From: wolff@zen.et.tudelft.nl (Rogier Wolff) Subject: Re: To cache or not to cache. Which machine to buy. Date: Sun, 14 Feb 1993 10:52:48 GMT
mcrocker@titan.ucs.umass.edu (MATTHEW S CROCKER) writes:
>on another note. I have 256kb external cache. With my micronics MB I
>can specify which region of RAM I want cached. If at all possible
>could I cache the area where the kernel lives only? this would in
>effect allow my to keep 90% of the kernel in the cache that should
>speed things up shouldn't it? Could linux do this itself on boot up?
Caches are designed to give best performance when used over the whole
memory. Some applications (for instance xv) use a lot of CPU time in
a reasonably small loop. This is where caches perform best. You may find
that your approach works for one or two applications, but give the system
the freedom to find often used regions all over the memory.....
Note that the internal 8k cache gives the best performance hit: say
a factor of 4. (cache cycle time 30ns, dram cycle time 120ns (although they
access in 60)). This should hold for around 90% of memory accesses.
Only the remaining 10% will hit on the external cache, giving a max
improvement around 30%.
No cache 100% * 4 clocks = 400
Internal cache 90% * 1 clock + 10 % * 4 clocks = 130
Perfect (8Mb)
External cache 90% * 1 clock + 10 % * 1 clock = 100.
Roger.
-- **** a 486 in V86 mode is like a VW buggy with a 6 litre V12 motor. **** EMail: wolff@duteca.et.tudelft.nl ** Tel +31-15-783643 or +31-15-142371