[linux-cifs-client] Re: sendmsg blocking with sendtimeout vs. non-blocking

[Jeff Layton]

On Thu, 23 Oct 2008 14:19:04 -0500
"Steve French" <smfrench at gmail.com> wrote:
> > His statement was basically "unless you know for sure that you don't
> > want to use more than X amount of memory, then there isn't much reason
> > to set the send and receive buffers".
> 
> I think that there is a problem still - cifs needs tcp autotuning but
> the buffers probably need to be at least as big as the largest SMB
> response (about 17K on receives, but configurable to be larger).   See
> comment below about NFS:
> 

On Thu, Oct 23, 2008 at 2:05 PM, Jim Rees <rees at umich.edu> wrote:
> There are two issues to be aware of.  One is that the socket buffers have to
> be big enough for the tcp congestion window.  In the old days, the
> application would have to know ahead of time how big this is, and call
> setsockopt(), which sets these numbers.
>
> Now however, the tcp stack "autotunes" the buffer sizes to the correct
> amount.  If you call setsockopt() to set a buffer size, or set sk_*buf, or
> set the SOCK_*BUF_LOCK bits in sk_userlocks, you defeat this autotuning.
> This is almost always a bad idea.
>
> The other issue is that at least for NFS, the receive buffer must be big
> enough to hold the biggest possible rpc.  If not, a partial rpc will get
> stuck in the buffer and no progress will be made.
>
> Issue one is easy to deal with, just don't muck with the socket internal
> data structure.  The second one is harder.  What's really needed is a new
> api into the tcp layer that will reserve a minimum amount of memory in the
> socket buffer so that receives won't stall.  For now, our patch sets the
> sk_*buf values without setting the lock flags.  


Agreed. Thanks Jim, for sending along that info...

It sounds like what we should do for CIFS is the same thing. Set the
buffer sizes but make sure the SOCK_*BUF_LOCK bits are cleared so that
they can grow as needed.

Does the kernel ever shrink these buffers?
-- 
Jeff Layton <jlayton at redhat.com>