linux-kernel - Re: [patch 14/22] pollfs: pollable futex

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Message-ID: <4638CC6D.6080505@haxent.com.br>
Date:	Wed, 02 May 2007 14:37:49 -0300
From:	Davi Arnaut <davi@...ent.com.br>
To:	Ulrich Drepper <drepper@...il.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 14/22] pollfs: pollable futex

Ulrich Drepper wrote:
> On 5/2/07, Davi Arnaut <davi@...ent.com.br> wrote:
>> thread A:
>>     int fd = plfutex(addr, 0);
>>     do
>>         poll(fdset+fd);
>>         process network events
>>         queue obj to thread B
>>         if fd:
>>             job processed
>>
>> thread B:
>>     wait_job();
>>     process_job();
>>     raise_event(addr);
>
> This is not the model you can implement with your changes.  Because
> every single waiter is woken you need one thread listening for the
> jobs and then distribute the work.  Otherwise you have thundering
> herds of threads and only one gets to do some work.
>

NO! Every single waiter of the _file descriptor_ is waked, not of the futex.
One can associate N fds with a single futex address. FUTEX_WAKE with
nproc = 1 will only wake one of the file descriptors. Its up to the user
to decide if he wants a broadcast or not.

Have you seen the email where I told you exactly this?

>> It simple as is, there is no need to overdesign.
>
> There is no reason to go with a limited, too-simple minded design if
> we've already identified a much better design.  The fact that poll is
> used today does not excuse piling on more and more code which makes
> additional functions which don't fit into the poll framework barely
> work.  Plus, poll/epoll itself is a problem.
>
epoll itself is a problem?! sorry, but i didn't know that. Care to 
elaborate?
I really need some guidance here. I just want to unify the epoll for various
event sources. It seems a lot of people like this, just look at the 
popularity
of libevent and other "unifying" event loops.

I don't think we need another epoll clone.

> And you cannot talk about little changes and no "overdesign".  You
> have 22 patches for all this.  It's not just limited to futexes, it's
> the whole thing which IMO is unnecessary ballast going forward.
davi@...mic:~/git/linux-2.6$ find patches/ -name *.patch |grep -v 
syscall | wc -l
10

davi@...mic:~/git/linux-2.6$ find patches/ -name *.patch |grep -v 
syscall |grep futex
patches/pollfs-futex-async-wait.patch
patches/pollfs-futex.patch

--
Davi Arnaut

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