From: Rusty Russell <ru...@rustcorp.com.au> Date: Thu, 21 May 2009 16:27:05 +0930
> On Tue, 19 May 2009 12:10:13 pm David Miller wrote: >> What you're doing by orphan'ing is creating a situation where a single >> UDP socket can loop doing sends and monopolize the TX queue of a >> device. The only control we have over a sender for fairness in >> datagram protocols is that send buffer allocation. > > Urgh, that hadn't even occurred to me. Good point. Now this all is predicated on this actually mattering. :-) You could argue that the scheduler as well as the size of the TX queue should be limiting and enforcing fairness. Someone really needs to test this. Just skb_orphan() every packet at the beginning of dev_hard_start_xmit(), then run some test program with two clients looping out UDP packets to see if one can monopolize the device and get a significantly larger amount of TX resources than the other. Repeat for 3, 4, 5, etc. clients. > I haven't thought this through properly, but how about a hack where > we don't orphan packets if the ring is over half full? That would also work. And for the NIU case this would be great because I DO have a marker bit for triggering interrupts in the TX descriptors. There's just no "all empty" interrupt on TX (who designs these things? :( ). > Then I guess we could overload the watchdog as a more general > timer-after-no- xmit? Yes, but it means that teardown of a socket can be delayed up to the amount of that timer. Factor in all of this crazy round_jiffies() stuff people do these days and it could cause pauses for real use cases and drive users batty. Probably the most profitable avenue is to see if this is a real issue afterall (see above). If we can get away with having the socket buffer represent socket --> device space only, that's the most ideal solution. It will probably also improve performance a lot across the board, especially on NUMA/SMP boxes as our TX complete events tend to be in difference places than the SKB producer. _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linux-foundation.org/mailman/listinfo/virtualization
