Intel IA-32 - Manual (Page 319)

Vol. 3A 7-51
MULTIPLE-PROCESSOR MANAGEMENT
The MONITOR and MWAIT instructions may be considered for use in the C0 idle state loops, if
MONITOR and MWAIT are supported.
Example 7-6 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
}
7.11.6.3 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