Re: modutils (Re: cipe 1.2.0 & kernel 2.2.x)

Malcolm Beattie (mbeattie@sable.ox.ac.uk)
Thu, 25 Mar 1999 14:37:24 +0000 (GMT)

Olaf Titz writes:
> I assume this is just this code in ciped:
> for (i=getdtablesize()-1; i>2; --i)
> (void) close(i);
> which makes sure all fds are closed (and deliberately ignores the
> errors). Are your 1000 really 253 or do you use an -ac kernel with
> higher fd limit? :-)
>
> This "close everything but standard fds" is employed by a number of
> programs, among them shells and perl, to avoid fd leakage. It is
> harmless.

On our heavily loaded Digital UNIX Alphaserver (which defaults to 4096
file descriptors per process), each "close everything but standard fds"
hits the system with another 4093 syscalls. When that's done frequently
(up to a few logins per second) it becomes a significant fraction of
the syscalls/sec which seems to correlate fairly well with the whole
system's performance.

Since Linux is now entering the realms of plenty of file descriptors,
I wonder if it's worth having a closefdset() syscall used as follows:

fd_set most;
FD_ZERO(&most);
for (i=getdtablesize()-1; i>2; --i)
FD_SET(i, &most);
closefdset(&most);

On the positive side, it reduces all those extra syscalls at a stroke
and it's simple to write. On the negative side, it's a non-standard
interface and could easily be considered a unnecessary wart. It also
can't return any errors beyond "at least one close failed" but that
doesn't matter for the purposes it would be used for.

--Malcolm

-- 
Malcolm Beattie <mbeattie@sable.ox.ac.uk>
Unix Systems Programmer
Oxford University Computing Services


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/