emmo@moncam.co.uk (Dave Emmerson) (08/01/89)
Perhaps I'm just displaying my ignorance in public, but personally I can't see that these huge transfer rates are of much use to Joe Public, since he still has to use DRAMs for any sizeable area of memory on the grounds of cost. Sure, he may have *some* fast SRAM in his system, probably only for the graphics, but the limiting factor in most *affordable* machines isn't going to be the disk's burst rate, it'll be the cycle time of the target memory. Byte for byte, I'd guess that SRAM costs 10x DRAM, and more for the very fast stuff. We already have the situation where stuffing faster processors into a system produces only a small improvement in performance because of the wait states needed to make use of even the fastest DRAMs. Anybody like to guess (quote?) the price of 4Mbytes of 35nS SRAM for a system? Compare it with my guestimate of 450 dollars for 100nS DRAM on ours. It seems that every time there's an improvement in processor speed/memory speed/memory size/ media size/media transfer rate, there follows a corresponding decrease in the efficiency with which we use it. Think back to what you were almost happy with 10 years ago. However did you manage? Oh yes, they'll improve, no doubt one day we'll get 25nS DRAMs free with our wheatie flakes, but till then the fast disk drive manufactors will have the same limited market as the fast processors, strictly where performance overrides almost any cost considerations. The wider transfer bus isn't just smart, it's the only affordable, workable solution for most of us. As such I welcome it, but I can't get all starry-eyed over the 8 bit performance claims, it's difficult to sustain 3Mb/sec, let alone 5! -Standard disclaimer- Dave E.
mo@prisma (08/03/89)
Sorry, but even on a Sun 4/280 you can get lots more through the machine than the disks can readily currently support, particularly if you tweak the system a bit. Amdahl's law says you need 1 megabyte per second of disk bandwidth per MIPS for a balanced system. Now I know much of the world is currently enamored with the MIPS rating of a machine as the only performance number, but a 20 MIPS cpu that has 2-4 MBs/sec of I/O is pretty unbalanced for some jobs, and as 20 goes to 200, it becomes untenable. Just remember that spinning the processor isn't the only thing computers get paid-for to to. In lots of jobs, I/O is a Big Deal, and you certainly don't have to use 35ns static rams for main memory to be able to do I/O (although you *will* have to build a real memory system). -Mike