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

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Date:	Wed, 14 Jan 2009 00:15:04 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Volker.Lendecke@...Net.DE, 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?

Andrew Morton a écrit :
> (cc's added)
> 
> On Thu, 8 Jan 2009 11:13:51 +0100
> Volker Lendecke <Volker.Lendecke@...Net.DE> wrote:
> 
>> Hi!
>>
>> While implementing splice support in Samba for better
>> performance I found it blocking when trying to pull data off
>> tcp into a pipe when the recvq was full. Attached find a
>> test program that shows this behaviour, on another host I
>> started
>>
>> netcat 192.168.19.10 4711 < /dev/zero
>>
>> vlendec@...ny:~$ uname -a
>> Linux lenny 2.6.28-06857-g5cbd04a #7 Wed Jan 7 10:10:42 CET 2009 x86_64 = GNU/Linux
>> vlendec@...ny:~$ gcc -o splicetest /host/home/vlendec/splicetest.c -O3 -Wall
>> vlendec@...ny:~$ ./splicetest out 65536 &
>> [1] 697
>> vlendec@...ny:~$ strace -p 697
>> Process 697 attached - interrupt to quit
>> splice(0x3, 0, 0x5, 0, 0x56a0, 0x1)     = 22176
>> splice(0x7, 0, 0x4, 0, 0x10000, 0x1^C <unfinished ...>

Volker, your splice() is a blocking one, from tcp socket to a pipe ?

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

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()

Maybe SPLICE_F_NONBLOCK splice() flag should only apply on pipe side.
tcp_splice_read() should not use this flag to select a blocking/nonbloking
mode on the source socket, but underlying file flag.

This way, your program could let socket in blocking mode, yet call splice()
with SPLICE_F_NONBLOCK flag to not block on pipe.

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