Re: Syslets, Threadlets, generic AIO support, v6

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> > I agree. What would be a good interface to allocate fds in such 
> > area? We don't want to replicate syscalls, so maybe a special new 
> > dup function?
> 
> I'd do it with something like "newfd = dup2(fd, NONLINEAR_FD)" or 
> similar, and just have NONLINEAR_FD be some magic value (for example, 
> make it be 0x40000000 - the bit that says "private, nonlinear" in the 
> first place).
> 
> But what's gotten lost in the current discussion is that we probably 
> don't actually _need_ such a private space. I'm just saying that if 
> the *choice* is between memory-mapped interfaces and a private 
> fd-space, we should probably go for the latter. "Everything is a file" 
> is the UNIX way, after all. But there's little reason to introduce 
> private fd's otherwise.

it's both a flexibility and a speedup thing as well:

flexibility: for libraries to be able to open files and keep them open 
comes up regularly. For example currently glibc is quite wasteful in a 
number of common networking related functions (Ulrich, please correct me 
if i'm wrong), which could be optimized if glibc could just keep a 
netlink channel fd open and could poll() it for changes and cache the 
results if there are no changes (or something like that).

speedup: i suggested O_ANY 6 years ago as a speedup to Apache - 
non-linear fds are cheaper to allocate/map:

  http://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg23820.html

(i definitely remember having written code for that too, but i cannot 
find that in the archives. hm.) In theory we could avoid _all_ fd-bitmap 
overhead as well and use a per-process list/pool of struct file buffers 
plus a maximum-fd field as the 'non-linear fd allocator' (at the price 
of only deallocating them at process exit time).

	Ingo
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