Re: [9fans] Virtual memory in BSD and Plan9

["John S. Dyson" <dyson@iquest.net>]

rob@plan9.bell-labs.com (rob pike) wrote in message news:<20011029123819.DCCD7199E4@mail.cse.psu.edu>...
> I'm a radical here, but I think if a machine is paging, you've lost.
> To me, VM is a pretty technique for memory management in the kernel,
> something distinct from paging, which is a way to get the system
> through temprorary overshoots in memory demand.
> 
In these days of 'inexpensive memory', I tend to agree with you, BUT:
VM is also a good tool to push out long unused pages so that they can
also be used for caching and other helpful purposes.   This is akin to
the 'temporary overshoot' issue, but slightly more active.

I have noticed in alot of cases, that such 'temporary overshoots' still
cause systems to thrash too much, trying to find the correct working
sets.  Some of the VM work in FreeBSD (not necessarily the whole package),
does alot of work to avoid the thrashing.

A system that is continually paging, even if the VM mgmt is really good,
will often not be a very friendly acting system.  The key to the VM
code is to make sure that in reasonable cases, the system isn't continually
paging.   Finding the reasonable working sets in a global sense (local
sense only causes less efficient behavior), so that paging activity is
minimized or converges to nil.

It is easy to define a 'finding' that the system will never actively
page, and assume that a system with 'insufficient' memory isn't well
configured.  This is a valid 'finding', but certainly can make the OS
VM less robust in a performance sense.   This is not necessarily a terrible
thing.

John