Boris Herman wrote:
I have compiled a single-threaded (and quite non MP aware) pascal program that does a lot of numerical computations. One calculation takes about 5 days on my 1.25 GHz DDR G4 (single cpu) and I need about 200 computations to do. Well, when I run the program on my G4 it takes up all CPU time (cca 93%-95%). I thought of speeding the whole thing by running it on a dual processor machine so I got someone to run it on a dual 2 GHz G5. So here comes the problem. If only one instance is run on a dual G5 the Activity monitor shows 50% CPU load (which is understandable) but the graph alternates between 100% load on 1st CPU and 100% load on 2nd CPU. I thought that is an anomaly of Activity Monitor.
I suppose that's normal. Either the system balances the load intentionally, or it just happens that it gets scheduled on both CPUs now and then.
However, when two instances are run, the time required to execute each task doubles even though the Activity monitor shows both cpus at 100% load.
My program doesn't really do anything much - it performs gazillions of computations in arrays that all together consume less than 20 Mb of memory.
Am I doing anything wrong or shouldn't I expect better performance on a dual cpu unit?
I don't really have much MP experience, but ISTM the memory speed is the bottleneck (as is often the case on single processor machines already). Are the programs very memory intensive? (I suppose so, i.e., the working set is generally larger than the processor cache.) Then I think the size of allocated memory doesn't matter much, if both processes have to wait for access to the memory bus. Localizing the memory accesses may help quite a bit -- if possible. Or run them on two separate machines with their own memory. ;-)
(I assume they don't use shared memory. If they do, this might be a factor as well, of course.)
Frank