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From Paul Richards <p.richa...@elsevier.co.uk>
Subject Re: fairy nuff
Date Fri, 01 Dec 1995 11:53:40 GMT
In reply to Rob Hartill who said
> 
>  
> > 72% vmstat
> >  procs   memory     page                         faults      cpu
> >  r b w   avm   fre  flt  re  pi  po  fr  sr s0   in   sy  cs us sy id
> >  0 0 0 21896  3780  132  15   7   0    0   0 1   258 194 110  4 11 85
> > 
> > As you can see, we're not thrashing or low on memory. The machine has 32
> > Mb RAM currently and soon will go to 48.
> 
> This has always confused me....
> 
> Where it says "avm", and the manpage describes this as "Active Virtual
> Memory", doesn't this mean that VM is being used and thus you don't
> have enough memory?
> 
> When I use vmstat under SunOs (32Mb) on a busy machine avm=0
>                         HPUX  (64Mb)      quiet        avm>0
> 
> Which confuses the hell out of me 'cos the HP machine has more memory
> and is idle when I do the test.
> 

If you see avm in the vmstat output then it's likely BSD based. If you
RTFM :-) it'll tell you that virtual memory is considered to be
"active" if it belongs to processes that are running or have run in the
last 20 seconds.

*DON'T* get confused by the idea that virtual memory is just your swap
space, IT IS NOT. Virtual memory is all the memory available for use
and that *includes* REAL memory. The fact that avm is >0 doesn't mean
your using swap space at all, it's just a measure of what memory is
being "actively" used. Inactive memory is that memory that's being used
up by processes that are sleeping or swapped out etc.

Sorry for the over zealous emphasis but some fundamental misconceptions
needed addressing :-)

SYSV boxes (at least SunOS5.X) doesn't have an avm entry, it has a 
swap entry instead which lists the amount of swap space free. Under
BSD there's a swapinfo command that does this though it's not
available with SunOS4.x.

Ohh and vmstat on sunOS4.x always seems to have avm=0, I guess it's not
doing what the man page says it should.

Hey, don't run SunOS unless you've no other choice. :-)


-- 
  Paul Richards. Originative Solutions Ltd.
  Internet: paul@netcraft.co.uk, http://www.netcraft.co.uk
  Phone: 0370 462071 (Mobile), +44 1225 447500 (work)



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