Timer Resolution

The profile provider uses arbitrary resolution interval timers in the operating system. On architectures that do not support truly arbitrary resolution time-based interrupts, the frequency is limited by the system clock frequency, which is specified by the hz kernel variable. Probes of higher frequency than hz on such architectures will fire some number of times every 1/hz seconds. For example, a 1000 hertz profile probe on such an architecture with hz set to 100 will fire ten times in rapid succession every ten milliseconds. On platforms that support arbitrary resolution, a 1000 hertz profile probe would fire exactly every one millisecond.

The following example tests a given architecture's resolution:

profile-5000
{
    /*
     * We divide by 1,000,000 to convert nanoseconds to milliseconds, and
     * then we take the value mod 10 to get the current millisecond within
     * a 10 millisecond window.  On platforms that do not support truly
     * arbitrary resolution profile probes, all of the profile-5000 probes
     * will fire on roughly the same millisecond.  On platforms that
     * support a truly arbitrary resolution, the probe firings will be
     * evenly distributed across the milliseconds.
     */
    @ms = lquantize((timestamp / 1000000) % 10, 0, 10, 1);
}

tick-1sec
/i++ >= 10/
{
    exit(0);
}

On an architecture that supports arbitrary resolution profile probes, running the example script will yield an even distribution:

# dtrace -s ./restest.d
 dtrace: script './restest.d' matched 2 probes
CPU     ID                    FUNCTION:NAME
  0  33631                       :tick-1sec 


           value  ------------- Distribution ------------- count    
             < 0 |                                         0        
               0 |@@@                                      10760    
               1 |@@@@                                     10842    
               2 |@@@@                                     10861    
               3 |@@@                                      10820    
               4 |@@@                                      10819    
               5 |@@@                                      10817    
               6 |@@@@                                     10826    
               7 |@@@@                                     10847    
               8 |@@@@                                     10830    
               9 |@@@@                                     10830

On an architecture that does not support arbitrary resolution profile probes, running the example script will yield an uneven distribution:

# dtrace -s ./restest.d
 dtrace: script './restest.d' matched 2 probes
 CPU     ID                    FUNCTION:NAME
  0  28321                       :tick-1sec 


           value  ------------- Distribution ------------- count    
               4 |                                         0        
               5 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  107864   
               6 |                                         424      
               7 |                                         255      
               8 |                                         496      
               9 |                                         0

On these architectures, hz may be manually tuned in /etc/system to improve the effective profile resolution.

Currently, all variants of UltraSPARC (sun4u) support arbitrary resolution profile probes. Many variants of the x86 architecture (i86pc) also support arbitrary resolution profile probes, although some older variants do not.