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

Andrew Morton
2009-01-13T20:37:41Z

(cc's added)

On Thu, 8 Jan 2009 11:13:51 +0100
Volker Lendecke <Volker.Lendecke@SerNet.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@lenny:~$ uname -a
> Linux lenny 2.6.28-06857-g5cbd04a #7 Wed Jan 7 10:10:42 CET 2009 x86_64 = GNU/Linux
> vlendec@lenny:~$ gcc -o splicetest /host/home/vlendec/splicetest.c -O3 -Wall
> vlendec@lenny:~$ ./splicetest out 65536 &
> [1] 697
> vlendec@lenny:~$ 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 ...>
> Process 697 detached
> vlendec@lenny:~$ netstat -nt | grep 4711
> tcp 69272 0 192.168.19.10:4711 192.168.19.1:33773 ESTABLISHED
> vlendec@lenny:~$
>
> Interestingly, whenever I start the strace, it gets another
> chunk of data and then blocks in the next splice call.
>
> If I start splicetest with a buffer size of 16384 instead of
> 65536, it does not block. I could not find a way to ask the
> kernel for the tipping point below which it does not block.
>
> What is a safe buffer size to use with splice?
>
> BTW, this kernel is from Steve French's linux-cifs.git repo.
>
> Thanks,
>
> Volker Lendecke
>
> Samba Team
>
> P.S: I'm not subscribed to linux-kernel, so if possible
> please CC me directly. If this is inappropriate behaviour,
> please give me a quick hint :-)
>