From: adisaacs@mtu.edu (Andy Isaacson) Newsgroups: comp.parallel.mpi Subject: Re: Celeron vs Pentium II Date: 10 Mar 1999 03:04:28 GMT Organization: Michigan Technological University Message-Id: References: <19990306115330.11135.00003808@ng-cb1.aol.com> Xref: ukc comp.parallel.mpi:4721 In article , Victor Eijkhout wrote: >adisaacs@mtu.edu (Andy Isaacson) writes: > >> The L2 cache in the Celeron is clocked at core speed (333 MHz in this > >What do you mean by the clock speed of a cache? I know of speed of >processors and of busses, but not of cache. (I'm not an expert on this stuff, I'm just parroting what I've read.) In many CPUs, the L2 cache runs at some divisor of the CPU core speed. Thus, on a 400 MHz PentiumII, for example, the CPU can read from the L1 cache at 400 MHz, but can only read from the L2 cache at 200 MHz, and main memory at 100 MHz. The Celeron 400, on the other hand, can read from L1 cache at 400 MHz and L2 cache at 400 MHz. (There's a few clock cycles extra latency for the L2 cache, which is why it's separate from the L1.) The main memory bus of a Celeron is only 66 MHz. So here's a little table: CPU Celeron PII PII Xeon L1 speed 1x 1x 1x L1 size 32k 32k 32k L2 speed 1x .5x 1x L2 size 128k 512k 512k, 1M, or 2M Mem. bus speed 66 MHz 100 MHz 100 MHz So, if your application fits into 128k, it will run faster on the Celeron than on the PII. If your application fits into 512k but not 128k, it will probably run faster on the PII than on the Celeron. If your application doesn't fit into 512k, it will probably run faster on the PII than on the Celeron, because of the faster memory bus. >Oh, and how does your 333 gell with other people claiming 66 or 100? >If there is a low number somewhere in your system, then the higher >ones are pretty much irrelevant. False. The CPU runs at some clock speed (probably between 200 and 500 MHz today), the various caches run at their various speeds, and main memory runs at some other speed (generally either 66 or 100 MHz, perhaps 83 MHz on some clone Super7 motherboards). The PCI bus runs at 33 MHz (or 66 MHz if you have an Alpha or UltraSPARC). Each of those busses has a purpose, and their different clock speeds impact different parts of the performance equation. Performance cannot be distilled to a single number, no matter how hard SPEC and the various vendors try. -andy