Cache performance on 386 boards running Unix

[David Hinds]

In article <416 at ssp2.idca.tds.philips.nl>, pb at idca.tds.PHILIPS.nl (Peter Brouwer) writes:
> This depends on the size/working set of your applications you use.
> Most caches are 64k = 16 pages. So if you have large applications with
> a working set ( number of pages it used during execution ) the cache is'nt
> a great help. 
> 
This is not really true.  A RAM cache constantly turns over to reflect
the current memory usage of whatever is running on a system.  It stores
recently used instructions and data.  It is most effective at speeding
up fairly small loops (a few K), or code which accesses the same data
repeatedly.  Fortunately, this represents the local activity of almost
all programs, almost all of the time.  I don't have any concrete data,
but I've seen quotes for 80386 systems with 64K caches of hit rates of
90-95% for typical real applications.  Context switching should not
degrade performance much, because the time scale over which the cache
works is much, much shorter than a task's time slice.  The cache turns
over fast enough that it recovers almost immediately from a context
switch.  RAM caches have been standard equipment on mainframes for
decades.  In fact, 64K is quite large as caches go; the 80486 chip has
a cache of something like 256 bytes, but I wouldn't be surprised if
this had hit rates of 80-90%, even with very large programs.
                             -David Hinds
                              dhinds at portia.stanford.edu