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Java Forum / General / May 2006strange performance behavior of a mathematical method: why?
I've found a difference of about 10% in the execution of 2 version of this method: public double[] evaluate(double input[]){ double a; //System.arraycopy(input,0,activation[0],0,input.length); activation[0]=input; //for (int i=0;i<input.length;i++) // activation[0][i]=input[i]; for (int i=1;i<layers;i++){ double activation_col[]=activation[i-1]; double activation_col_res[]=activation[i]; double weight_matr[][]=weight[i-1]; for (int j =0; j< activation_col_res.length;j++ ){ double weight_col[]=weight_matr[j]; double acc=0; for (int k=0; k<activation_col.length;k++){ *************variant go here************* } activation_col_res[j]=g(acc); } } setChanged(); notifyObservers(); return activation[layers-1]; } variant 1: a= activation_col[k]; a*=weight_col[k]; acc+=a; variant 2: acc+= activation_col[k]*weight_col[k]; variant1 is 10% faster than variant2. I've a matrix of about 1200x400 elements (weight matrix) variant1 avarage is 4.82ms variant2 avarage is 5.24ms does this performance difference makes any sense? does someone has any tips to write always the faster code? thanks Dimitri > I've found a difference of about 10% in the execution of 2 version of > this method: [quoted text clipped - 38 lines] > > does this performance difference makes any sense? If you are curious look at the generated Java bytecodes (use javap). But the difference isn't important. Another compiler, jit or processor architecture could give you different results. > does someone has any tips to write always the faster code? - Measure before optimizing - Focus on algorithmic improvements  SignatureDiomidis Spinellis Code Quality: The Open Source Perspective (Addison-Wesley 2006) http://www.spinellis.gr/codequality?cljp what do you tjink of this method implementation? any tip? must i pass to c++ & asm to get better performance? note that the g method called inside the loop is final and the compiler does inline it, or it looks to be so from the profiling data. > what do you tjink of this method implementation? > any tip? > must i pass to c++ & asm to get better performance? > note that the g method called inside the loop is final and the compiler > does inline it, or it looks to be so from the profiling data. From what I can see this particular code manipulates floating point numbers. This is one of the (not common) cases where a good Java compiler and JIT-based runtime system should be able to give you the same performance as the one you would achieve with (say) C. You are unlikely to gain anything from moving to assembly, unless you can utilize instructions that compilers don't typically support. For example, I see that your code calculates some type of dot product: if your data allows it, you could gain by using Intel's SSE SIMD extensions or AMD's 3DNow. Another possibility would be to move your code to be executed by a 3D graphic card's hardware. SignatureDiomidis Spinellis Code Quality: The Open Source Perspective (Addison-Wesley 2006) http://www.spinellis.gr/codequality?cljp thanks diobidis, do you have any link where i can found such an approach? thanks > thanks diobidis, > do you have any link where i can found such an approach? http://www.ics.forth.gr/eHealth/publications/papers/2005/PCI2005.pdf SignatureDiomidis Spinellis Code Quality: The Open Source Perspective (Addison-Wesley 2006) http://www.spinellis.gr/codequality?cljp this paper is very interesting, like other resources on gpgpu.org, but do you, or any one else, know of an interface for java like brook for c++ to use the gpu as a vector coprocessor? Thanks Dimitri On 23 Apr 2006 13:17:08 -0700, "Dimitri Ognibene" <dimitri.ognibene@gmail.com> wrote, quoted or indirectly quoted someone who said : >variant 1: > a= activation_col[k]; [quoted text clipped - 4 lines] > >variant1 is 10% faster than variant2. that is a puzzle. First have a look at the byte codes generated. see http://mindprod.com/jgloss/disassembler.html SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. On 23 Apr 2006 13:17:08 -0700, "Dimitri Ognibene" <dimitri.ognibene@gmail.com> wrote, quoted or indirectly quoted someone who said : >does this performance difference makes any sense? > >does someone has any tips to write always the faster code? You try variants and measure. Use an AOT compiler or java -server which optimises harder, but takes longer to come up to speed. see http://mindprod.com/jgloss/aot.html http://mindprod.com/jgloss/javaexe.html SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > I've found a difference of about 10% in the execution of 2 version of > this method: [...] > variant 1: > a= activation_col[k]; [quoted text clipped - 4 lines] > > variant1 is 10% faster than variant2. It's very difficult to estimate exactly what will have marginal effects at this level of optimisation. The JIT is pretty clever, the CPU does lots of optimisations of its own, cache effects and alignment effects combine to confound estimation too. I have no idea what the explanation might be here; it could be an effect of the (JITed form of the) first expression being able to make better use of multiple arithmetic units within the CPU, but that's nothing more than a guess. You don't mention whether you are using the -client or -server JVM, so I'm guessing that you aren't aware of how that choice affects the kind of optimisations that the JIT will do. Rule of thumb: -server is a good deal better at optimising arithmetic code. FWIW, on the only micro-benchmark I've run recently (and one micro-benchmark means almost nothing on its own), the server JVM was doing essentially the same optimisations as the best C++ compiler I have access to (the one in MS VS.2003). Similarly, you don't mention what CPU you are running on, so I'm guessing that you are not aware of how details of chip architecture affect how fast a specific expression of an algorithm can run. Agner Fog has a fascinating (but long) guide to optimisation for Pentium family processors on this page: http://www.agner.org/assem/ which also has some links. If nothing else, it should put you off the idea of re-implementing in assembly ;-) -- chris thanks very much Chris, however i don't want to rewrite it in assembly but build an jni interface to Math kernel libraries... but i'll look at the links that you and diomidis suggest, and after some trial i'll decide thanks agani dimitri ok you were right with the -server option the two version are about 2 time faster and have the same speed 2.70 ms avarage thanks > variant 1: > a= activation_col[k]; [quoted text clipped - 9 lines] > > does this performance difference makes any sense? How exactly did you measure performance? Did you give the JVM some warm up runs before doing the real measurement? Cheers robert yes i do, i've tried to wait a little and this are the "steady" value. however i was not using the -server option.. i'll post the result later thanks dimtiri > yes i do, > i've tried to wait a little and this are the "steady" value. What exactly do you mean by "wait a little"? In order for the JVM's optimization to kick in you have to actually execute the code several times before you start measuring. Also you should execute every variant several times to get better measurement accuracy. robert yes, i was in steady state... When I was working on some code optimization I read somewhere that copying global variables to local variables does increase performance. Don't ask me how/why, but its a behind the scenes thing. I will try to find the article and post link. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. 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