Syscall Overhead

System calls overhead

For example, if we consider getpid(), when a system call is made to getpid() my guess is that if the control is currently in the child process then a context switching has to be made to enter the parent process to get the pid.

No context switch to the child process should be necessary here — the kernel should have all of the necessary data available to itself. In most cases, the kernel will only switch contexts to a userspace process in the scheduler, or when returning from a system call.

Also when getpid() is called, there will be some metadata transfer across the user-space boundary and enters and exits the kernel. So will the constant switching between user space and kernel also cause some overhead?

Yes, if getpid() was being called often, the overhead would certainly build up. There are some approaches available which can avoid this overhead for simple "getter" system calls like getpid() and gettimeofday(); one such approach which was at one point used under Linux was to store the (known) result of the system call in a special memory page. (This mechanism was known as vsyscall.)

Syscall overhead

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).

Unix system call overhead

It really depends on the system call and on the hardware.

The overhead of making a syscall is not very big! this gives less than a microsecond (i.e. a hundred processor clock cycles). Some syscalls are using vdso(7) to reduce that overhead.

However, for a given syscall, the kernel is doing some work. Depending on the actual involved syscall, this may take a lot of time.

For setsockopt(2) it should depend on which options you are setting.

In general, Linux socket implementation is quite good, and most of the time is spent on the actual network (i.e. Ethernet, etc....).

Overhead of times() system call - relative to file operations

On Linux, you could write a little program that does lots of calls to times() and fread() and measure the syscall times with strace -c

e.g

for (i = 0; i < RUNS; i++) {
        times(&t_buf);
        fread(buf,1,BUF,fh);
}

This is when BUF 4096 (fread will actually call read() every time)

# strace -c ./time_times
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 59.77    0.001988           0    100000           read
 40.23    0.001338           0     99999           times

and this is when BUF 16

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 99.00    0.001387           0     99999           times
  1.00    0.000014           0       392           read

Fastest Linux system call

One that doesn't exist, and therefore returns -ENOSYS quickly.

From arch/x86/entry/entry_64.S:

#if __SYSCALL_MASK == ~0
    cmpq    $__NR_syscall_max, %rax
#else
    andl    $__SYSCALL_MASK, %eax
    cmpl    $__NR_syscall_max, %eax
#endif
    ja  1f              /* return -ENOSYS (already in pt_regs->ax) */
    movq    %r10, %rcx

    /*
     * This call instruction is handled specially in stub_ptregs_64.
     * It might end up jumping to the slow path.  If it jumps, RAX
     * and all argument registers are clobbered.
     */
#ifdef CONFIG_RETPOLINE
    movq    sys_call_table(, %rax, 8), %rax
    call    __x86_indirect_thunk_rax
#else
    call    *sys_call_table(, %rax, 8)
#endif
.Lentry_SYSCALL_64_after_fastpath_call:

    movq    %rax, RAX(%rsp)
1:

How can I reduce system call overhead when sending a large number of UDP packets? (both Windows & Linux)

On Windows you can use the new Windows Registered I/O API (RIO) on Server 2012 and Windows 8 and later.

I've written quite a lot about it here and have done several performance comparisons with the previous APIs that are available on Windows. The performance tests can be found here.

In summary: "The Registered I/O Networking Extensions, RIO, is a new API that has been added to Winsock to support high-speed networking for increased networking performance with lower latency and jitter. These extensions are targeted primarily for server applications and use pre-registered data buffers and completion queues to increase performance. The increased performance comes from avoiding the need to lock memory pages and copy OVERLAPPED structures into kernel space when individual requests are issued, instead relying on pre-locked buffers, fixed sized completion queues, optional event notification on completions and the ability to return multiple completions from kernel space to user space in one go."

The results of my performance tests seem to imply that it works.

Measure overhead without pthread in C

You should try different approach.

As it was stated, you are trying to measure

overhead of a context switch >between the kernel and userspace

Context switch from user to kernel is done via syscall. For sure printf underneath uses write syscall, but this syscall is too heavy to get reliable estimation. To improve this estimation you should answer to the question - what is the fastest syscall in linux? And the answer is - syscall with invalid parameter.

P.S.
Don't forget about measurement accuracy. Also you should divide your result by 2 because syscall is a round-trip.

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