netdev - Re: autotuning of send buffer size of a socket

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Date:	Mon, 12 May 2008 17:47:27 -0700
From:	Sridhar Samudrala <sri@...ibm.com>
To:	Shirish Pargaonkar <shirishpargaonkar@...il.com>
Cc:	linux-net@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: autotuning of send buffer size of a socket

On Mon, 2008-05-12 at 14:00 -0500, Shirish Pargaonkar wrote:
> Hello,
> 
> kernel_sendmsg fails with error EAGAIN, yet I no matter how long I try,
> I still get the same error and do not see the send buffer size of a socket
> changing (increasing)
> 
> The initial buffer sizes are 16384 for send side and 87380 for the receive
> side but I see receive side buffer tuning but do not see the same with
> send side.
> 
> If tcp does not see a need to increase the send buffer size, wonder why I
> get EAGAIN error on this non-blocking socket for kernel_sendmsg!

I think the send buffer auto-tuning doesn't happen here because there is 
already congestion window worth of packets sent that are not yet acknowledged. 
See tcp_should_expand_sndbuf().

Also, the comments for tcp_new_space() says that sndbuf expansion does
not work well with largesends. What is the size of your sends?

Adding netdev to the CC list.

Thanks
Sridhar

> 
> I do subscribe to this mailing list so, please send your responses to this
> mail address.
> 
> Regards,
> 
> Shirish
> 
> --------------------------------------------------------------------------------------------------
> uname -r
> 2.6.18-91.el5
> 
>  sysctl -a
> 
> net.ipv4.tcp_rmem = 4096        87380   4194304
> net.ipv4.tcp_wmem = 4096        16384   4194304
> net.ipv4.tcp_mem = 98304        131072  196608
> 
> net.core.rmem_default = 126976
> net.core.wmem_default = 126976
> net.core.rmem_max = 131071
> net.core.wmem_max = 131071
> 
> net.ipv4.tcp_window_scaling = 1
> net.ipv4.tcp_timestamps = 1
> net.ipv4.tcp_moderate_rcvbuf = 1
> 
> 
> cat /proc/sys/net/ipv4/tcp_moderate_rcvbuf
> 1
> 
> 
> CIFS VFS: sndbuf 16384 rcvbuf 87380
> 
> CIFS VFS: sends on sock 0000000009903100, sendbuf 34776, rcvbuf 190080
> stuck for 32 seconds,
> error: -11
> CIFS VFS: sends on sock 0000000009903a00, sndbuf 34776, rcvbuf 138240
> stuck for 32 seconds,
> error: -11
> 
> 
> CIFS VFS: sends on sock 0000000009903100, sndbuf 34776, rcvbuf 126720
> stuck for 64 seconds,
> error: -11
> 
> CIFS VFS: sends on sock 0000000009903100, sndbuf 34776, rcvbuf 222720
> stuck for 256 seconds,
> error: -11
> 
> I see the socket receive buffer size fluctuating (tcp_moderate_rcvbuf
> is 1) but not
> the socket send buffer size.
> The send buffer size remains fixed, the auto-tuning for send side is
> enabled by default,so I do not see it happening here no matter how
> long the c ode tries to
> kernel_sendmsg after receiving EAGAIN return code.
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