🚀 go-pugleaf

On Sun, 19 Sep 2004 17:51:38 -0500, ANT...@zimage.com wrote:
>
>I am going to upgrade my old gaming box to an Athlon 64 system. I
>noticed the memory CAS speeds. Does CAS 2 (faster) and CAS 3 (slower) 
>make any big speed differences for 1 GB of PC3200 RAM on a Socket 754 
>ATX motherboard with an AMD Athlon 64 3000+/2.0 Ghz to 3400+ 2.4 Ghz 
>(both with 512 KB socket and 754 CPU)? CAS 2 is expensive so... I am
>wondering if getting 3 is really worth the price. I am mainly gaming,
>watching movies, using the Internet, etc. Gaming is the big one to
>note.

Generally speaking you're looking at about a 0-3% difference in
performance, probably about 1.5% on average.  It makes sense if you're
going for a high-end system, where the extra $50 for CAS2 memory makes
about as much difference as an extra $200 worth of CPU speed.
However, given that you're looking at a 3000+ to 3400+, the saved on
CAS3 memory would be better spent elsewhere.

If you were to compare a system with an Athlon64 3000+ and CAS2 memory
to one with an Athlon64 3200+ with CAS3 memory, the two systems would
cost about the same amount but the latter system would usually be
faster.


Note that the difference is definitely *NOT* 33% as another poster
mentioned.  From the test bellow, an Athlon64 3200+ (2.0GHz) has a
total latency for an uncached memory access of 83ns, or 166 clock
cycles for the processor (this is a slight over-simplification, but it
will suffice for the example).

http://www.aceshardware.com/read.jsp?id=60000258

This test used CAS2 memory.  Going to CAS3 memory would add a single
200MHz clock cycle on the memory side of things, or 10 clock cycles as
far as the processor is concerned.  The result would be an uncached
memory access time of 88ns.  So we get a performance improvement of
about 6% when going from CAS3 memory to CAS2.

Of course, uncached memory access is only part of the story.  In
reality, well over 90% of all memory access is handled by the L1 and
L2 cache, though even at that the MUCH slower main memory access takes
a disproportionate amount of time (for comparison to that 166 clock
cycles for memory, L1 cache normally takes 2 clock cycles and L2 cache
takes usually 7-20 clock cycles).

In the end, main memory access makes up at most about 50% of
performance.  So, 6% faster memory gets you, at most, a 3% speed
boost.  Typically the benefit is actually smaller than that, or about
the 1.5% improvement I mentioned above.

-------------
Tony Hill
hilla <underscore> 20 <at> yahoo <dot> ca