linux-kernel - Re: PROPOSAL: extend pipe() to support NULL argument.

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Message-ID: <20090703071504.GG5880@cr0.nay.redhat.com>
Date:	Fri, 3 Jul 2009 15:15:04 +0800
From:	Amerigo Wang <xiyou.wangcong@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Amerigo Wang <xiyou.wangcong@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PROPOSAL: extend pipe() to support NULL argument.

On Fri, Jul 03, 2009 at 01:42:12PM +0800, Changli Gao wrote:
>On Fri, Jul 3, 2009 at 1:19 PM, Amerigo Wang<xiyou.wangcong@...il.com> wrote:
>>
>>
>> Yes, exactly.
>> Inventing a new API is better than modifying pipe(2), IMO.
>>
>> BUT I still don't agree that you really need this... I think you
>> can add a flag or something like that to an fd to do this, e.g.
>>
>>   fcntl(fd, F_SETFD, FD_PIPERW);
>>
>> Isn't this better?
>You don't understand my need. I don't want to change a RDONLY or
>WRONLY pipe to a RW one, but I want to pipe() return just one RW pipe
>instead.

This is exacly what I meant by that example.

>It seems you prefer pipe0() to pipe(NULL).

pipefd() seems better.

>>>You don't know my meaning. As a proxy server, there maybe lots of
>>>connections to maintain, and these connections will keep open for a
>>>long time. If the data received can be sent in a relay cycle, the
>>>kernel buffer can be reused. If not, the kernel buffer must be
>>>reserved. When there are lots of these kinds of connections, lots of
>>>kernel buffers must be reserved. At this time, whether two fds per
>>>kernel buffer or one fds per kernel buffer matters.
>>
>> SHow us the code, please.
>>
>If you don't know my need after reading my words, I don't think you
>can understand the fake code.

No, code is *much* better than any of your words.

>The fake code:
>
>main thread:
>
>while (1) {
>   fd = accept();
>   pthread_create(worker, fd);
>}
>
>worker thread(assume data is transfered just from client to server for
>simplification):
>
>serv_fd = connect();
>while (1) {
>  select(fd, RD);
>  pipe = get_pipe_from_poll();
>  n = splice(fd, pipe);
>  while (n > 0) {
>     n -= splice(pipe, serv_fd);
>  }
>  put_pipe_to_poll();
>}
>close(serv_fd);
>close(fd);

If saving one fd really helps here, probably you want to
save more, you will need a syscall like:

  int splice_without_new_fd(int infd, int outfd);

But splice(2) is designed to be as it is. You need to increase
your fd limit, instead of saving one by pipe().
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