. It's likely going to be an issue for discussion when we're up for mainline inclusion. thanks, Nivedita