Syscall overhead

Question

How big is (approximately) an I/O syscall overhead on Linux from C program, I mean how bad is running e.g. many small read / write operations compa

Answer 1

You can measure this yourself. Just open /dev/zero and do some reading and writing while measuring the time. Also vary the number of bytes you put into each call - e.g. 1 bytes, 2 bytes, 128 bytes, .. 4096bytes. Also take care to use the read(2) and write(2) syscalls and not anything using internal buffers.

Answer 2

Syscalls take at least 1-2 microseconds on most modern machines just for the syscall overhead, and much more time if they're doing anything complex that could block or sleep. Expect at least 20 microseconds and up to the order of milliseconds for IO. Compare this with a tiny function call or macro that reads a byte from a userspace buffer, which is likely to complete in a matter of nanoseconds (maybe 200 ns on a bad day).