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tirwhan's scratchpad

by tirwhan (Abbot)
on Oct 11, 2005 at 12:19 UTC ( #499133=scratchpad: print w/ replies, xml ) Need Help??

The load_balance() function and related methods are fairly large and complicated, although the steps they perform are comprehensible:

1. First, load_balance() calls find_busiest_queue() to determine the busiest runqueue. In other words, this is the runqueue with the greatest number of processes in it. If there is no runqueue that has at least 25% more processes than the current, find_busiest_queue() returns NULL and load_balance() returns. Otherwise, the busiest runqueue is returned.

2. Second, load_balance() decides from which priority array on the busiest runqueue it wants to pull. The expired array is preferred because those tasks have not run in a relatively long time and thus are most likely not in the processor's cache (that is, they are not "cache hot"). If the expired priority array is empty, the active one is the only choice.

3. Next, load_balance() finds the highest priority (smallest value) list that has tasks, because it is more important to fairly distribute high-priority tasks than lower-priority ones.

4. Each task of the given priority is analyzed to find a task that is not running, not prevented to migrate via processor affinity, and not cache hot. If the task meets this criteria, pull_task() is called to pull the task from the busiest runqueue to the current runqueue.

5. As long as the runqueues remain imbalanced, the previous two steps are repeated and more tasks are pulled from the busiest runqueue to the current. Finally, when the imbalance is resolved, the current runqueue is unlocked and load_balance()returns.

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