Why is my Cgo code slower than expected Go code?

Why is cgo so slow?

Your testing code compares the execution time of C and Go functions run 100 million times each. The C function takes longer than the Golang function, and you are concerned that something is wrong with your testing code.

The high overhead of calling C/C code via CGo makes CGo calls best minimized. In the provided example, it might be more efficient to create a C loop instead of calling a CGo function repeatedly in a loop.

The Go runtime's setup for its threads can break the expectations of C code in several ways:

1. Relatively small goroutine stacks
2. Improper interaction with libpthread's thread local storage implementation
3. Interference with the Go runtime's UNIX signal handler
4. Potential detriment to other goroutines if C code monopolizes a thread

Therefore, CGo takes the safe approach of running C code in a separate thread with a traditional stack.

Unlike languages like Python where rewriting code hotspots in C was common for speeding up programs, Go's performance gap between C and Go code is much smaller. Consider reserving CGo for interfacing with existing libraries, possibly with small C wrapper functions to reduce the number of calls required from Go.

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