Why my programs is taking longer to execute with more threads allocated?

I wrote two programs that computes area of a function with a certain number of rectangles using Riemann sums, one is written in Go and the other with C++.

The objective was to measure execution time and see which language is the faster on multithreading.

I ran the program on a 32-cores server (Dual Intel Xeon) using a bash script4 to run it with 1, 2, 4, 8, 16 and 32 threads. The script use time --format %U to get the execution time .

But as you can see in the results running the Go version using 1 core is 1.19 seconds and with 32 cores it's 1.69 seconds ! I thought using more cores would have made computations faster...

Did I made an error writing my programs ? Is the measure of time accurate ? Or maybe the results are good but how ?

Thank you in advance for your answers !

Sources :

Go code : https://github.com/Mistermatt007/Benchmark-go-vs-cpp/blob/master/CalculGo/Calcul.go

C++ code : https://github.com/Mistermatt007/Benchmark-go-vs-cpp/blob/master/CalculCpp/Calcul.cpp

launch script : https://github.com/Mistermatt007/Benchmark-go-vs-cpp/blob/master/script/Launch.sh

Riemann sums : http://mathworld.wolfram.com/RiemannSum.html

first results : https://github.com/Mistermatt007/Benchmark-go-vs-cpp/blob/master/results/GoVSCpp.txt

According to man time:

> U Total number of CPU-seconds that the process used directly (in user mode), in seconds.

You are measuring CPU-seconds, i.e. time spent by each CPU cumulatively, not "wallclock" seconds. This measure won't go down with additional threads, because it is proportional to amount of work which is constant. On the other hand, this may go up with number of threads, as every new thread incurs some additional bookkeeping.

If you want to list "real" time, use %e specifier.