Author: Matt Taylor
Date: 12:26:00 12/05/02
Go up one level in this thread
On December 05, 2002 at 09:02:16, Vincent Diepeveen wrote: >On December 04, 2002 at 18:52:19, Matt Taylor wrote: > >>Yes, 1.3% speed increase is significant when dealing with algorithm analysis, >>and 13% is even more incredible. However, 13% is barely significant when you're >>comparing the speeds of hardware, and that's what you're doing. >> >>The following is how RDRAM works -the way I understand it-. I could have some >>facts grossly wrong. My interests have been in the AMD platform because I can >>build a cluster that operates much faster for the same cost. That said, I am >>-fairly- certain that I have all my facts straight about RDRAM. > >How fast is your cluster RAM latency? > >Are we talking about a default 1Gbit network >with milliseconds latency or so really not capable of running >programs that do some inter process communications, or >something faster than that? Yes, I'm talking about parallel problems that don't need interprocess communication. Chess isn't the only thing that's computable, and it's not the only thing I enjoy computing. :-) >>No RDRAM part operates on a 100 MHz clock. The pc800 part operates at 400 MHz on >>a DDR bus with a width of 2-bytes. This yield a maximum bandwidth of 400*2*2 = >>1.6 GB/sec. The pc1066 part operates at 533 MHz on the same bus, and 533*2*2 = >>2.1 GB/sec. The P4 FSB is a 16-byte DDR bus running at 100 or 133 MHz (100*2*16 >>= 3.2 GB/sec, 133*2*16 = 4.2 GB/sec). > >This is not true for latency. You are quoting bandwidth calculation here. > >It operates at 100Mhz internally but it is quad pumped. > >That quad pumped increases bandwidth, but not latency. > >So for latency you must face the fact that it is 2 times slower. > >For latency DDR ram is 2 times faster: it is 133/100 * (15T/10T) = 2.0 >times faster. > >This is why DDR ram is way faster for me than RDRAM at the same >processor. > >Note that the bandwidth i do not believe either, but as i said before >we can discuss forever here. If you go do some big matrix calculation >(say from 2 gigabyte) where bandwidth is important, then the testresults >i see is that DDR ram has bigger practical bandwidth. > >theoretical it is clear that RDRAM has more bandwidth. > >But i do not want to get into that discussion, it is an endless discussion. > >For latency things is very clear. > >If you still do not understand it, then test it yourself. I do agree about the latency bit, though I am not sure why you use the pc800 part instead of the pc1066 part. I don't think latency has ever been in dispute. RDRAM has been criticized for that from the very beginning. I am not sure how the RDRAM bandwidth discussion is endless; I have never seen anyone claim that RDRAM has lower bandwidth than DDR. I'm not talking about lower performance in some application -- I'm talking about the synthetic tests that measure the amount of bandwidth of RDRAM vs. DDR. Certainly my own tests parallel everything I have read. A synthetic benchmark is important here because we want to measure performance of the hardware to use as a predictor for performance in a real-world application. If we wanted to measure real-world performance of the application, we would take his end-game database software and run it on two systems and compare throughput. >>Note that the P4 has a higher FSB speed than the single RDRAM chip. This is >>intentional. What good chipsets do is issue parallel requests. This means that 2 >>RDRAM modules get twice the bandwidth of a single RDRAM module. Coincidentally, >>you are required to add them in pairs. The same technique -can- be applied to >>DDR, but at present I have not heard of this. (This is why I don't have to >>purchase my DDR modules in pairs -- much to my relief.) > >You have pretty old knowledge then. try AMD 760MPX chipset which requires >also 2 modules. I own an AMD 760MPX-based board, and I am currently running off of (1) Samsung pc2100 1GB CL=2.5 Reg/ECC module. At home I use a Tyan Tiger MPX, and at work I have an Iwill MPX2. Both are based on the AMD 760MPX chipset. I borrowed an Unregistered Samsung 256 MB module and paired it with an Unregistered Micron 256 MB module that I own, and my bandwidth is within ~10% of what I had from the single DIMM. My machine at work uses 2 512 MB CL=2.5 Micron modules, and its bandwidth is within 20-30% of what I get at home. If the chipset had any sort of mux, I should see more than 20-30% gain. The bandwidth calculation is done by copying large amounts of memory, a fairly standard algorithm. Consistent, repeatable results (within 1%) in addition to confirmation from other tests reassure me that the test is accurate. >>Chip-for-chip, DDR modules may sustain higher transfer rates, but I assure you >>that empirical data shows RDRAM systems winning the bandwidth war. > >No. > >This looks like a RAMBUS propaganda talk you write down here. > >For DIEP DDR ram, even at the P4, it is 13% faster or something than >RDRAM. > >This where the cpu speed is most important for DIEP. DIEP uses hash-tables. DIEP is latency-dependent. DIEP will run a little slower on RDRAM because it has higher latency. >>The P4 Williamette system I used at work for a while had a practical bandwidth >>of 2.8 GB/sec on pc800 RDRAM, a 100 MHz bus. The faster DDR-based boards use >>pc2700 which, as I understand, really isn't standard. This is a maximum >>theoretical bandwidth lower than what I have measured. I have a dual AthlonMP >>1600 with about 1.3 GB/sec bandwidth. The SMP factor is probably biasing the >>measurement, but in either case I'm not convinced that there is any DDR system >>that even matches that old P4 on RDRAM. > >Why not buy some chess software and compare a P4 RDRAM system with DDR ram. Chess software isn't necessarily an accurate benchmark of bandwidth, which is why synthetic benchmarks aren't necessarily accurate for chess. The original question wasn't, "Which type of ram runs chess faster?" The question was, "Which type of ram performs best in end-game computation?" For the computation of the database, the bandwidth is likely the most important factor, particularly since the WC memory type allows a lazy-commit style of memory writing. For table queries like DIEP uses, DDR will probably be faster. >>-Matt >> >>On December 04, 2002 at 18:19:43, Vincent Diepeveen wrote: >> >>>On December 04, 2002 at 17:40:13, Matt Taylor wrote: >>> >>>i hope you realize that good programmers/designers work months >>>to get 1.3% speedup. Both chessprogrammers for their program >>>and hardware designers for their chips. >>> >>>13% is really a lot then if you understand that the speed >>>of DIEP isn't depending only upon memory speed, but even more >>>upon processor speed. >>> >>>So the actual speedup of DDR ram over SDRAM in latency is >>>more like 100% faster, which is actually true. >>> >>>DDR ram needs 10T versus RDRAM 15T. That's already 50%. >>> >>>RDRAM initially was clocked 100Mhz and >>>the DDR ram is clocked 133Mhz. >>> >>>Nowadays there is also RDRAM clocked to higher speeds than 100Mhz >>>(quad pumped of course), but still it is of course 50% slower in >>>timing than DDR ram. >>> >>>So where RDRAM might win it nowadays perhaps on bandwidth (tests >>>which try to pump actual terabytes of data through the ram suggest >>>that fastest DDR ram can pump through more than fastest RDRAM, >>>despite theoretical specifications of the RDRAM versus theoretical >>>specifications of DDR ram, but i don't want to get in the middle >>>of a battle there which is getting fought out non-stop; and the >>>truth is simply that you have to choose to believe either technical >>>specifications or the actual tested speeds by experts so it is >>>a forever 'yes' 'no' fight), there is not a single doubt on >>>what is the better latency. >>> >>>DDR ram has over 50% faster latency than RDRAM. This is very clear. >>>The bus of most of the tested old P4s was 100Mhz, versus K7 soon >>>already 133Mhz. So also that speed difference we must take into >>>account. >>> >>>If that total of 1.33 * 1.5 = 2.0 times faster latency is >>>then giving a 13% speedup of DIEP, then that is quite a lot IMHO. >>> >>>>On December 04, 2002 at 13:32:01, Vincent Diepeveen wrote: >>>> >>>>>On December 04, 2002 at 11:42:17, Matt Taylor wrote: >>>>> >>>>>>On December 04, 2002 at 10:43:59, Vincent Diepeveen wrote: >>>>>> >>>>>>>On December 04, 2002 at 10:21:08, James T. Walker wrote: >>>>>>> >>>>>>>>On December 04, 2002 at 08:00:35, martin fierz wrote: >>>>>>>> >>>>>>>>>hi, >>>>>>>>> >>>>>>>>>i'm on the lookout for a new PC for endgame database computations. i'll probably >>>>>>>>>be buying a lot of ram, 2-3GB. i see that there is a big price difference >>>>>>>>>between DDRAM and SDRAM. IIRC the main difference is that you get a larger >>>>>>>>>bandwidth, but about the same latency with DDR - so i suppose i'm better off >>>>>>>>>buying SDRAM for my application. any opinions of the experts? >>>>>>>>> >>>>>>>>>thanks in advance >>>>>>>>> martin >>>>>>>> >>>>>>>>For what it's worth: I purchased one stick (256M) of DDR ram to compare to my >>>>>>>>cheap SDRAM. I found no noticable difference in chess performance (just price). >>>>>>>> I did not do any extensive testing. I simply compared Fritz marks. I suspect >>>>>>>>that in the future most motherboards will not accept the SDRAM. >>>>>>>>Jim >>>>>>> >>>>>>>I see a big difference. 64 versus 32 bytes cache lines matters >>>>>>>a lot for DIEP and all software that doesn't fit within L1 cache. >>>>>>> >>>>>>>Best regards, >>>>>>>Vincent >>>>>> >>>>>>Cache line size is a part of the CPU, not the ram. There are a number of >>>>>>transitional products, both P4 and Athlon, that accept both SDRAM and DDR SDRAM. >>>>>>(However, I have never heard of anyone happy with these products.) >>>>> >>>>>the P4 ended up being a lot faster for DIEP when i tested a p4 with ddr ram >>>>>isntead of RDRAM. >>>>> >>>>>P4 with ddr ram (northwood) is like 1.5 : 1 for a K7 >>>>>used to be 1.7 : 1 to a k7 with rdram. >>>>> >>>>>So 1.7 Ghz P4 rdram == 1.0Ghz K7 for DIEP >>>>> 2.4 Ghz P4 ddr == 1.6Ghz K7 for DIEP (both ddr). >>>>> >>>>>DDR is a big step forward!! >>>>> >>>>>i don't know where the processor gets 64 bytes instead of 32 bytes in >>>>>the design. I just know it gets 64 bytes, versus SDRAM 32. >>>>> >>>>>Best regards, >>>>>Vincent >>>> >>>>By your figures, DDR SDRAM speed compared to RDRAM speed on a P4 platform is >>>>1.7/1.5 = 113%. I wouldn't call 13% a "big step forward." >>>> >>>>This also makes the assumption that both the 1 GHz K7 and 1.6 GHz K7 run equally >>>>fast. The 1 GHz K7 is the Thunderbird chip. The 1.6 GHz K7 is the AthlonXP 1900. >>>>Thunderbirds report that they are model 4, whereas AthlonXP 1900 may report >>>>model 6 (palomino) or 8 (thoroughbred). Model 4 and Model 6 are not the same >>>>thing, and they differ in MORE than just instructions. One change that I have >>>>observed is that the model 6 L2 cache is slightly faster. Other timings have >>>>probably changed, too. >>>> >>>>I will also mention that a 2.4 GHz P4 is the P4 Northwood. The 1.7 GHz P4 may be >>>>a Northwood, but I suspect (based on the numbers) that it was probably the older >>>>Williamette. The major difference is that the P4 Williamette had a smaller L2 >>>>cache (256KB instead of 512KB). >>>> >>>>I will have to agree with Jeremiah, here. If DDR SDRAM is faster, DIEP is >>>>latency-dependant. If RDRAM is faster, it would be bandwidth-dependant. I have >>>>measured pc800 RDRAM bandwidth on one of my systems, and it exceeds theoretical >>>>bandwidth on any standard part DDR SDRAM. (I am not completely sure, but I don't >>>>think pc2700 is part of the JDEC specification.) >>>> >>>>I am not sure what you're saying about 64-bytes vs. 32-bytes, but I assure you >>>>that SDRAM-based, DDR-based, and RDRAM-based P4s all have the cache line size. >>>>The information is available from the cpuid instruction. The vector is >>>>documented in both Intel and AMD literature, but off-hand I don't know which >>>>vector it is. There are many utilities, especially for Windows, that will give >>>>this information. I -believe- wcpuid is one such utility, but I usually end up >>>>writing a program every time I get curious about cpuid information. >>>> >>>>If you would like, I will write such a program and post it. >>>> >>>>-Matt
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