Re: sunrpc: socket buffer size tuneable

[Ben Myers <bpm@sgi.com>]

Hey Bruce & Jim & Olga,

On Fri, Jan 25, 2013 at 02:16:20PM -0500, Jim Rees wrote:
> J. Bruce Fields wrote:
> 
>   On Thu, Jan 24, 2013 at 06:59:30PM -0600, Ben Myers wrote:
>   > At 1020 threads the send buffer size wraps and becomes negative causing
>   > the nfs server to grind to a halt.  Rather than setting bufsize based
>   > upon the number of nfsd threads, make the buffer sizes tuneable via
>   > module parameters.
>   > 
>   > Set the buffer sizes in terms of the number of rpcs you want to fit into
>   > the buffer.
>   
>   From private communication, my understanding is that the original
>   problem here was due to memory pressure forcing the tcp send buffer size
>   below the size required to hold a single rpc.

Years ago I did see wrapping of the buffer size when tcp was used with many
threads.  Today's problem is timeouts on a cluster with a heavy read
workload... and I seem to remember seeing that the send buffer size was too
small.

>   In which case the important variable here is lock_bufsize, as that's
>   what prevents the buffer size from going too low.

I tested removing the lock of bufsize and did hit the timeouts, so the overflow
is starting to look less relevant.  I will test your minimal overflow fix to
see if this is the case.

>   Cc'ing Jim Rees in case he remembers: I seem to recall discussing this
>   possibility, wondering whether we needed a special interface to the
>   network layer allowing us to set a minimum, and deciding it wasn't
>   really necessary at the time as we didn't think the network layer would
>   actually do this.  Is that right?  In which case either we were wrong,
>   or something changed.
> 
> I do remember discussing this. My memory is that we needed it but no one
> wanted to implement it and it never happened. But I could be
> mis-remembering. Maybe ask Olga, she's the one who put the bufsize
> autotuning patch in, commit 96604398.
> 
> I'll go back through my old mail and see if I can figure out what happened
> with this.

Thanks,
	Ben