On Thu, 2 Jun 2016 20:47:25 +0200 Jesper Dangaard Brouer <bro...@redhat.com> wrote:
> On Tue, 24 May 2016 23:34:14 +0300 > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > On Tue, May 24, 2016 at 07:03:20PM +0200, Jesper Dangaard Brouer wrote: > > > > > > On Tue, 24 May 2016 12:28:09 +0200 > > > Jesper Dangaard Brouer <bro...@redhat.com> wrote: > > > > > > > I do like perf, but it does not answer my questions about the > > > > performance of this queue. I will code something up in my own > > > > framework[2] to answer my own performance questions. > > > > > > > > Like what is be minimum overhead (in cycles) achievable with this type > > > > of queue, in the most optimal situation (e.g. same CPU enq+deq cache > > > > hot) > > > > for fastpath usage. > > > > > > Coded it up here: > > > https://github.com/netoptimizer/prototype-kernel/commit/b16a3332184 > > > > > > https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/skb_array_bench01.c > > > > > > This is a really fake benchmark, but it sort of shows the > > > overhead achievable with this type of queue, where it is the same > > > CPU enqueuing and dequeuing, and cache is guaranteed to be hot. > > > > > > Measured on a i7-4790K CPU @ 4.00GHz, the average cost of > > > enqueue+dequeue of a single object is around 102 cycles(tsc). > > > > > > To compare this with below, where enq and deq is measured separately: > > > 102 / 2 = 51 cycles > > The alf_queue[1] baseline is 26 cycles in this minimum overhead > achievable benchmark with a MPMC (Multi-Producer/Multi-Consumer) queue > which use a locked cmpxchg. (SPSC variant is 5 cycles, thus most cost > comes from locked cmpxchg). > > [1] > https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/include/linux/alf_queue.h > > > > > Then I also want to know how this performs when two CPUs are involved. > > > > As this is also a primary use-case, for you when sending packets into a > > > > guest. > > > > > > Coded it up here: > > > https://github.com/netoptimizer/prototype-kernel/commit/75fe31ef62e > > > > > > https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/skb_array_parallel01.c > > > > > > This parallel benchmark try to keep two (or more) CPUs busy enqueuing or > > > dequeuing on the same skb_array queue. It prefills the queue, > > > and stops the test as soon as queue is empty or full, or > > > completes a number of "loops"/cycles. > > > > > > For two CPUs the results are really good: > > > enqueue: 54 cycles(tsc) > > > dequeue: 53 cycles(tsc) > > As MST points out, a scheme like the alf_queue[1] have the issue that it > "reads" the opposite cacheline of the consumer.tail/producer.tail to > determine if space-is-left/queue-is-empty. This cause an expensive > transition for the cache coherency protocol. > > Coded up similar test for alf_queue: > https://github.com/netoptimizer/prototype-kernel/commit/b3ff2624f1 > > https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/alf_queue_parallel01.c > > For two CPUs MPMC results are, significantly worse, and demonstrate MSTs > point: > enqueue: 227 cycles(tsc) > dequeue: 231 cycles(tsc) > > Alf_queue also have a SPSC (Single-Producer/Single-Consumer) variant: > enqueue: 24 cycles(tsc) > dequeue: 23 cycles(tsc) > > > > > Going to 4 CPUs, things break down (but it was not primary use-case?): > > > CPU(0) 927 cycles(tsc) enqueue > > > CPU(1) 921 cycles(tsc) dequeue > > > CPU(2) 927 cycles(tsc) enqueue > > > CPU(3) 898 cycles(tsc) dequeue > > > > It's mostly the spinlock contention I guess. > > Maybe we don't need fair spinlocks in this case. > > Try replacing spinlocks with simple cmpxchg > > and see what happens? > > The alf_queue uses a cmpxchg scheme, and it does scale better when the > number of CPUs increase: > > CPUs:4 Average: 586 cycles(tsc) > CPUs:6 Average: 744 cycles(tsc) > CPUs:8 Average: 1578 cycles(tsc) > > Notice the alf_queue was designed with the purpose of bulking, to > mitigate the effect of this cacheline bouncing, but it was not covered > in this test. Added bulking to the alf_queue test: https://github.com/netoptimizer/prototype-kernel/commit/e22a0d8745 This does help significantly, but requires use-cases where there are packets to be bulk deq/enq. On the other hand, the skb_array also requires that objects in the queue/array exceed one cacheline, before it starts to scale. For two CPUs we need bulk=4 before beating skb_array. See benchmark adjusting bulk size: CPUs:2 bulk=step:1 Average: 231 cycles(tsc) CPUs:2 bulk=step:2 Average: 118 cycles(tsc) CPUs:2 bulk=step:3 Average: 65 cycles(tsc) CPUs:2 bulk=step:4 Average: 48 cycles(tsc) CPUs:2 bulk=step:5 Average: 40 cycles(tsc) CPUs:2 bulk=step:6 Average: 37 cycles(tsc) CPUs:2 bulk=step:7 Average: 29 cycles(tsc) CPUs:2 bulk=step:8 Average: 24 cycles(tsc) CPUs:2 bulk=step:9 Average: 23 cycles(tsc) CPUs:2 bulk=step:10 Average: 20 cycles(tsc) Keeping bulk=8, and increasing the CPUs does show better scalability, due to bulking. This system (i7-4790K CPU @ 4.00GHz) only had 8-core CPUs: CPUs:2 bulk=step:8 Average: 25 cycles(tsc) CPUs:4 bulk=step:8 Average: 71 cycles(tsc) CPUs:6 bulk=step:8 Average: 100 cycles(tsc) CPUs:8 bulk=step:8 Average: 185 cycles(tsc) Found a (slower) 24-core CPU system (E5-2695v2-ES @ 2.50GHz): CPUs:2 bulk=step:8 Average: 50 cycles(tsc) CPUs:4 bulk=step:8 Average: 101 cycles(tsc) CPUs:6 bulk=step:8 Average: 214 cycles(tsc) CPUs:8 bulk=step:8 Average: 347 cycles(tsc) CPUs:10 bulk=step:8 Average: 468 cycles(tsc) CPUs:12 bulk=step:8 Average: 670 cycles(tsc) CPUs:14 bulk=step:8 Average: 698 cycles(tsc) CPUs:16 bulk=step:8 Average: 1149 cycles(tsc) CPUs:18 bulk=step:8 Average: 1094 cycles(tsc) CPUs:20 bulk=step:8 Average: 1349 cycles(tsc) CPUs:22 bulk=step:8 Average: 1406 cycles(tsc) CPUs:24 bulk=step:8 Average: 1553 cycles(tsc) I still think skb_array is the winner, when the normal use-case is two CPUs, and we cannot guarantee CPU pinning (thus cannot use SPSC). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer
