From mboxrd@z Thu Jan  1 00:00:00 1970
From: daniel.baluta@gmail.com (Daniel Baluta)
Date: Fri, 25 Feb 2011 19:15:59 +0200
Subject: syscalls performance
In-Reply-To: <AANLkTimZac++X4jUsz2-bTTtEJwOR45NHAQ=GR3xCVnG@mail.gmail.com>
References: <AANLkTimZac++X4jUsz2-bTTtEJwOR45NHAQ=GR3xCVnG@mail.gmail.com>
Message-ID: <AANLkTiniirL=jN=yOmsev6MF3a1mpwGUkm-_Qu7zh6AR@mail.gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

Hi Mauro,

On Fri, Feb 25, 2011 at 7:03 PM, Mauro Romano Trajber <trajber@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.