Author: Jeremiah Penery
Date: 10:48:15 09/06/01
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On September 06, 2001 at 13:30:32, Robert Hyatt wrote: >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. Here is an interesting article: http://www.hardocp.com/articles/memory/ddrovr/ While not as fast as I thought, it's still a lot faster than they were 10 years ago.
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