Re: Should __FD_SETSIZE still be set to 1024 ?

[Chris Wedgwood <cw@f00f.org>]

On Fri, Jun 22, 2001 at 06:25:41PM +0200, Dag Wieers wrote:

    So if someone wants to increase it for an application he needs to
    be sure that everything that it is linked with is compiled with a
    similar __FD_SETSIZE ?

Well... only things that care about __FD_SETSIZE :)
    
    Why can you safely increase the value in Squid then ?

Because it has been tested and works. I assume libc doesn't care is
this value is increased, not sure about other libraries.

Using libc5 and linux 2.1.9x + scts large-fd patch (which got merged
in later kernels), I manged to work with 100K FDs without too many
problems.

The defintion of 'fd_set' changes with __FD_SETSIZE, so anything
makes these kind of assumptions might break.

For example, if you have some code which assumes this is 1024 and you
try to copy a value into a preallocated array of structure it
defined, then it will break.

So will precompiled code such as:

struct cookie {
	int	type;
	fd_set	chocolate_feeds;
	int	cc_count;
}monster;

sort of thing; because fd_set's size will be wrong.

    Yes, but still, why 1024 ?

Because thats what is has been for a very long time, a safe default.

If your application knows better, it can change it.

    No, squid takes the lowest of both (FD_SETSIZE and SQUID_MAXFD)
    in main.c.  And the Squid configure gets the FD_SETSIZE value
    from linux/posix_types.h ;(

Really? I though for the last couple of years squid would use poll
over select anyhow?
    
    I'm still not convinced that something might break, since everybody
    advices to increase __FD_SETSIZE before compiling Squid.

So do it then.
    
    And if linux/posix_types.h defines the limit of open file
    descriptors of the system, 1024 is (IMO) a wrong number. But then
    again, nobody bothered to change it...

Because 1024 works everywhere, a large or smaller value might not.



  --cw