Thanks Enrico and Daniel, you're right. glibc was caching getpid(); but this
is not the root cause of this behavior.

Going further, I decide to use call getpid without glibc, using
 syscall(SYS_getpid) to test this behavior and it happened again.
Calling it once, the test consumes about 7k CPU cycles and 10 calls consumes
about 10k CPU cycles.

Any ideas ?

On Fri, Feb 25, 2011 at 2:15 PM, Daniel Baluta [off-list ref]wrote:

Hi Mauro,

On Fri, Feb 25, 2011 at 7:03 PM, Mauro Romano Trajber [off-list ref]
wrote:

quoted

I was doing some performance tests of system calls and I find an

interesting

quoted

behavior.
Using RDTSC to count the CPU cycles, a single call to the getpid()

consumes

quoted

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

quoted

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.

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