Le lundi 15 février 2010 à 12:41 -0800, David Daney a écrit : > On 02/15/2010 12:27 PM, Eric Dumazet wrote: > > Le lundi 15 février 2010 à 12:13 -0800, David Daney a écrit : > >> If we wait for the once-per-second cleanup to free transmit SKBs, > >> sockets with small transmit buffer sizes might spend most of their > >> time blocked waiting for the cleanup. > >> > >> Normally we do a cleanup for each transmitted packet. We add a > >> watchdog type timer so that we also schedule a timeout for 150uS after > >> a packet is transmitted. The watchdog is reset for each transmitted > >> packet, so for high packet rates, it never expires. At these high > >> rates, the cleanups are done for each packet so the extra watchdog > >> initiated cleanups are not needed. > > > > s/needed/fired/ > > > > or perhaps s/are not needed/are neither needed nor fired/ > > > Hmm, but re-arming a timer for each transmited packet must have a cost ? > > > > The cost is fairly low (less than 10 processor clock cycles). We didn't > add this for amusement, people actually do things like only send UDP > packets from userspace. Since we can fill the transmit queue faster > than it is emptied, the socket transmit buffer is quickly consumed. If > we don't free the SKBs in short order, the transmitting process get to > take a long sleep (until our previous once per second clean up task was > run). I understand this, but traditionaly, NIC drivers dont use a timer, but a 'TX complete' interrupt, that usually fires a few us after packet submission on Gigabit speed. A fast program could try to send X small udp packets in less than 150 us, X being greater than the size of your TX ring. So your patch makes the window smaller, but it still is there (at physical layer, we'll see a burst of packets, a ~100us delay, then a second burst)
