Author: Vincent Diepeveen
Date: 08:56:56 11/01/02
Go up one level in this thread
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. 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. You'll be a lot slower than searching with a single thread! 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'. As you can see from graphs. Usually SMT brings zero speedup. 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... Just try it like i tried at Jan Louwman's 2.4Ghz P4s and 2.53Ghz P4s. I can't measure *any* speedup *anyhow*. Also theoreticlaly i see major problems for the P4 chip even if you have software which could theoretically profit.
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