Author: Robert Hyatt
Date: 10:30:32 09/06/01
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On September 06, 2001 at 13:06:24, Jeremiah Penery wrote: >On September 06, 2001 at 10:17:15, Robert Hyatt wrote: > >>More accurately, note that memory speeds (random access speeds) have not >>increased _at all_. DRAM was 100ns (or slightly less) 10 years ago. It is >>_still_ that slow today. > > >You can get 5ns DRAM (or possibly faster) today. Look again. There is no memory on the market that will let you _randomly_ access any byte and get the result back in 5ns. They can transfer large chunks and make it appear to be very fast. But it still takes just as long today to dump a capacitor and make the 0/1 determination as it did 10 years ago. The Cray's are the best indicator. Just compare the hardware timings for memory access starting at the Cray-1 (8 clocks) and ending up with the T90 (50 clocks) Cray 1 ran at 12.5 ns clock, T90 at a 2ns clock. The memory speed over that 20 year period is pretty constant. 8*12.5 = 100ns. 2*50 = 100ns. If a PC had a 5ns memory, it would be able to sustain 1.6 gigabytes of memory transfers per second (that is 200 million cycles per second * 8 bytes per cycle). The PC can actually sustain more like 100 megabytes per second of memory bandwidth. Which is closer to an average of 8 bytes every 100 nanoseconds than it is to 8 bytes every 5 nanoseconds. Ignore the "PC400" specification. That is not for the first byte. That is for the synchronous transfer clock speed _after_ the DRAM data has been dumped into an SRAM on-chip cache. Once you get the data into SRAM, you can dump it onto the bus at most any speed you can afford. but getting it from the DRAM is _still_ a big problem. That is one reason early Cray's didn't even bother with DRAM and used bipolar memory. But eventually cost drove them to DRAM and a static memory access time.
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