Messages in this thread | | | From | David Laight <> | Subject | RE: epoll_wait() performance | Date | Wed, 27 Nov 2019 16:04:12 +0000 |
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From: Jesper Dangaard Brouer > Sent: 27 November 2019 15:48 > On Wed, 27 Nov 2019 10:39:44 +0000 David Laight <David.Laight@ACULAB.COM> wrote: > > > ... > > > > While using recvmmsg() to read multiple messages might seem a good idea, it is much > > > > slower than recv() when there is only one message (even recvmsg() is a lot slower). > > > > (I'm not sure why the code paths are so slow, I suspect it is all the copy_from_user() > > > > and faffing with the user iov[].) > > > > > > > > So using poll() we repoll the fd after calling recv() to find is there is a second message. > > > > However the second poll has a significant performance cost (but less than using recvmmsg()). > > > > > > That sounds wrong. Single recvmmsg(), even when receiving only a > > > single message, should be faster than two syscalls - recv() and > > > poll(). > > > > My suspicion is the extra two copy_from_user() needed for each recvmsg are a > > significant overhead, most likely due to the crappy code that tries to stop > > the kernel buffer being overrun. > > > > I need to run the tests on a system with a 'home built' kernel to see how much > > difference this make (by seeing how much slower duplicating the copy makes it). > > > > The system call cost of poll() gets factored over a reasonable number of sockets. > > So doing poll() on a socket with no data is a lot faster that the setup for recvmsg > > even allowing for looking up the fd. > > > > This could be fixed by an extra flag to recvmmsg() to indicate that you only really > > expect one message and to call the poll() function before each subsequent receive. > > > > There is also the 'reschedule' that Eric added to the loop in recvmmsg. > > I don't know how much that actually costs. > > In this case the process is likely to be running at a RT priority and pinned to a cpu. > > In some cases the cpu is also reserved (at boot time) so that 'random' other code can't use it. > > > > We really do want to receive all these UDP packets in a timely manner. > > Although very low latency isn't itself an issue. > > The data is telephony audio with (typically) one packet every 20ms. > > The code only looks for packets every 10ms - that helps no end since, in principle, > > only a single poll()/epoll_wait() call (on all the sockets) is needed every 10ms. > > I have a simple udp_sink tool[1] that cycle through the different > receive socket system calls. I gave it a quick spin on a F31 kernel > 5.3.12-300.fc31.x86_64 on a mlx5 100G interface, and I'm very surprised > to see a significant regression/slowdown for recvMmsg. > > $ sudo ./udp_sink --port 9 --repeat 1 --count $((10**7)) > run count ns/pkt pps cycles payload > recvMmsg/32 run: 0 10000000 1461.41 684270.96 5261 18 demux:1 > recvmsg run: 0 10000000 889.82 1123824.84 3203 18 demux:1 > read run: 0 10000000 974.81 1025841.68 3509 18 demux:1 > recvfrom run: 0 10000000 1056.51 946513.44 3803 18 demux:1 > > Normal recvmsg almost have double performance that recvmmsg. > recvMmsg/32 = 684,270 pps > recvmsg = 1,123,824 pps
Can you test recv() as well? I think it might be faster than read().
... > Found some old results (approx v4.10-rc1): > > [brouer@skylake src]$ sudo taskset -c 2 ./udp_sink --count $((10**7)) --port 9 --connect > recvMmsg/32 run: 0 10000000 537.89 1859106.74 2155 21559353816 > recvmsg run: 0 10000000 552.69 1809344.44 2215 22152468673 > read run: 0 10000000 476.65 2097970.76 1910 19104864199 > recvfrom run: 0 10000000 450.76 2218492.60 1806 18066972794
That is probably nearer what I am seeing on a 4.15 Ubuntu 18.04 kernel. recvmmsg() and recvmsg() are similar - but both a lot slower then recv().
David
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