netdev - Re: setsockopt()

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Date:	Mon, 7 Jul 2008 14:24:08 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Olga Kornievskaia <aglo@...i.umich.edu>
Cc:	netdev@...r.kernel.org, Jim Rees <rees@...ch.edu>,
	"J. Bruce Fields" <bfields@...ldses.org>
Subject: Re: setsockopt()

On Mon, 07 Jul 2008 14:18:38 -0400
Olga Kornievskaia <aglo@...i.umich.edu> wrote:

> Hi,
> 
>     I'd like to ask a question regarding socket options, more 
> specifically send and receive buffer sizes.
> 
>     One simple question: (on the server-side) is it true that, to set 
> send/receive buffer size, setsockopt() can only be called before 
> listen()? From what I can tell, if I were to set socket options for the 
> listening socket, they get inherited by the socket created during the 
> accept(). However, when I try to change send/receive buffer size for the 
> new socket, they take no affect.
> 
>     The server in question is the NFSD server in the kernel. NFSD's code 
> tries to adjust the buffer size (in order to have TCP increase the 
> window size appropriately) but it does so after the new socket is 
> created. It leads to the fact that the TCP window doesn't open beyond 
> the TCP's "default" sysctl value (that would be the 2nd value in the 
> triple net.ipv4.tcp_rmem, which on our system is set to 64KB). We 
> changed the code so that setsockopt() is called for the listening socket 
> is created and we set the buffer sizes to something bigger, like 8MB. 
> Then we try to increase the buffer size for each socket created by the 
> accept() but what is seen on the network trace is that window size 
> doesn't open beyond the values used for the listening socket.

It would be better if NFSD stayed out of doign setsockopt and just
let the sender/receiver autotuning work?

>     I looked around in the code. There is a variable called 
> "window_clamp" that seems to specifies the largest possible window 
> advertisement. window_clamp gets set during the creation of the accept 
> socket. At that time, it's value is based on the sk_rcvbuf of the 
> listening socket. Thus, that would explain the behavior that window 
> doesn't grow beyond the values used in setsockopt() for the listening 
> socket, even though the new socket has new (larger) sk_sndbuf and 
> sk_rcvbuf  than the listening socket.
> 
>     I realize that send/receive buffer size and window advertisement are 
> different but they are related in the way that by telling TCP that we 
> have a certain amount of memory for socket operations, it should try to 
> open big enough window (provided that there is no congestion).
> 
>     Can somebody advise us on how to properly set send/receive buffer 
> sizes for the NFSD in the kernel such that (1) the window is not bound 
> by the TCP's default sysctl value and (2) if it is possible to do so for 
> the accept sockets and not the listening socket.
> 
>     I would appreciate if we could be CC-ed on the reply as we are not 
> subscribed to the netdev mailing list.
> 
> Thank you.
> 
> -Olga
> 
>    
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