B6. I’ve run a calculation on my PC and the reported time per block oscillates wildly by factors of 2–10. OR I’ve run a calculation on my PC and the reported CPU time is 2–10 times longer than I expected. Why is this?

Your Linux computer is dynamically changing the processor frequency to save power. It should set the frequency to the maximum when you run CASINO, but by default it ignores processes with a nice value greater than zero (CASINO’s default is +15). To fix this, supply the ‘--user.nice=0‘ option to runqmc.

You will see wild fluctuations in the block timings only if some other process triggers a change in the frequency, otherwise you will only notice slowness.

The above problem should not appear in modern Linux distributions (from 2008 onwards).

Other than this, block times are bound to oscillate, since during the course of the simulation particles are moved in/out of (orbital/Jastrow/ backflow) cutoff regions, which increases/reduces the expense of calculating things for particular configurations. However, provided the blocks contain a sufficient number of moves, the block times should be equally long on average.

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Category: B: Using CASINO

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