Limits on jumbo mbuf cluster allocation

[Garrett Wollman]

<<On Tue, 12 Mar 2013 23:48:00 -0400 (EDT), Rick Macklem <rmacklem at uoguelph.ca> said:

> Basically, this patch:
> - allows setting of the tcp timeout via vfs.nfsd.tcpcachetimeo
>   (I'd suggest you go down to a few minutes instead of 12hrs)
> - allows TCP caching to be disabled by setting vfs.nfsd.cachetcp=0
> - does the above 2 things you describe to try and avoid the livelock,
>   although not quite using an lru list
> - increases the hash table size to 500 (still a compile time setting)
>   (feel free to make it even bigger)
> - sets nfsrc_floodlevel to at least nfsrc_tcphighwater, so you can
>   grow vfs.nfsd.tcphighwater as big as you dare

Thanks, this looks very good.  One quibble with the last bit: I'd do
that in a sysctl() handler rather than checking it every time through.
If somebody uses a debugger rather than sysctl to change tcphighwater,
they deserve what's coming to them.  Also, I might suggest adding a
counter for how many times we had to go through the "try harder"
phase, so that the sysadmin has some indication that the defaults need
adjustment.

I will test this out later this week and see how it performs.  I have
a user who has been able to reproducibly clobber servers before, so if
he has time and cycles available it should be pretty easy to tell
whether it's working or not.

-GAWollman