linux-kernel - RE: sendfile to nonblocking socket

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Message-ID: <MDEHLPKNGKAHNMBLJOLKIEDFDEAC.davids@webmaster.com>
Date:	Tue, 24 Apr 2007 02:19:49 -0700
From:	"David Schwartz" <davids@...master.com>
To:	<linux-kernel@...r.kernel.org>
Subject: RE: sendfile to nonblocking socket


> David Schwartz пишет:
> > You have a misunderstanding about the semantics of 'sendfile'. 
> The 'sendfile' function is just a more efficient version of a 
> read followed by a write. If you did a read followed by a write, 
> it would block as well (in the read).
> >
> > DS

> sendfile function is not just a more efficient version of a read 
> followed by a write.  It reads from one fd and write to another at tha 
> same time. Please try to read 2G, and then write 2G - and how much 
> memory you will be need and how much time you will loose while reading 
> 2G from disk, but not writing them to socket.

You are correct. What I meant to say was that it's just a more efficient version of 'mmap'ing a file and then 'write'ing from the 'mmap'. The 'write' to a non-blocking socket can still 'block' on disk I/O.

> If you know more 
> efficient method to transfer file from disk to network - please advise. 
> Now all I want is really non-blocking sendfile. Currently sendfile is 
> non-blocking on network, but not on disk i/o. And when I have network 
> faster than disk - I get block.

There are many different techniques and which is correct depends on what direction you want to go. POSIX asynchronous I/O is one possibility. Threads plus epoll is another. It really depends upon how much performance you need, how much complexity you can tolerate, and how portable you need to be.

DS


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