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. -- 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
