On Fri, Apr 24, 2015 at 6:50 AM, Eric Dumazet <[email protected]> wrote: > On Thu, 2015-04-23 at 21:40 -0700, Dave Taht wrote: >> and of course, after writing the previous email, I go reading the >> original commit for this option. Yea, that is a huge increase in >> context switches... >> >> https://lwn.net/Articles/560082/ >> >> ... but totally worth it for many apps that can do something else >> while their connection congests, and totally awesome for tcp vpns, >> x11, screen sharers, etc.... > > It all depends on how many bytes are pushed by the application per > sendmsg() > > To keep the amount of unsent bytes low, the application should not issue > a large write, but it still can if it needs to for whatever reason. > > netperf -t TCP_STREAM" uses a default size of 16384 bytes per sendmsg. > > So obviously, if a wakeup is needed per sendmsg(), number of context > switches is exactly bandwidth_in_bytes_per_second / 16384 > > Normally, without this TCP_NOTSENT_LOWAT option, number of wakeups is > more like bandwidth_in_bytes_per_second / SO_SNDBUF, because kernel > wakes up the blocked task when output buffers size occupancy reached 50% > > >
I think a "userspace janitors" project is needed, where we identify everything that could benefit from TCP_NOTSENT_LOWAT[1], and go patch it. I did a little of this for using IPV6_TCLASS right on a ton of applications and (for example) have some long standing patches submitted to rsync for selecting congestion control and setting IP_TOS/IPV6_TCLASS (sigh - still not accepted). Maybe GSOC? Getting, say just one college class to up and go do it, for a week or two, together, analyzing the the results as they go, would make a dent.... [1] I think userspace vpns could use an internal fq+codel algorithm, or perhaps the kernel socket read buffer could gain a socket option to present one -- Dave Täht Open Networking needs **Open Source Hardware** https://plus.google.com/u/0/+EricRaymond/posts/JqxCe2pFr67 _______________________________________________ Bloat mailing list [email protected] https://lists.bufferbloat.net/listinfo/bloat
