Author: Matt Taylor
Date: 09:53:42 03/19/03
Go up one level in this thread
On March 18, 2003 at 23:09:08, Robert Hyatt wrote: >On March 18, 2003 at 19:45:43, Tom Kerrigan wrote: > >>On March 18, 2003 at 18:20:14, Robert Hyatt wrote: >> >>>On March 18, 2003 at 17:46:10, Tom Kerrigan wrote: >>> >>>>On March 18, 2003 at 16:37:35, Robert Hyatt wrote: >>>> >>>>>>>1. no interleaving, which means that the raw memory latency is stuck at >>>>>>>120+ns and stays there. Faster bus means nothing without interleaving, >>>>>>>if latency is the problem. >>>>>> >>>>>>Uh, wait a minute, didn't you just write a condescending post to me about how >>>>>>increasing bandwidth improves latency? (Which I disagree with...) You can't have >>>>>>it both ways. >>>>>> >>>>>>Faster bus speed improves both latency and bandwidth. How can it not? >>>>> >>>>>It doesn't affect random latency whatsoever. It does affect the time taken to >>>>>load a >>>>>cache line. Which does affect latency in a different way. However, >>>>>interleaving does >>>>>even better as even though it doesn't change latency either, it will load a >>>>>cache line even >>>>>faster. >>>> >>>>Are you kidding me? How can FSB speed _not_ affect latency? >>> >>>Very simple. Latency is caused _in_ the memory system, only a tiny part of >>>latency >>>is caused by the delay of shipping the data over the bus. If you ran the bus >>... >>>Run the test. This discussion was held on r.g.c.p a while back. And the _same_ >>>results were found. Memory has 120ns latency no matter _what_ memory you >>>use. RDRAM is even slower in terms of latency. If you can get your memory to >>>sub-100ns latency, you've done a miracle in modern electronics. >> >>I guess I'm sitting in front of one miraculous computer, then, because it can >>randomly access a word in 75ns. Just ran the test. (RDRAM, BTW.) > >Yes you are. You have the fastest single CPU on the planet. Notice that to >do this test, you have to access a byte, skip down 128 bytes and access another >and repeat this for a _long_ set of addresses. If you _still_ get 75ns >you _do_ have the fastest PC latency ever reported by any serious tester. AMD thinks so too. The most accurate figure I've found is about 70 ns for the on-die memory controller that Clawhammer has. (I saw some claims of sub-40 ns, but I find that hard to believe.) >>If you have a 133MHz DIMM that's rated at 2-1-1-1, it can obviously access a >>word in 15ns. > >I don't believe 15ns for a second. Just look at current specs for DRAM and >tell me how that is going to happen? Again, look at any memory benchmarking >done on the internet by folks that do this for a living. _nobody_ has reported >sub 100ns latency for any test I have seen, when talking about the PC. Or >when talking about a sixty million dollar Cray. 15 ns is believable. You must remember that ram is configured as rows and columns. The full 100-120 ns is the latency of opening a new row and reading. You and Tom seem to be talking about different things here. A completely random access is going to hit RAS and stall the full 100-120 ns. Reloading the column will only hit CAS and stall for 15 ns. >> If the system gets that word in 75ns (ignoring RDRAM vs. DIMM >>latency for now) that means 20% of the latency is from the memory and 80% (not >>"a tiny part") is from "shipping the data over the bus" (and through the >>northbridge). Conventional wisdom says there's a 10ns wire/pin delay for a >>signal going into or out of a chip, so into northbridge + out of northbridge + >>into processor = 30ns. That means 30ns of processing is done on the northbridge >>and processor. That's why everybody is so worked up about Hammer's on-die memory >>controller--it reduces memory latency by, well, somewhere between 20 and 50ns, >>or roughly 50%. >> >>End of today's lecture... > >Now to get some _real_ data before giving the _next_ lecture. As I said, >access 1M bytes, with a 128 byte stride so cache-line pre-fetching won't >artificially bias the result downward. > >I'll try to run this on a group of dual xeons here tomorrow, starting with my >2.8's and also trying the 3.06's. > >Several of us did this on R.G.C.P a few months back however, and 120+ ns >was the _best_ time reported when the test was run correctly. I got 133 ns as well. Aaron was running tests like crazy this morning on his nForce 2, and he reported times as low as 70 ns. I find that -very- impressive. Of course, that was with massive memory overclocking. -Matt
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