Re: [patch 7/8] fdmap v2 - implement sys_socket2

From: Linus Torvalds
Date: Sat Jun 09 2007 - 23:50:14 EST

On Sun, 10 Jun 2007, Al Viro wrote:
> >
> > And that means that libraries currently MUST NOT open their own file
> > descriptors, exactly because they mess with the "application file
> > descriptor namespace", namely the linear POSIX-defined fd allocation
> > rules!
> Unless it does so in a thread that has unshared its descriptor table.

Agreed. That was actually part of the reason why I thought clone() was
much better than the pthreads interface.

That said, the Linux !CLONE_FILES does have downsides:

- it is potentially much slower to do than sharing everything (if you
have lots of file descriptors, incrementing the refcounts etc is
actually a real overhead)

- it simply doesn't work, if the library wants to run in the same
execution context, and just wants to open one (or more) file
descriptors for some helper thing.

IOW, the most common case for libraries is not that they get invoced to do
one thing, but that they get loaded and then used over and over and over
again, and the _reason_ for wanting to have a file descriptor open may
well be that the library wants to cache the file descriptor, rather than
having to open a file over and over again!

For example, a library routine that does a full

fd = open();
.. do something with it ..

generally doesn't need any private file descriptors at all (although there
are the threading issues with exec etc) - it will temporarily use a normal
file descriptor, and the caller won't be any wiser. Lots of current
library routines do this all the time.

But let's say that you want to do a library that does name resolution, and
you actually want to create the socket that binds to the DNS server just
once, and then re-use that socket across library calls. It's not that the
library is a thread of its own - it's not - but with the normal linear fd
space it really cannot do this. Sure, it could try to hide it up somewhere
in high fd space, but that would slow down other operations, and there's
no way to guarantee it doesn't clash with some _other_ library doing the
same thing, so it really isn't a good idea.

Now, when you do a DNS query, the setup cost of opening the socket is the
least of your worries, so the above example is not a very good one. I'm
really just giving it as a concrete example of a _conceptual_ problem,
where some other library really had more pressing performance reasons why
they cannot keep re-opening a file descriptor and closing it each time.

So _that_ is the kind of situation where I think "anonymous file
descriptors" make sense.

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