Author: Robert Hyatt
Date: 12:30:25 11/21/01
Go up one level in this thread
On November 21, 2001 at 14:04:57, Slater Wold wrote: >On November 21, 2001 at 13:33:14, Robert Hyatt wrote: > >>On November 21, 2001 at 11:52:09, Slater Wold wrote: >> >>>On November 21, 2001 at 11:19:08, Robert Hyatt wrote: >>> >>>>On November 21, 2001 at 11:05:00, Slater Wold wrote: >>>> >>>>> >>>>>As I have found positions where the NPS search is 2.5x faster, but it solves the >>>>>solution in 4x faster than a single cpu. >>>>> >>>>>Dann and I had this "super" linear discussion before. >>>>> >>>>>Seems like it would even out, eventually. But like I said, I believe you. And >>>>>I'll do it to solution now. (But of course, I'll still look at the NPS!) :) >>>>> >>>> >>>> >>>>First, two cpus is going to have a _hard_ time searching 2.5x the raw >>>>nodes per second. I have no idea how that might happen, unless there is a >>>>bug in the node-counting that sometimes counts nodes twice. >>> >>>Sorry. I've seen positions where it will search 1.8x the NPS and solve it 4x >>>faster. >> >>That is not uncommon at all... >> >> >> >>> >>>I was just reversing your comment. >>> >>>>Second, "super-linear" can happen on occasional positions. But as you said, >>>>it will average out over multiple positions so that the speedup simply can not >>>>be >2.0 for two processors on average. I was in the middle of the super-linear >>>>speedup discussion. I hope it stays "at rest" now. :) >>> >>>I've seen it happen a time or two. I've found a solution one go, and can never >>>get it again. (Happened more than once with DJ7.) >>> >>>>I have seen several cases of spectacular speedups, but then I have also seen >>>>an equal number of horrible speedups. Bruce once sent me one that produced >>>>a particularly ugly result on Crafty, But I can't seem to locate the thing >>>>at present... >>> >>>I think I've only seen 1 or 2. But I have seen a LOT that are greater than the >>>speedup of the NPS speedup. In other words, it takes the SMP less nodes to find >>>the solutions, that the 1 CPU. >>> >> >>That is the classic super-linear speedup situation. bad move ordering >>corrected by the parallel search taking 'em two at a time. >> >> >> >>>>In any case, NPS is kind of like engine RPM. It should increase linearly with >>>>the number of processors, assuming the parallel algorithm is good at keeping >>>>both cpus busy all the time and doesn't have one (or more) sitting around >>>>waiting excessively. But RPM has nothing to do with vehicle speed, because >>>>of losses along the drive train. The MPH value (time to solution) is the thing >>>>that wins races (or games). >>> >>>HUM. Now you're picking a subject I am _very_ familiar with. RPM's and and MPH >>>aren't _DIRECTLY_ related. >>> >>>IE: >>> >>>If you have a car that has a 4.11 gear ratio with 351c.i. motor, that is getting >>>400 HP at 5750RPM, it SHOULD go 12's in the 1/4 mile. (Depending on weight.) >>>Let's just say, it goes 12.5 @ 119MPH. >>> >>>Now, let's say you install an aluminum driveshaft. Your RPM's are going to >>>increase. Same setup, same everything, it will probably get 400HP at 5900 RPM. >>>And if you run the 1/4 mile again, I would guarantee 12.3's at the SAME MPH. >>>Maybe, you'll get 1 or 2 more MPH. You're going .2 seconds faster, but the MPH >>>isn't changing. RPM doesn't = MPH, but the faster you can rev, the faster you >>>can get to that top MPH. Unless your gears are wacky. >> >>You do better than I do there. I've never seen an alum drive shaft increase >>RPM. I have seen it decrease the time to reach a particular RPM, but I don't >>see why it would increase the actual RPM since it doesn't take much energy to >>keep a driveshaft rotating, it just takes energy to accelerate it down the >>track, and energy to accelerate the RPM level of the shaft itself. > >On a fox body Mustang, an aluminum driveshaft over the stock driveshaft will >improve the cars 1/4 mile times by around .2 seconds. I've seen it done more >than a dozen times. I never meant to imply that the RPM's would go higher, but >that the time to get to X RPM would get shorter. And I know a lighter >driveshaft will put the powerband of the car slightly higher. I also know it >will not help your cars top speed. Only off the line accleration. > >>Your above example can't work however, as you didn't change the rear end ratio, >>and if you reach a higher rpm you will _definitely_ reach a higher top speed as >>well. RPM and MPH are exactly proportional if everything else (tire diameter >>and final drive ratio) remains constant. > >You're correct. If you're in 4th gear at 3k RPM, you will always be going X >MPH. It's only if you change gears that will change. However, the off the line >accleration is what's going to get you those better times. You're getting to X >RPM faster, and therefore X MPH faster, which means you're going to gain SOME >ground. That's actually what we are racing, in fact. :) 1990 coupe. Goes like the blazes...
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