I found some info about this on this page:

http://www.cs.wisc.edu/~adesmet/linux_scalability.html

It looks like you can do it, but it isn't just echoing 
1 or 0 to a /proc/ filesystem location. I have not tested 
it.

>>> JAustad at temgweb.com 07/17/03 11:54AM >>>
> That's simply untrue, there are many IRC servers out there that handle
> thousands of clients running on Linux, and have done so since 
> the 2.2.x
> kernels.
I've argued this before on this list.  :)  With FreeBSD, you just add
-DFD_SETSIZE when you compile, and this works.  However, with linux, you
cannot do this.  There is a hard limit of 1024 descriptors in FD_SET().
Apparently, you can't just change it and recompile as it will break a
bunch
of things.  I would assume that these IRC servers are forking multiple
copies or using poll() instead of select() (qmail uses select()).  
If you can get qmail running on linux with a conf-spawn of more than 509,
I'd like to see it.  I spent weeks trying to get this to work, I tried
recompiling the kernel, using ulimit, changing things in /proc, and there
is
still a hard set limit for FD_SET of 1024.  Anyway, if I remember
correctly,
it's qmail-queue that actually dies when you go over the limit.  It
communicates with every qmail-send process that's running, and has a file
descriptor open for each one, rather than qmail-send opening the
descriptor
itself.  


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