On Tue, Mar 03, 2015 at 11:21:32AM +0200, Vasile Catalin-B50542 wrote: > 1. Why does virtio-net have a timer to handle tx virtqueue?
It's a performance optimization. By delaying tx virtqueue processing by 150 microseconds and turning guest->host virtqueue notifications off, QEMU is able to batch transmit. > 2. Why isn't the virtqueue kick enough? (I see that both are set to handle > jobs from virtqueue.) It is enough. The timer is not for functionality or correctness, it is purely a performance optimization. The optimization is a gamble, there are workloads where the extra 150 microseconds of latency causes worse performance. > 3. What is the difference between regular timers and bh (bottom halves)? Why > would you choose one over another? A bottom half is scheduled in the main loop "as soon as possible". A timer has a specific duration (150 microseconds by default in this case). 150 is a magic number. BH is another magic number, essentially. This is just a bet that performance will be better if we delay a bit to allow batching. Hope this helps, Stefan
pgppsiye9twp6.pgp
Description: PGP signature
