Re: > 64 MB of RAM

Kevin Buettner (
Wed, 8 May 1996 00:59:19 -0700 (MST)

> On Sat, 4 May 1996, Kevin Buettner wrote:
> > Memory prices seem to be falling and I am contemplating buying more
> > memory.
> >
> > In Documentation/, it says the following:
> >
> > : Limit memory to low 16MB
> > : CONFIG_MAX_16M
> (clip)
> > : You also need at least 512kB
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > : of RAM cache if you have more than 64MB of RAM. Some other things
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > : to try when experiencing seemingly random, "weird" problems: 1)
> (clip)
> >
> > Is the line about needing a 512kB cache true?
> >
> > In theory, my motherboard can hold up to 128MB of memory, yet I am
> > fairly certain that I am limited to a 256kB cache. (I have a Plato
> > motherboard.)
> >
> > Kevin
> I have a motherboard which has 8MB with an 8KB internal CPU cache, with a
> 486(DX)33... right now I have 16.59 Bogomips, and I have another 8MB SIMM
> ordered... How much of a speed hit from undercaching should I expect? What
> is a good "main_memory/cache" ratio?

I think that you'll benefit *immensely* from an additional 8MB of
memory (though your Bogomips won't go up). I seriously doubt that
you'll experience any negative performance impact due to the size of
the cache. (Of course bigger is always better.) As far as the ratio
goes, it seems to me that with most motherboards, you don't have a lot
of choice. 256k L2 cache seems to quite common on contemporary
Pentium motherboards. It doesn't seem that there is any easy way to
upgrade this without getting a new motherboard. And even then it's
kind of hard. The largest cache I've seen on a Pentium based system
(from poking around in Computer Shopper) is only 512 kB. OTOH, I've
seen some Alpha (not experimental ;-) motherboards with as much as 4MB
of cache!

Now as to my original query...

I have received a number of responses and I thank everyone who has
responded. I will attempt to summarize...

I received one report of a site with (at least) two linux machines
with in excess of 64 MB of memory. Their machines are running fine
and there is no reported performance degradation.

I have also received several responses from people who say that it
depends on the cache (apparently on the width of the tag). They say
that unless you try it, there is no effective way of knowing whether
or not performance will suffer or not. (Apparently everything will
still work though; it's just that a portion of your memory is

I have also received a somewhat ambiguous response which stated
(paraphrasing) that a 512kB cache is needed for best performance.
I interpret this to mean the same thing as was described in the
previous paragraph. (Otherwise, it seems patently true.)

I also received some interesting advice which was that instead of
buying more memory, buy a new motherboard and get a guarantee that it
will support a certain amount of memory without loss of performance.
(And then buy more memory.)

So what am I going to do? Don't know yet. I'll probably buy more
memory, but I won't go over 64 MB total. Does anyone know whether
memory prices are supposed to continue to fall?