Same behavior for a new syscall created from scratch.

On Friday, February 25, 2011, Enrico Granata [off-list ref] wrote:

We already ruled that out this morning. The code is now using syscall() directly:inline long mygetpid() { ? ? return syscall(SYS_getpid);}


Enrico GranataComputer Science & Engineering Department (EBU3B) -?Room 3240office phone 858 534 9914University of California, San Diego


On Feb 25, 2011, at 12:59 PM, Jim Kukunas wrote:
On Fri, Feb 25, 2011 at 12:42 PM, Enrico Granata [off-list ref] wrote:


I modified the source code to show exactly how many clock ticks it is taking
for each call. It seems that the behavior hinted by Mauro Romano Trajber is
actually there:
[enrico at espresso ~]$ ./syscallperf 15
4925
1190
942
942
935
942
636
577
627
621
580
591
565
580
565
I am starting to wonder if this depends on the syscall itself OR on some
call optimization.. any gcc experts around?

From the getpid(2) manpage:

??????"Since glibc version 2.3.4, the ?glibc ?wrapper ?function ?for ?getpid()
???????caches ?PIDs, ?so ?as ?to ?avoid additional system calls when a process
???????calls getpid() repeatedly."


Enrico Granata
Computer Science & Engineering Department (EBU3B) -?Room 3240
office phone 858 534 9914
University of California, San Diego
On Feb 25, 2011, at 12:30 PM, Mauro Romano Trajber wrote:

Sure, the code is attached.

On Fri, Feb 25, 2011 at 5:15 PM, Daniel Baluta [off-list ref]
wrote:

On Fri, Feb 25, 2011 at 8:22 PM, Mauro Romano Trajber [off-list ref]
wrote:
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 ?

Can you post a pointer to your code and information about how you got
this numbers?

thanks,
Daniel.

<syscallperf.c>