Path: utzoo!utgpu!watserv1!watmath!att!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!van-bc!ubc-cs!news-server.csri.toronto.edu!helios.physics.utoronto.ca!ists!yunexus!xrtll!silver
From: silver@xrtll.uucp (Hi Ho Silver)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Faster RAM <==> Faster operation
Message-ID: <1990Nov4.221653.4823@xrtll.uucp>
Date: 4 Nov 90 22:16:53 GMT
References: <1718@kielo.uta.fi>
Reply-To: silver@xrtll.UUCP (Hi Ho Silver)
Organization: Not around here, pal!
Lines: 39

In article <1718@kielo.uta.fi> jackin@vehka.uta.fi (Markku M{enp{{) writes:
$I am wondering about how faster ram affects perfomance.
$I understand that non-cache boards probably benefit from
$faster ram but is it the same with cache MBs too ?

   Yes, but nowhere near as much.  The idea of a cache is that the vast
majority of memory references occur in a fairly small area of memory
during any given time period.  For example, in a matrix multiplication
routine, you'd have some loop code that gets executed repeatedly, and
your data fetches into the matrices would also be done repeatedly.  By
loading these frequently-accessed areas into a fast cache, your computer
doesn't have to wait for main memory as often.

   If 90% of your memory accesses are serviced out of the cache, and your
main memory runs at 2 wait states (and assuming cache misses take no
additional clock cycles of their own), your average number of wait states
would be

	.9 * 0		+ .1 * 2		= .2
	(the cache)	(main memory)

   If you reduce your main memory wait states to 1 by replacing the
chips with faster memory, you then end up with an average of .1 wait
states per memory access.

   On my old 286 system, running at 0 ws was about 10% faster than
running at 1 ws; if this holds for 386 systems as a guideline, then
in the above example, you'd only speed your machine up by 1%.  Admittedly,
the numbers are entirely made-up, but they should show you that you are
not likely to gain much by speeding up main memory on a cached motherboard.
Keep in mind that although a 90% hit rate may sound pretty high, you
actually don't need a very large cache to achieve that kind of rate.  I
don't have the figures available, but that rate should be achievable with
a 64K cache, and perhaps even with a 32K one.
-- 
HI ROGER |Nikebo says "Nikebo knows how to post.  Just do it."| silver@xrtll
_________|-----------------------|_______________|------------|_____________
yunexus!xrtll!silver (L, not 1)  | Hi Ho Silver  | costing the net thousands
Silver:  Ever Searching for SNTF |i need a grilf | upon thousands of dollars