🚀 go-pugleaf

John Jay Feiler writes
>> I've got a large image that I need to keep all in ram, at all times.

dan@az.stratus.com writes:

> any attempt to fool [the OS demand paging] results in slightly increased performance for
> one particular application at the expense of  all the others.   

> the consensus [...] was that attempts to defeat the LRU demand paging algorithm  
> were generally detrimental to overall system throughput and response time.

Dan gives alot of good arguments why its usually not a good idea to
fight the demand paging system.  It does pay to understand how it works so you can
take better advantage of it.  That means it pays to learn about zones, 
copy on write optimizations and perhaps OS tuning parameters.

There is one very legitimate reason for wanting to wire a page in physical
memory irregardless of how frequently its accessed.  Thats is when it is needed
to support a task with hard real time deadlines.  Here the need is for predictability
rather than speeed. Demand paging introduces uncertain delays in individual cases even if it
increases the overall performance.  This uncertainty can make it impossible to prove that
you will meet your deadlines.  (most of the time is not good enough)

except in this special case, where you are using your computer (typically) to
control or track some physical system with hard real-time constraints,
Dan's advice is quite sound. 

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Alex Blakemore alex@cs.umd.edu   NeXT mail accepted