linux-kernel - Re: Re[2]: sendfile to nonblocking socket

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Date:	Tue, 24 Apr 2007 14:48:27 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Alex Vorona <voron@...ost.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Re[2]: sendfile to nonblocking socket

On Tue, 24 Apr 2007 14:33:48 +0400
Alex Vorona <voron@...ost.net> wrote:

> Hello David,
> 
> Tuesday, April 24, 2007, 1:19:49 PM, you wrote:
> 
> >> 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.
> 
> DS> You are correct. What I meant to say was that it's just a
> DS> more efficient version of 'mmap'ing a file and then 'write'ing
> DS> from the 'mmap'. The 'write' to a non-blocking socket can still
> DS> 'block' on disk I/O.
> 
> How can I avoid that blocking? Or maybe another question - how can I
> deliver data from disk to network with minimal copy operations, etc.
> 

I believe the modern way would be to use splice() system call :

loop
 splice(from disk , to pipe) with SPLICE_F_NONBLOCK
 wait_event_from_epoll (pipe ready for reading)
 splice(from pipe, to socket)
 wait_event_from_epoll (socket ready for writing)

But :

1) I am not sure epoll can get an event when page(s) is/are ready in pipe (ie disk delivered the page(s) to cache)
2) I am not sure splice() to socket is actually implemented with 0-copy
3) I am not sure pipe capacity (16 pages) would be enough to get a good readahead.

Eric



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