Date: Tue, 1 Oct 1996 22:25:31 -0700 (PDT) From: "Rodney W. Grimes" <rgrimes@GndRsh.aac.dev.com> To: richard@pegasus.com (Richard Foulk) Cc: freebsd-hardware@FreeBSD.ORG Subject: Re: H/W recommendation Message-ID: <199610020525.WAA20652@GndRsh.aac.dev.com> In-Reply-To: <199610020227.QAA22441@pegasus.com> from Richard Foulk at "Oct 1, 96 04:26:50 pm"
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> } > As can be seen the best parts to be using are the 100, 133, 166 and 200, > } > with the exception that at a mulitplier of 3 the CPU starves for memory. > } > } Depends on the cache, and whether you get a Pentium or a Pentium Pro. A > } Pentium Pro with a built-in 512 KB level 2 cache usually won't starve, even > } on UNIX boxes. (To put things in perspective, a typical FreeBSD kernel, > } with unnecessary drivers removed, is about that size.) But the bargain > } basement version of the Pentium Pro, with the 256 KB cache, will drag in > } the same configuration. Unfortunately, far too many clone vendors just > } HAPPEN not to mention in their ads that they're including the cheaper CPU. > } > } I'd like to see a megabyte cache on board. > } > > This isn't quite the way cache works. A board with 512K of cache > won't hold 512K of code, even if that were necessary -- most of the > code in the kernel or any other large program seldom gets run. > > But the cache isn't just a mirror of memory, probably half the > cache is available for code storage (depends on how it's implemented.) > > More importantly, diminishing returns sets in real quick after 256K, > (actually before.) I wish I had the time to go calculate what a 1MB 4 set associtive seperate I/D write allocating cache would do for a Pentium 200 running FreeBSD, but I suspect quite a lot! It would probably do very little for the Pentium Pro, given that the internal 256K cache is already set associative and non-blocking. The current design of direct map external caches does pretty much top out at 256K, but when you add associative sets and split the I and D you eliminate some real bottlenecks (like large bcopy'ies that end up displacing all your cached code in the external cache :-(). An external really fast memory to memory DMA engine might be cheaper and give you the same overall system performance boost :-). Too bad the dreaded DMA on x86 machines is still based on the stupid old ISA DMA controller. Hummm... can I make a 2940 into a DMA engine, nahh.. the PCI bus is too slow compared to the 176 or so MB/s of usable memory bandwidth :-(. -- Rod Grimes rgrimes@gndrsh.aac.dev.com Accurate Automation Company Reliable computers for FreeBSD
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