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Message-ID: <18027.40637.672203.14305@cargo.ozlabs.ibm.com>
Date: Sun, 10 Jun 2007 16:48:29 +1000
From: Paul Mackerras <paulus@...ba.org>
To: Kyle Moffett <mrmacman_g4@....com>
Cc: Davide Libenzi <davidel@...ilserver.org>,
Alan Cox <alan@...rguk.ukuu.org.uk>,
Theodore Tso <tytso@....edu>,
Ulrich Drepper <drepper@...hat.com>,
Eric Dumazet <dada1@...mosbay.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 7/8] fdmap v2 - implement sys_socket2
Kyle Moffett writes:
> 1) Linear FD allocation makes it IMPOSSIBLE for libraries to
> reliably use persistent FDs behind an application's back. For
That's not completely true; for example, openlog() opens a file
descriptor for the library's own use, as does sethostent(). I agree
that it creates difficulties if the library implementor wants to use a
file descriptor in a set of functions that didn't previously use one,
but with a bit of assistance from the kernel, that can be solved
without breaking the ABI.
> for (i = 0; i < NR_OPEN; i++)
> if (!fd_is_special_to_us(i))
> close(i);
>
> Note that this is conceptually buggy, but occurs in several major C
> programming books, most of the major shells, and a lot of other
> software to boot.
Buggy in what way? In the use of the NR_OPEN constant?
> 3) In order to allocate new FDs, the kernel has to scan over a
> (potentially very large) bitmap. A process with 100,000 fds (not
> terribly uncommon) would have 12.5kbyte of FD bitmap and would trash
> the cache every time it tried to allocate an FD.
For specialized programs like that we can offer alternative fd
allocation strategies if necessary (although I expect that with
100,000 fds other things will limit performance more than
allocation).
None of those things is an excuse for breaking the ABI, however.
As I said to Davide, I was really protesting about the attitude that
we can just break the ABI however and whenever we like and force
programs to adapt.
Paul.
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