matlab - Performance of index creation -

in trying choose indexing method recommend, tried measeure performance. however, measurements confused me lot. ran multiple times in different orders, measurements remain consistent. here how measured performance:

for n = [10000 15000 100000 150000]     x =  round(rand(n,1)*5)-2;     idx1 = x~=0;     idx2 = abs(x)>0;      tic     t = 1:5000         idx1 = x~=0;     end     toc      tic     t = 1:5000         idx2 = abs(x)>0;     end     toc end 

and result:

elapsed time 0.203504 seconds. elapsed time 0.230439 seconds.  elapsed time 0.319840 seconds. elapsed time 0.352562 seconds.  elapsed time 2.118108 seconds. % strange part elapsed time 0.434818 seconds.  elapsed time 0.508882 seconds. elapsed time 0.550144 seconds. 

i checked , values around 100000 happens, @ 50000 strange measurements occur.

so question is: else experience range, , causes this? (is bug?)

this may due automatic optimization matlab uses basic linear algebra subroutine.

just yours, configuration (osx 10.8.4, r2012a default settings) takes longer compute idx1 = x~=0 x (10e5 elements) x (11e5 elements). see left panel of figure processing time (y-axis) measured different vector size (x-axis). see lower proceesing time n>103000. in panel, displayed number of cores active during calculation. see there no drop one-core configuration. means matlab not optimize execution of ~= when 1 core active (no parallelization possible). matlab enables optimization routines when 2 conditions met: multiple cores , vector of sufficient size.

the right panel displays results when feature('accel','on'/off') set off (doc). here, 1 core active (1-core , 4-core identical) , therefore no optimization possible.

finally, function used activating/deactivating cores maxnumcompthreads. according loren shure, maxnumcompthreads controls both jit , blas. since feature('jit','on'/'off') did not play role in performance, blas last option remaining.

i leave final sentence loren: "the main message here should not need use function [maxnumcompthreads] @ all! why? because we'd make matlab best job possible you."

accel = {'on';'off'}; figure('color','w'); n = 100000:1000:105000;  ind_accel = 2:-1:1     eval(['feature(''accel'',''' accel{ind_accel} ''')']);     telapsed = zeros(4,length(n));     ind_core = 1:4         maxnumcompthreads(ind_core);         n_core = maxnumcompthreads;         ii = 1:length(n)             fprintf('core asked: %d(true:%d) - n:%d\n',ind_core,n_core, ii);             x =  round(rand(n(ii),1)*5)-2;             idx1 = x~=0;             tstart = tic;             t = 1:5000                 idx1 = x~=0;             end             telapsed(ind_core,ii) = toc(tstart);         end     end     h2 = subplot(1,2,ind_accel);     plot(n, telapsed,'-o','markersize',10);     legend({('1':'4')'});     xlabel('vector size','fontsize',14);     ylabel('processing time','fontsize',14);     set(gca,'fontsize',14,'ylim',[0.2 0.7]);     title(['accel ' accel{ind_accel}]); end