getting to yaws, devpoll
Fri Jul 5 11:09:53 CEST 2002
> Joe Armstrong <> writes:
> > If you are doing [yaws] benchmarks then the following question is
> > very interesting.
> > 3) Measure the throughput as you massivly overload the system.
> > (say the page rate is 1000 pages/sec)
> > Subject the system to 10000 requests/sec.
> I wonder, wouldn't this situation be helped by kernel polling?
To clearify why kernel polling suites erlang extremely well one just
have to look on the way the scheduler works.
On a system with load it will sit in a loop where it executes a number
of reductions and then goes away to check for i/o with zero timeout.
If you run a unix that supports poll and have, say 15000, active
TCP/IP connections you will have to copy 12*15000 bytes from user
space to kernel space. The kernel then have to do a driver poll for
each and every of those file descriptors.
Note that this will occur even if there is no actual i/o taking over
the connections, like many http 1.1 sessions.
It is not uncommon to see 70-80% kernel load already at 2000-3000
With kernel polling it is a totally different story.
Instead of having each connections fd in the pollset the sockets fds
are bound in kernel state.
Only a single fd is included in the pollset reducing the user-kernel
space copying but more important it is not necessary for the kernel to
execute the driver poll for each fd since it keeps state.
The system load now depends on actual traffic and not established
connections which makes erlang + kernel an ideal platform for massive
multiuser internet services.
> Per Bergqvist was working on a patch, but - I don't see devpoll
> stuff in a recent snapshot (otp_src_P9_2002-06-30).
I haven't updated the patch for P9. Will probably happen during the
summer. There is still some weird behaviour with FreeBSD kqueue
support but the Solaris and Linux+/dev/kpoll variants are flawless.
/dev/kpoll is a derivate work from Davide Libenskis devpoll driver.
It mimics the semantics of poll better than devpoll.
It is still unreleased to a wider audience. It should really have been
released a few months ago but nobody seemed interested in the kernel
Let me know if your interested and I'll fix it.
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