kmem_size / arc_max ratio

Rolf Nielsen listreader at
Wed Feb 3 03:18:24 UTC 2010

Hello everyone,

I recently installed more RAM in my computer; increased it from 4GB to 
8GB. Mainly to be able to use a larger arc for zfs. Now my question is, 
how much memory does the kernel need apart from the arc? I currently 
have kmem_size_max 512MB bigger than arc_max. I haven't run into any 
problems so far. I use my computer as desktop only, running WindowMaker 
and normally having xconsole, emiclock, firefox with 5+ tabs, 
thunderbird, amsn, sunbird, thunar, ktorrent and upto 5 or 6 xterms.

My loader.conf looks like this:


I am considering increasing kmem_size, kmem_size_max and arc_max by 
1024MB each. The idea behind that, is that when I had 4GB RAM I had 
kmem_size and kmen_size_max set to 2GB and never had any trouble with 
user memory running out, so I believe 2GB user memory should still be 
enough. Please correct me if I'm wrong. ;)
Anyway, the main question is, should 512MB difference between kmem and 
arc be enough, or should I increase it? Or could I even decrease it? 
I've had very noticably increased performance with larger arc size, so I 
would like to set it as high as possible without causing problems for or 
degradation of other kernel tasks and/or user applications.
Please let me know if you need any further information about my system.


Rolf Nielsen

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