I would begin by looking at the generated assembler code. You would use "objdump -d" for C, and the "code_native" function for Julia. Can you post the results?
The discussion below is beside your points, but nevertheless: Your sqrt implementation is very slow for two reasons: (1) You are using a while loop to check for termination. If you use exponent manipulation, then your starting guess can be within sqrt(2) of the result, and then you can determine the necessary number of iterations ahead of time. You probably need 5 or 6 iterations for the accuracy you chose. (2) You are using a division in your algorithm, where divisions are expensive. If you calculated 1/sqrt(x) instead of sqrt(x), then you don't need a division. Later you can calculate sqrt(x) = x * (1/sqrt(x)). A good sqrt routine should take less than 10^-8 seconds. -erik > On Jan 27, 2015, at 17:39 , Miles Lubin <[email protected]> wrote: > > I'm working on a microbenchmark and found a surprising result (maybe not?) > that Julia is about 2x faster than the same algorithm hand-coded in C. I > wanted to check if I'm doing anything obviously wrong here before reporting > these results. The timings reproduce across different systems and compiler > options (clang/gcc -O2/-O3). > > The test is just to compute square root using newton's method. The relevant > code is in this gist: https://gist.github.com/mlubin/4994c65c7a2fa90a3c7e. > > On Julia 0.3.5, each function call takes 8.85*10^-8 seconds. The best timing > I've seen from C is 1.61*10^-7 using gcc -O2 -march=native. > > I did my best to check for common mistakes: > - Julia and C use the exact same timing routine with 10,000 repetitions > - Both give the correct answer, and the important code isn't being optimized > away. > > Any ideas as to why Julia is faster on this very simple code? I know that > performance comparisons with runtimes on the order of nanoseconds are > probably not too meaningful, but people still like absolute numbers, and it's > a bit surprising that I can't match the performance of Julia from C. > > Thanks, > Miles -- Erik Schnetter <[email protected]> http://www.perimeterinstitute.ca/personal/eschnetter/ My email is as private as my paper mail. I therefore support encrypting and signing email messages. Get my PGP key from https://sks-keyservers.net.
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