usleep not respected on rhel5

Issue

Simple test:

#include <stdio.h>
#include <time.h>
#include <unistd.h>

#define NS_PER_SECOND 1000000000LL
#define TIMESPEC_TO_NS( aTime ) ( ( NS_PER_SECOND * ( ( long long int ) aTime.tv_sec ) ) + aTime.tv_nsec )

int main()
{

 struct timespec lLastTime;
        struct timespec lCurrentTime;
        clock_gettime( CLOCK_REALTIME, &lLastTime );

   int i;
   int nr=1000;
   for (i = 0; i < nr; i++)
    {

        usleep(500);
    }


        clock_gettime( CLOCK_REALTIME, &lCurrentTime );
        long long int lDifference = ( TIMESPEC_TO_NS( lCurrentTime ) - TIMESPEC_TO_NS( lLastTime ) );
        printf( "Waited (usecs): %lld\n", lDifference  / 1000 / nr  );

    return 0;
}

compiled as

# gcc -o usleep2 usleep2.c -lrt

the results are quite different in rhel5 and rhel6

on rhel5 (clock source jiffie)

linxdev20,/home/grucher/malloc # ./usleep2
Waited (usecs): 999
linxdev20,/home/grucher/malloc # ./usleep2
Waited (usecs): 1000
linxdev20,/home/grucher/malloc # ./usleep2
Waited (usecs): 1988
linxdev20,/home/grucher/malloc # ./usleep2
Waited (usecs): 2000
linxdev20,/home/grucher/malloc # ./usleep2
Waited (usecs): 1497

on rhel6 (nohz=0 and tsc)

ny380g8l,/root # ./usleep2
Waited (usecs): 550
ny380g8l,/root # ./usleep2
Waited (usecs): 550
ny380g8l,/root # ./usleep2
Waited (usecs): 550
ny380g8l,/root # ./usleep2
Waited (usecs): 550
ny380g8l,/root # ./usleep2
Waited (usecs): 550

We think the difference can be related to the tick time.