8-50 Vol. 3A

MULTIPLE-PROCESSOR MANAGEMENT

// shown below
}

}

}
// C1 handler uses a Halt instruction
VOID C1Handler() 
{ STI

HLT

}

The MONITOR and MWAIT instructions may be considered for use in the C0 idle state loops, if MONITOR and MWAIT are supported. 

Example 8-25.  An OS Idle Loop with MONITOR/MWAIT in the C0 Idle Loop

// WorkQueue is a memory location indicating there is a thread 
// ready to run.  A non-zero value for WorkQueue is assumed to
// indicate the presence of work to be scheduled on the processor.
// The following example assumes that the necessary padding has been 
// added surrounding WorkQueue to eliminate false wakeups
// The idle loop is entered with interrupts disabled.

WHILE (1) {

IF (WorkQueue) THEN {

// Schedule work at WorkQueue.
} 

ELSE {

// No work to do - wait in appropriate C-state handler depending 

// on Idle time accumulated.

IF (IdleTime >= IdleTimeThreshhold) THEN {

// Call appropriate C1, C2, C3 state handler, C1 
// handler shown below
MONITOR WorkQueue  // Setup of eax with WorkQueue

// LinearAddress, 
// ECX, EDX = 0

IF (WorkQueue ≠ 0) THEN {

MWAIT
}

}

}

}
// C1 handler uses a Halt instruction.

VOID C1Handler() 
{ STI

HLT

}

8.10.6.3   Halt Idle Logical Processors

If one of two logical processors is idle or in a spin-wait loop of long duration, explicitly halt that processor by means 
of a HLT instruction. 
In an MP system, operating systems can place idle processors into a loop that continuously checks the run queue 
for runnable software tasks. Logical processors that execute idle loops consume a significant amount of core’s 
execution resources that might otherwise be used by the other logical processors in the physical package. For this 
reason, halting idle logical processors optimizes the performance.

12

 If all logical processors within a physical 

package are halted, the processor will enter a power-saving state.