:> >(sysctl-ized) in FBSD (Some work have been done in Linux, since a
:> >well-known comparative benchmark offense). Would be even more usefull
:> >in SMP context.
:
:I don't think the wake-many problem was ever the cause of the poor numbers
:that comparitve benchmark unearthed. This is only a problem if you have a
:whole slew of children sitting around waiting for new connections, rather
:than doing real work. this sure isn't the environment a heavily loaded
:server is under :) If you're still curious, check out
:
:http://www.kegel.com/mindcraft_redux.html
:
:specifically
:
:http://kernelnotes.org/lnxlists/linux-kernel/lk_9906_04/msg01100.html
:
:-- zach
Well, the wake-many problem hit me several times at BEST both with
Apache and with the WWW server I wrote. We had the problem under both
FreeBSD and IRIX. These were heavily loaded web servers and the wakeup
issue turned into an O(N^2) problem. Every time a connection was
accepted it woke up N processes where N effectively scaled to the
connection rate. For example, shellx (the IRIX box) was getting
40 connections/second and had over 600 active connections at any given
moment. There would perhaps be 200 processes waiting to accept a new
connection. Without a fix the result was 200x40 = 8000 wakeups/second
which is significant. Not only that, but the wakeup's themselves were
O(N) resulting in O(2*N^2) operation. All sorts of scaling problems
occured inside the kernel!
The solution Apache takes is to surround the accept() with a file lock
so only one process blocks in accept() at any given point (the file lock
uses wakeup_one and is safe).
The solution that I took with BestWWWD was to have just one process
accept all the connections and then have it dole the descriptor out to the
appropriate sub-processes over a unix-domain socket.
-Matt
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