NFS calculation of max commit size

[John Baldwin]

On Sunday, August 07, 2011 6:47:46 pm Rick Macklem wrote:
> A recent PR (kern/159351) noted that the following
> calculation results in a divide-by-zero when
> desiredvnodes < 1000.
> 
> 	nmp->nm_wcommitsize = hibufspace / (desiredvnodes / 1000);
> 
> Just fixing the divide-by-zero is easy enough, but I'm not
> sure what this calculation is trying to do. Making it a fraction
> of "hibufspace" makes sense (nm_wcommitsize is the maximum # of
> bytes of uncommitted data in the NFS client's buffer cache blocks,
> if I understand it correctly), but why divide it by
> 
>                 (desiredvnodes / 1000) ??
> 
> Maybe thinking that fewer vnodes means sharing it with fewer
> other file systems or ???
> 
> Anyhow, it seems to me that the formulae is bogus for small
> values of desiredvnodes (for example desiredvnodes == 1500
> implies nm_wcommitsize == hibufspace, which sounds too large
> to me).
> 
> I'm thinking that putting an upper limit of 10% of hibufspace
> might make sense. ie. Change the above to:
> 
> 	if (desiredvnodes >= 11000)
> 		nmp->nm_wcommitsize = hibufspace / (desiredvnodes / 1000);
> 	else
> 		nmp->nm_wcommitsize = hibufspace / 10;
> 
> Anyone have comments or insight into this calculation?
> 
> rick
> ps: jhb, I hope you don't mind. I emailed you first and then
>     thought others might have some ideas, too.

Oh no, this is fine.  A broader discussion is probably warranted.  I honestly 
don't know what the goal is.  I do think it is an attempt to share with other 
file systems, but I'm not sure how desiredvnodes / 1000 is useful for that.
It also seems that we can end up setting this woefully low as well.  That is, 
I wonder if we need a minimum of 10% of hibufspace so that it can scale 
between 10% and 90% of hibufspace (but I'm not sure what you would use to
pick the scaling factor sanely).  To my mind what you really want to do is 
something like 'hibufspace / (number of active mounts)', but that will not 
really work correctly unless we recalculate the value on each mount and 
unmount operation.

-- 
John Baldwin