🚀 go-pugleaf

Adrian Gentle (adrian@vw.maths.monash.edu.au) wrote:
: I know this isn't strictly linux related....
: We're in the process of buying a new machine, which will
: be used largely to run linux - and I have heard that some
: systems can slow down as more memory is added if the 
: cache isn't sufficient.

This one seems to raise it's head at regular intervals. Is there
some FAQ about this anywhere? The reason this used to be an issue was bad
cache design. It has to do with which address bits are used for what in a
set associative cache. 

I have a fairly detailed thingo' I wrote up about this when I was trying
to figure out why this could happen. If anyone is interested, send me
mail.

I'm pretty sure that all the newer motherboards with pipeline-burst
cache have fixed the problems older motherboards used to have.

: We're looking at 64M of RAM, and as usual getting conflicting
: messages from different shops.  256k cache on the chip is the 
: minimum, but I was after 512k.  Is this necessary/recommended?
: The remaining 256k seems to live on the motherboard - but some
: shops claim that you can't put it on the m/b (I assume this
: says more about their m/b's than the cache!).

It doesn't matter where the cache is. I belive most motherboards have 256k
onboard, and use a seperate cache module to make 512k. Still, it would
surprise me if there aren't some HX motherboards with 512k onboard. As
long as you insist on pipelined burst cache, you should be right.

: So do I need 512k?  Should it live half on the m/b and half on 
: the chip?

The best size for your cache doesn't depend on how much RAM you have
(given the cache design is flexible enough to handle it). It depends on
your working set size, which depends on what applications you run. Tuning
your system for best bang-per-buck is a delicate balence between disk,
RAM, and cache for your particular application. Adding more RAM or cache
will not speed things up if you are not using most of what you've already
got.

Having said all that, probably 256k cache is sufficient for most sytems
that have (and use) up to 64M RAM. If you find you have a system that
benefits from adding more than 64M, you will probably also benefit from
adding more cache.

ABO