Author: Robert Hyatt
Date: 09:20:14 11/01/02
Go up one level in this thread
On November 01, 2002 at 11:56:56, Vincent Diepeveen wrote: >On November 01, 2002 at 10:41:25, Robert Hyatt wrote: > >>On October 31, 2002 at 10:53:07, Vincent Diepeveen wrote: >> >>>On October 30, 2002 at 06:59:21, Terje Vagle wrote: >>> >>>>Hi all, >>>> >>>>The new cpu from intel will have a new function called >>>>hyper-threading. >>>> >>>>This will make the operating system able to recognize the cpu as if it was >>>>2 cpu's. >>>> >>>>Could the programs with smp-support make use of this? >>>> >>>>Regards, >>>> >>>>Terje Vagle >>> >>>No chessprograms cannot make use of that feature at all. It is sad but >>>the truth. Hyperthreading is a cool thing for the future but the P4 >>>processor is a too small processor to allow hyperthreading from getting >>>to work. >>> >>>Apart from that a major problem is that even if we have a great processor >>>which really allows hyperthreading to be effective, that the threads >>>run at unequal speeds. >>> >>>Hyper threading is supposed to work for 2 threads where 1 is a fast >>>thread and the other is some kind of background thread eating little cpu >>>time. >>> >>>In chessprograms having a second search thread which just runs now and >>>then in the background is simply impossible to use. >> >> >>It is not impossible at all. The only problem was spinlocks and Eugene >>posted a link to an Intel document that describes how to solve this problem. >> >>Given that solution, hyper-threading will work just fine since spinlocks >>won't confuse the processor... >> >>It won't be 2x faster, but it will certainly be faster if you can run a second >>thread while the first is blocked on a memory access... > >No it won't be 2 times faster. suppose you start crafty with 2 threads. I didn't say it would be _two_ times faster. I said it would be _faster_. And it will. > >thread A starts search and has 1.e4,e5 >thread B starts and continues with 1.d4 > >now when A is ready, B will still be busy with its own search space, >and delay thread A time and again. > >that'll slow down incredible. > Except that isn't how it works. The threads co-execute in an intermingled way as one blocks for a memory read the other fills in the gap. It is something like having 1.5 cpus... and it does work. >You'll be a lot slower than searching with a single thread! > Not very likely... >Also note that there is just 8 KB data cache and just like >40 registers to rename variables. then another 12KB tracecache. > >*both* threads are eating from that 8 KB and 12KB tracecache, >that is an additional problem they 'overlook'. > That is a problem on an SMP machine. But _both_ threads are executing the _same_ code anyway... so that isn't a problem. At least for me. For you it is different because you are not using "shared everything" in lightweight threads, so your results might be different. But all my threads share the exact same executable instruction code... >As you can see from graphs. Usually SMT brings zero speedup. I have seen numbers around 1.3 up to 1.5... which is not to be ignored. > >Try crafty on a 2.4Ghz single cpu P4 or P4-Xeon please (northwood) or >above. Not on a slower P4 or P4-Xeon. Of course we go for the latest >hardware... Why does it matter? Hyper-Threading is Hyper-Threading, unless you are going to start that memory speed nonsense. And, in fact, the faster the processor vs memory speed, the better hyperthreading should perform. Just like the greater the difference in processor speed vs disk speed, the better normal operating systems do at running multiple processes. > >Just try it like i tried at Jan Louwman's 2.4Ghz P4s and 2.53Ghz P4s. That says it all. "Like I tried it". As if that is a comprehensive and exhaustive testing? > >I can't measure *any* speedup *anyhow*. > Why am I not surprised??? >Also theoreticlaly i see major problems for the P4 chip even if you >have software which could theoretically profit. "theoretically". :) :) :) Theory from someone that doesn't know theory. :) :)
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