Re: Any answer for this ParallelDo error

*To*: mathgroup at smc.vnet.net*Subject*: [mg108884] Re: Any answer for this ParallelDo error*From*: Zach Bjornson <bjornson at mit.edu>*Date*: Tue, 6 Apr 2010 07:22:52 -0400 (EDT)

Hello Pratip, You can use ParallelTable instead of ParallelDo. Even in non-parallel mode, Table is faster than Do (here it worked out to about 1.4x faster). a = ParallelTable[If[PrimeQ[(i^3 + j^5)] == True, 1, 0], {i, 1, 20}, {j, 1, 50}] However, when I tested this with your tiny range of i and js, there was no speedup. Change to {i, 200} and {j, 500} gave a 2.42x speedup in parallel. -Zach On 4/5/2010 8:01 AM, pratip wrote: > Hallo Group, > > Here is a harmless piece of code. > > Clear[a]; > t=AbsoluteTime[]; > a=ParallelTable[0,{i,1,20},{j,1,50}]; > DistributeDefinitions[a]; > mat=ParallelDo[a[[i,j]]=If[PrimeQ[(i^3+j^5)]==True,1,0],{i,1,20},{j, > 1,50}]; > AbsoluteTime[]-t > > The error is something like > > Set::noval: Symbol a in part assignment does not have an immediate value. > Set::noval: Symbol a in part assignment does not have an immediate value. > > I know there is a way of using SetSharedVariable but that version of > the code is very slow. > > Clear[a]; > t=AbsoluteTime[]; > a=ParallelTable[0,{i,1,20},{j,1,50}]; > SetSharedVariable[a]; > mat=ParallelDo[a[[i,j]]=If[PrimeQ[(i^3+j^5)]==True,1,0],{i,1,20},{j, > 1,50}]; > Print[a//ArrayPlot]; > AbsoluteTime[]-t > > Time taken: 6.1443514 > > The single processor version is much faster > > Clear[a]; > t=AbsoluteTime[]; > a=Table[0,{i,1,20},{j,1,50}]; > mat=Do[a[[i,j]]=If[PrimeQ[(i^3+j^5)]==True,1,0],{i,1,20},{j,1,50}]; > Print[a//ArrayPlot]; > AbsoluteTime[]-t > > Time taken: 0.0360021 > > My question is why I cant distribute the definition of a array to my > processor kernels. > Hope someone can give me an answer. I need to make this Do loop > parallel. > > Yours, > > Pratip > >