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SubjectRe: [PATCH] async poll for 2.5
At 10:18 AM 15/10/2002 -0700, Dan Kegel wrote:
>Benjamin LaHaise wrote:
> >
> > On Tue, Oct 15, 2002 at 10:06:22AM -0700, Dan Kegel wrote:
> > > Doesn't the F_SETSIG/F_SETOWN/SIGIO stuff qualify as a scalable
> > > alternative?
> >
> > No.
>What's the worst part about it? The use of the signal queue?

there are four things that really suck about sigio.
in order of most-significant-suckage to least-significant-suckage, i see
them as:

[1] signals are very heavy.
thousands of signals/second does not scale on SMP due to the
serialization of them in the kernel.
(just look at the code path for delivering a signal)

signals also resulted in 128 u32's being transferred from kernel
to userspace for every signal. thats a lot of memory i/o
bandwidth consumed at 1000's of concurrent sockets
and tens-of-thousands of events/sec happening.

[2] SIGIO only exposes half of the POLL_OUT semantics.
with poll(), you can use POLL_OUT to indicate if there
is free buffer space to write into or not.
with SIGIO, for most applications, you can only find out
by issuing a write() and get back a -EWOULDBLOCK.
to indicate !POLL_OUT.
(perhaps that has been addressed in the last 12 months or so;
but i doubt it)

[3] SIGIO had no easy recovery path if you hit the maximum-
queue-limit for number of signals queued to userspace.
(ok, you *could* do a poll() and start again, but it couldn't
be done in a 100% race-free manner)

[4] you couldn't enable SIGIO on a incoming socket
accept()ed without there being a race window where
something happened to that socket between accept() and
enable-SIGIO. [sort-of related to (3); you could work-around
it by doing a poll() on the socket after enable-SIGIO, but
it makes a clean interface a horrible interface]

other miscellaneous things that makes SIGIO less usable in the real-world:
- you can only get one event at a time -- that means tens-of-thousands
to hundreds-of-thousands of system calls / second just to get event
status, when it'd probably make more sense to poll for multiple signals
at the same time
- SIGIO only addressed "event notification". it did nothing to address the
other large scalability that you typically hit when writing
high-performance i/o
systems: overhead of memory-copy from userspace<->kernelspace.
various zerocopy mechanisms help address that side of thing, but if you're
comparing aio to SIGIO, aio *is* addressing a much larger problem than just
SIGIO on its own



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