Re: VM Requirement Document - v0.0

[Helge Hafting <helge.hafting@idb.hist.no>]

Martin Knoblauch wrote:

> 
>  maybe more specific: If the hit-rate is low and the cache is already
> 70+% of the systems memory, the chances maybe slim that more cache is
> going to improve the hit-rate.
> 
Oh, but this is posible.  You can get into situations where
the (file cache) working set needs 80% or so of memory
to get a near-perfect hitrate, and where
using 70% of memory will trash madly due to the file access
pattern.  And this won't be a problem either, if
the working set of "other" (non-file) 
stuff is below 20% of memory.  The total size of
non-file stuff may be above 20% though, so something goes
into swap.

I definitely want the machine to work under such circumstances,
so an arbitrary limit of 70% won't work.

Preventing swap-trashing at all cost doesn't help if the
machine loose to io-trashing instead.  Performance will be
just as much down, although perhaps more satisfying because
people aren't that surprised if explicit file operations
take a long time.  They hate it when moving the mouse
or something cause a disk access even if their
apps runs faster. :-(

Helge Hafting