Date: Sun, 17 Jun 2001 14:38:36 -0700 (PDT) From: Matt Dillon <dillon@earth.backplane.com> To: Jonathan Lemon <jlemon@flugsvamp.com> Cc: sascha@schumann.cx, hackers@FreeBSD.ORG Subject: Re: poll(2)'s arbitrary limit Message-ID: <200106172138.f5HLcaS08280@earth.backplane.com> References: <local.mail.freebsd-hackers/20010617153129.N1832@superconductor.rush.net> <200106172033.f5HKXEn56198@prism.flugsvamp.com>
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:Well, I have to disagree, as the conclusion in the above paper is
:misleading. Basically, they say that if a listen socket returned
:by select() becomes ready, then it is more efficient to call accept()
:on the socket multiple times, rather than once per select() call.
:
:Well gee, that's nice, and quite true, but has nothing to do with
:the efficiency (or lack thereof) of select() itself. This is simply
:the old 'polling' vs 'interrupt' argument that has been around for
:ages (see also: 'interrupt livelock').
:--
:Jonathan
Well, not quite... select() isn't quite as bad as it appears. Consider
the case of a heavily loaded web server which is select()ing on 500
descriptors. If the select() returns 20 ready descriptors and the
web server then proceeds to process those descriptors by the time it
gets around to calling select() again it is likely that another 20
descriptors will be ready.
So rather then having O(N^2) overhead, select() winds up with O(25N)
overhead (500/20 = 25), which isn't all that bad. The actual overhead
of making the select() call relative to doing other work winds up
going *DOWN* under heavier loads and *UP* in lighter loads.
Where select() breaks down is when you have hundreds of descriptors
but the load is not high enough to cause more then a few to be 'ready' at
any given moment. The select() based program will still be reasonably
responsive, but it will wind up eating a considerably greater amount of
cpu to get that responsiveness. Something like kqueue() has low cpu
overhead and high efficiency under all load conditions, select() has
high overhead under lightly loaded conditions and while it gets much
better with load, it still will be nowhere near as good as kqueue().
But that doesn't necessarily mean that a select() based loop will become
unmanageable!
And when you get into the thousands of descriptors, then select's
scanning overhead can really take its toll.
An argument can also be made for the increased efficiency you get with
batch processing. Take the web server example again, this time focusing
an the listen descriptor. From the point of view of efficiency, allowing
a number of new connections to build up on the listen socket and then
accept()ing them in a tight loop is in fact more cpu-efficient then
waking up the process for each one. You get this effect under heavier
loads whether you are using select(), poll(), or kqueue().
-Matt
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