how big can kern.maxvnodes get?

[John Baldwin]

Chris Peiffer wrote:
> I have a backend server running 8-2-PRERELEASE with lots of
> independent files that randomly grow and then get truncated to
> zero. (Think popserver.)  
> 
> Roughly 2.5 million inodes on each of 4 Intel SSD disks. 24 gb of RAM
> in the system. I want to maximize the buffer cache in RAM.
> 
> I doubled kern.maxvnodes to 942108 and reads/second went down and
> memory use went up, (as I expected) but right now there's still about
> 15g RAM marked as free.
> 
> vfs.numvnodes crept up to 821704 and has hovered there. The file
> sizes range to 1 mb but most are in the range 0-10k. Since the server
> operations are so simple kern.openfiles hovers in the range 100-200.
> 
> Obviously, all things being equal I'd like to give the filesystem
> buffer cache access to that free RAM by allowing more vnodes to stay
> cached. 
> 
> Can I increase kern.maxvnodes by another factor of 2? more? Are there
> any known problems with stepping it up, besides general memory
> exhaustion? With so much free RAM I'd like to turn the dial a little
> bit but I wonder if there are other linked things I should watch out
> for.

You can increase it, but if numvnodes is less than maxvnodes then it 
won't help you as the system hasn't had to recycle any vnodes yet.  It 
is already caching all the vnodes you have accessed in that case.

If the files are frequently truncated then you may end up with a lot of 
free RAM simply because there isn't enough recently used data to cache. 
  The VM system will cache everything that is accessed until either 1) 
the pages become invalid (e.g. due to an unlink or truncate) or 2) free 
memory runs low enough to trigger pagedaemon to harvest some inactive 
pages.  If you have 15G of free RAM, then 2) isn't happening and your 
working set is less than your RAM.

-- 
John Baldwin