Author: Vincent Diepeveen
Date: 10:06:53 11/01/02
Go up one level in this thread
On November 01, 2002 at 12:20:14, Robert Hyatt wrote:
Feel free to ship a version of crafty that doesn't do spinlock
or whatever you want to modify. I'll extensively test it for you
at all P4s i can get my hands on...
I would be really amazed if you get even 0.1% faster in nodes a
second...
...of course it must be a fair compare in contradiction to what
intel shows. They do next comparision
a) some feature called 'SMT' in the bios turned on
- just running 2 threads then
b) turning it off
- also running 2 threads at it
Like everyone who is not so naive we know that you also need
to do next test:
a) some feature called 'SMT' in the bios turned on
- just running 1 thread eating all system time
b) turning it off
- also running 1 thread eating all system time
There shouldn't be a speed difference between a and b of course.
That verification step is missing.
>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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