Re: epoll_wait() performance

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Wed, Nov 27, 2019 at 11:04 AM David Laight <David.Laight@aculab.com> wrote:
>
> 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().

Indeed, surprising that recv(from) would be less efficient than recvmsg.

Are the latest numbers with CONFIG_HARDENED_USERCOPY?

I assume that the poll() after recv() is non-blocking. If using
recvmsg, that extra syscall could be avoided by implementing a cmsg
inq hint for udp sockets analogous to TCP_CM_INQ/tcp_inq_hint.

More outlandish would be to abuse the mmsghdr->msg_len field to pass
file descriptors and amortize the kernel page-table isolation cost
across sockets. Blocking semantics would be weird, for starters.