Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!gatech!prism!loligo!mccalpin From: mccalpin@loligo.cc.fsu.edu (John McCalpin) Newsgroups: comp.arch Subject: Re: Do you have bandwidth? Keywords: memory bandwidth latency Message-ID: <592@loligo.cc.fsu.edu> Date: 15 Apr 89 21:36:24 GMT References: <407@bnr-fos.UUCP> <7766@thorin.cs.unc.edu> Reply-To: mccalpin@loligo.cc.fsu.edu (John McCalpin) Organization: Supercomputer Computations Research Institute Lines: 35 In article <7766@thorin.cs.unc.edu> davis@cs.unc.edu (Mark Davis) writes: >You can always improve bandwidth with silicon (and wires). To double >bandwidth, double the data bus size. > >On the other hand, latency (roughly the number of cycles to get the >data after you figure out its address), is a much more difficult problem. >Making the latency twice as good (50% as long) can be very tough. >Mark (davis@cs.unc.edu or uunet!mcnc!davis) One place where the distinction between latency and bandwidth shows up very clearly is in the CDC/ETA line of supercomputers. These machines (the Cyber 205 and ETA-10) use a memory-to-memory vector architecture. The machine being installed now at FSU (an ETA-10G) supports a sustained bandwidth of 6.85 GByte/s from each CPU's 32 MByte (soon 128 MByte) local memory through the vector pipes and back to memory. This bandwidth consists of two 64-bit loads and one 64-bit store for each of 2 vector pipes on each CPU every 7 ns cycle. Total: 6 words/clock * 8 Bytes/word * 143 M clock/s = 6.85 GB/s. I think that this memory is implemented in 35 ns SRAM. Latency on the 10.5ns machine is about 6-8 cycles, or 60-80 ns. I don't know how this will scale with the faster CPU's. The second-level memory consists of 1 GByte of DRAM. It has 8 ports capable of sustaining one 64-bit word/clock transfers. The aggregate transfer rate is thus 9.1 GByte/s. The hardware setup time for a transfer is supposed to be about 256 cycles, but I don't know what fraction of this is the actual memory latency. Disclaimer: I don't work for CDC/ETA. In fact, I don't work much at all.... -- ---------------------- John D. McCalpin ------------------------ Dept of Oceanography & Supercomputer Computations Research Institute mccalpin@masig1.ocean.fsu.edu mccalpin@nu.cs.fsu.edu --------------------------------------------------------------------