cliffhanger@cup.portal.com (Cliff C Heyer) (09/07/89)
I'm planning to buy a 80386 PC for use with UNIX, MSDOS, OS/2, and WINDOWS/386. After studying the trade papers and marketing literature, I've made the following conclusions: (feel free to comment!) 1. Price: 33MHz hardware about same ballpark as 25MHz hardware. 2. 33MHz hardware not yet reviewed in key areas of: bus speed, paged/interleaved memory, shadow (BIOS/video) RAM, disk cache(memory or controller), extended memory speed, wait states. 3. 100% of 33MHz hardware gives 10-20% better MIPS than 25MHz. 4. 33MHz hardware disk I/O only 0-5% better than 25MHz. In other words, it might as well be the same. 5. 80386 portables are about the same price as 33MHz desk hardware but are 50% slower in CPU and 70% slower in I/O. QUESTIONS: 80386 PCs used with UNIX... (This is where I need the help!) 1. UNIX (or any multitasking OS) and the effects of the on-board cache: While multitasking, does flushing the cash waste a measurable amount of run time or is it insignificant compared to swapping, paging, and/or other overhead? In other words, is the cache still beneficial even though it is being flushed? (I assume "yes" since minicomputers such as all VAX models have them.) 2. Is memory technology (cost/speed ) lagging behind microprocessor technology? All the newest 33MHz 80386 PCs are using 70+ ns DRAMs when the 386 is running at 30 ns and the on-board caches are rated at 25 ns. You can't get 0 wait states 100% of the time with this approach. 3. Is it impractical (cost and/or size) to put 1MB 25 ns RAM (SRAM-no refresh overhead and cycle time=access time) up for main memory? In other words, is it cheaper to implement paged (PMRAM, SCRAM) or interleaved schemes to reduce wait states rather than use faster memory? Is there 25 ns DRAM? 4. Are any board makers making (or have made) motherboards with ESDI and/or SCSI interfaces ON BOARD to bypass the 8MHz AT bus? Also hopefully this mfg. would include shadow RAM (BIOS & video) and extended/expanded memory that is as fast as main memory. (eg. add on memory boards have same cycle time as the first 2MB.) 5. I assume the ONLY thing that makes the 33MHz PCs faster is the 25 ns cache. Otherwise, with 70 ns DRAM the BEST you could do would be run as fast as a 16MHz 80386 PC (62 ns) but with lots of wait states. In other words, memory cycle time limits non-cache CPU performance to that of a 16MHz 80386. 6. If you whipped out your trusty soldering gun and anti-static gear and changed all your memory chips to 25 ns (on a 33MHz machine w/no cache) would the wait states go away? OR is the timing part of the hardware architecture? 7. The PC manufacturers never talk about parity error checked memory, ECC memory, separate data/instruction cache, data write-thru cache, write buffers (CPU can go on after issuing initial memory instructions), and multi-word memory transfers. Are they behind the times? 8. Is there ANY manufacturer who has fully exploited the power of the 80386 chip? That is, at 33MHz is there any hardware that... >can support sustained disk I/O >1MB/sec by bypassing the AT bus via on-board controllers, or using VME, etc., >has 25 ns cache, main memory, AND expanded/extended memory boards (no wait states 100% of the time), >(for PCs) has shadow RAM (BIOS & video), >gives you several "real" 32-bit "backplane" slots and controllers for them, >operates FCC class B.
davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr) (09/07/89)
In article <21931@cup.portal.com>, cliffhanger@cup.portal.com (Cliff C Heyer) writes: [ this is an answer to part of his posting ] | 1. UNIX (or any multitasking OS) and the effects of | the on-board cache: | | While multitasking, does flushing the cash waste a | measurable amount of run time or is it | insignificant compared to swapping, paging, and/or | other overhead? In other words, is the cache still | beneficial even though it is being flushed? (I | assume "yes" since minicomputers such as all VAX | models have them.) I assume you mean disk cache. Yes it helps a lot, yes it causes a big slowdown in the system when it flushes. Tune you disk cache size to fit your needs. [ questions about memory ] 64k cache gives 90+% cache hits. Coupled with interleave or column static gives something like .05-.30 wait state average depending on what you're doing. | 4. Are any board makers making (or have made) | motherboards with ESDI and/or SCSI interfaces ON | BOARD to bypass the 8MHz AT bus? Also hopefully | this mfg. would include shadow RAM (BIOS & video) | and extended/expanded memory that is as fast as | main memory. (eg. add on memory boards have same | cycle time as the first 2MB.) Mylex makes a controller which goes on the 32 bit bus and has a load of cache on it. You can set the UNIX cache very small and still have good performance. After looking at cache performance I'm not sure you buy anything this way, at least not $4k worth, but it's there. | | 5. I assume the ONLY thing that makes the 33MHz PCs | faster is the 25 ns cache. Otherwise, with 70 ns | DRAM the BEST you could do would be run as fast as | a 16MHz 80386 PC (62 ns) but with lots of wait | states. In other words, memory cycle time limits | non-cache CPU performance to that of a 16MHz 80386. On longer instructions the faster speed still buys performance. Not as much as you get with faster memory, but still some. It depends on your instruction mix, since pure CPU stuff will run full speed even with several wait states due to the pipeline. When you start branching and accessing data in memory you slow down. | | 6. If you whipped out your trusty soldering gun and | anti-static gear and changed all your memory chips | to 25 ns (on a 33MHz machine w/no cache) would the | wait states go away? OR is the timing part of the | hardware architecture? Some boards can have the w/s changed. It's not clear how much you would gain. | | 7. The PC manufacturers never talk about parity error | checked memory, ECC memory, separate | data/instruction cache, data write-thru cache, | write buffers (CPU can go on after issuing initial | memory instructions), and multi-word memory | transfers. Are they behind the times? As far as I know the PC based machines all use parity. I believe there are ECC AT bus memory boards available, but I didn't save the article. Conclusion: if you want a big jump in power, wait for a 486. Although it's not obvious, a hardware trace on the wait line of most cached 386s will show that you are only losing 10-15% *at worst*. Before spending a lot of money trying to get faster everything you can start with something inherently faster. -- bill davidsen (davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen) "The world is filled with fools. They blindly follow their so-called 'reason' in the face of the church and common sense. Any fool can see that the world is flat!" - anon