syscalls performance

[Mauro Romano Trajber]

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 <daniel.baluta at gmail.com>wrote:

> 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.
>
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