[manwe150]

ucontext modifies the signal mask, requiring a syscall. That’s very expensive (as shown by the boost benchmark above), and usually unnecessary. “emulate” is a rather negative sounding way to describe running effective the same code as ucontext does (which also happens to typically be the same as sigsetjmp) - it’s not like ucontext has some privledged permissions. It’s “just” a context switch.

[bgongfu]

I was under the impression that Boost Context is more than that; at least that's what the amount of assembler code tells me; but I'll be the first to admit I don't have much patience for deciphering modern C++. I get that it's possible to invoke the same functionality as ucontext minus signal masks manually, but I'm not convinced it would save enough cycles to pay for the added complexity.

[gpderetta]

Saving the signal mask easily cost hundreds to thousand cycles. One or two order of magnitude more than the rest of contex switching.

[bgongfu]

For the record: I just benchmarked GnuPth with NULL sigmask against ucontext for the example in the post and it's slightly (3.4/3.2s) slower.