site_archiver@lists.apple.com Delivered-To: darwin-kernel@lists.apple.com Andrew, Thanks, Vincent On Jan 10, 2007, at 1:50 PM, Andrew Gallatin wrote: /* * Determine a reasonable value for maxseg size. * If the route is known, check route for mtu. * If none, use an mss that can be handled on the outgoing * interface without forcing IP to fragment; if bigger than * an mbuf cluster (MCLBYTES), round down to nearest multiple of MCLBYTES * to utilize large mbufs. If no route is found, route has no mtu, <....> And in tcp_mss() FreeBSD actually does this: tp->t_maxseg = mss; #if (MCLBYTES & (MCLBYTES - 1)) == 0 if (mss > MCLBYTES) mss &= ~(MCLBYTES-1); #else if (mss > MCLBYTES) mss = mss / MCLBYTES * MCLBYTES; #endif tp->t_maxseg = mss; Any idea why this code is missing from darwin? While trying to get decent 10GbE jumbo frame performance numbers from MacOSX 10.4.8 between a pair of 2.6Ghz Mac Pros, I found I was limited to about 7Gb/s for a single threaded test, or 8.8Gb/s for a multi-threaded test. If I manually set the MTU such that the MSS winds up as 8192, then the single threaded performance jumps to 8Gb/s and the multi-threaded to 9.08Gb/s. Eg: % netperf242 -H macpro01-m -l10 -P 0 524288 524288 524288 10.00 7047.07 % sudo route change -mtu 8244 macpro01-m change host macpro01-m % netperf242 -H macpro01-m -l10 -P 0 524288 524288 524288 10.00 8009.17 (I'm quoting a small sample size for this email, but it is reproducable over a large number of runs..) _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-kernel mailing list (Darwin-kernel@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-kernel/site_archiver%40lists.a... This is an optimization that is simple missin in xnu -- thanks for bringing it to our attention. Feel free to file an enhancement request at http:// bugreporter.apple.com/ to make sure this won't fall through the cracks. Most BSDs "clamp" the MSS to a multiple of MCLBYTES.

From netinet/tcp_input.c in both darwin and and FreeBSD:

Drew This email sent to site_archiver@lists.apple.com