RE: epoll_wait() performance

[David Laight]

From: Jesper Dangaard Brouer
> Sent: 27 November 2019 15:48
> On Wed, 27 Nov 2019 10:39:44 +0000 David Laight <David.Laight@xxxxxxxxxx> 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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