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Re: Compilation: Avoiding inlining

  • To: mathgroup at smc.vnet.net
  • Subject: [mg121873] Re: Compilation: Avoiding inlining
  • From: "Oleksandr Rasputinov" <oleksandr_rasputinov at hmamail.com>
  • Date: Wed, 5 Oct 2011 04:03:41 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201109250234.WAA26203@smc.vnet.net> <j5s88c$pf1$1@smc.vnet.net>

On Tue, 04 Oct 2011 07:45:30 +0100, DmitryG <einschlag at gmail.com> wrote:

> On Sep 27, 6:24 am, Oliver Ruebenkoenig <ruebe... at wolfram.com> wrote:
>> On Sat, 24 Sep 2011,DmitryGwrote:
>>
>> > A potentially very important question: I have noticed that the program
>> > we are discussing, when compiled in C, runs on both cores of my
>>
>> Only when compiled to C? You could try to set Parallelization->False
>> and/or it might be that MKL runs some stuff in parallel.
>>
>> Try
>>
>> SetSystemOptions["MKLThreads" -> 1] and see if that helps.
>>
>> > processor. No parallelization options have been set, so what is it?
>> > Automatic parallelization by the C compiler (I have Microsoft visual
>> > under Windows 7) ?  Do you have this effect on your computer?
>>
>> I can not test that since I use Linux/gcc.
>>
>> > However, the programs of a different type, such as my research
>> > program, still run on one core of the processor. I don't see what
>> > makes the compiled program run in different ways, because they are
>> > written similarly.
>>
>> I understand that you'd want to compare the generated code with the
>> handwritten code on the same number of threads but I can not resist to
>> point out that the parallelization of the C++ code is something that  
>> need=
> s
>> to be developed but that parallelization via Mathematica come at almost
>> not additional cost.
>>
>> On a completely different note, here is another approach that could be
>> taken.
>>
>> CCodeGenerator/tutorial/CodeGeneration
>>
>> Oliver
>
> This behavior is not new to me. Calculating matrix exponentials also
> leads to a 100% processor usage on multiprocessor computers without
> any parallelization. I have observed it on my Windows 7 laptop and on
> a Mac Pro at work. The system monitor shows that only one Mathematica
> kernel is working but the load of this kernel is much greater than
> 100%, especially on the Mac Pro that has 8 cores. My laptop may switch
> off (because of overheating?) during such calculations while the Mac
> is OK.
>
> Also I've seen such a behavior solving PDEs with NDSolve in some
> cases.
>
> I wonder what is happening and I do not know whether this effect is
> good or bad. As I cannot control it, I cannot measure if such an
> extensive processor usage leads to a speed-up.
>
> I am going to get Mathematica for Linux and test it there, too.
>
> Best,
>
> Dmitry
>

In the case of the matrix exponentials (or really any numerical linear  
algebra), this behaviour is undoubtedly due to MKL threading and can be  
controlled by the option Oliver gives above. Obviously it is not good if  
your laptop switches off due to overheating, but this is not so much a  
problem of Mathematica as badly designed cooling in the laptop. MKL's  
threading is carefully done and scales well for moderate numbers of cores,  
so you should be seeing considerably increased performance as a result of  
it on an 8-core machine. In regard to NDSolve, I don't know how this is  
implemented internally and so can't comment on any parallelization that  
might exist.



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