netdev - Re: maximum buffer size for splice(2) tcp->pipe?

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Date:	Wed, 14 Jan 2009 08:40:08 +0100
From:	Volker Lendecke <Volker.Lendecke@...Net.DE>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Steven French <sfrench@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>, netdev@...r.kernel.org
Subject: Re: maximum buffer size for splice(2) tcp->pipe?

On Wed, Jan 14, 2009 at 12:15:04AM +0100, Eric Dumazet wrote:
> Volker, your splice() is a blocking one, from tcp socket to a pipe ?

Yes, it is.

> If no other thread is reading the pipe, then you might block forever
> in splice_to_pipe() as soon pipe is full (16 pages).

Why does it block when the pipe is full? Why doesn't it
return a short read, just like the read(2) call does? We
need to cope with that behaviour anyway.

> As pages are not necessarly full (each skb will use at least one page, even if 
> its length is small), it is not really possible to use splice() like this.
> 
> In your case, only safe way with current kernel would be to call splice()
> asking for no more than 16 bytes, that would be really insane for your needs.
> 
> You may prefer a non blocking mode, at least when calling splice_to_pipe()

Which fd do I have to set the nonblocking flag on? The TCP
socket I read from, or the pipe I write to?

Thanks for the hint anyway :-)

Volker

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