syscalls performance

[Daniel Baluta]

Hi Mauro,

On Fri, Feb 25, 2011 at 7:03 PM, Mauro Romano Trajber <trajber at gmail.com> wrote:
> I was doing some performance tests of system calls and I find an interesting
> behavior.
> Using RDTSC to count the CPU cycles, a single call to the getpid() consumes
> about 7k of CPU clock cycles and ten calls consume approximately 9,800
> cycles.
> The fact is that from the second call, the CPU cycles grows at a rate of
> about 350 CPU cycles per call.
> Why does this happen? There is some hardware optimization when the syscall
> ID is already in EAX register ?

Use strace and check to number of getpid() syscalls.
You'll notice that only 1 system call is made, glibc caching the pid value.

thanks,
Daniel.