Messages in this thread | | | Date | Mon, 8 Apr 2024 19:14:05 +0800 | From | Kuan-Wei Chiu <> | Subject | Re: [PATCH net-next] net: sched: cake: Optimize number of calls to cake_heapify() |
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On Sun, Apr 07, 2024 at 06:10:04PM +0200, Toke Høiland-Jørgensen wrote: > Kuan-Wei Chiu <visitorckw@gmail.com> writes: > > > Improve the max-heap construction process by reducing unnecessary > > heapify operations. Specifically, adjust the starting condition from > > n / 2 to n / 2 - 1 in the loop that iterates over all non-leaf > > elements. > > Please add an explanation for why this change is correct, and why it is > beneficial. "Improve" and "unnecessary" is way too implicit. > > pw-bot: cr
For correctness: To build a heap, we need to perform heapify operations on all non-leaf nodes, so we need to find the index of the first non-leaf node. In a heap, the index of node i, the left child's index is 2 * i + 1, and the right child's index is 2 * i + 2. The left and right children of node CAKE_MAX_TINS * CAKE_QUEUES / 2 are at indexes CAKE_MAX_TINS * CAKE_QUEUES + 1 and CAKE_MAX_TINS * CAKE_QUEUES + 2, respectively. Both children's indexes are beyond the range of the heap, indicating that CAKE_MAX_TINS * CAKE_QUEUES / 2 is a leaf node. The left child of node CAKE_MAX_TINS * CAKE_QUEUES / 2 - 1 is at index CAKE_MAX_TINS * CAKE_QUEUES - 1, and the right child is at index CAKE_MAX_TINS * CAKE_QUEUES. Therefore, we know the left child exists, but the right child does not. Since it's not a leaf node, the loop should start from it.
For benefit: We can reduce 2 function calls (one for cake_heapify() and another for cake_heap_get_backlog()) and decrease 5 branch condition evaluations (one for iterating through all non-leaf nodes, one inside the while loop of cake_heapify(), and three more inside the while loop with if conditions). The only added operation is an extra subtraction.
If you're satisfied with the explanation above, I can attempt to rewrite the commit message and send the v2 patch.
Thanks, Kuan-Wei
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