kmem_map too small with ZFS and 8.2-RELEASE

[Jeremy Chadwick]

On Fri, Mar 04, 2011 at 08:23:29AM -0500, Adam McDougall wrote:
> On 03/04/11 04:48, Mickaël Canévet wrote:
> >>I'd use vm.kmem_size="32G" (i.e. twice your RAM) and that's it.
> >
> >Should I also increase vfs.zfs.arc_max ?
> >
> >Do you have any idea why the kernel panicked at only 8GB allocated ?
> >
> >Thank you
> 
> I believe ARC allocations in kmem can become fragmented, so when it
> is searching for a place to store a new contiguous segment of
> memory, the remaining fragmented free spaces may all be too small.
> I also set
> vm.kmem_size to about twice the amount of ram to help it avoid this
> issue.  I suspect if kmem is badly fragmented then performance of
> ZFS can downgrade, so that is another reason to keep kmem bigger.

My findings on 8.2-RELEASE indicate that doing this results in very
unexpected behaviour regarding the ARC maximum.  As such, I cannot
recommend this model.

For example, on an amd64 system with 8GB physical RAM and these two
settings in /boot/loader.conf:

vm.kmem_size="8192M"
vfs.zfs.arc_max="6144M"

kstat.zfs.misc.arcstats.size tops out at around 6240986896, with Wired
in top(1) showing ~6.3GB.  This is expected behaviour and fits (I
think) what people expect.

However, on the exact same system with these two settings:

vm.kmem_size="16384M"
vfs.zfs.arc_max="6144M"

The above ARC numbers are exactly *half* that amount.  This is easily
reproducible.

Can someone 1) justify the "2x the amount of RAM for vm.kmem_size"
setting, and 2) explain in detail the above behaviour?

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.               PGP 4BD6C0CB |