Kuan-Wei Chiu <visitor...@gmail.com> writes: > On Sun, Apr 07, 2024 at 06:10:04PM +0200, Toke Høiland-Jørgensen wrote: >> Kuan-Wei Chiu <visitor...@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.
Yes, sounds reasonable. Did you measure any real-world performance benefit, or is this purely a theoretical optimisation? Either way, please indicate this in the updated patch description. -Toke _______________________________________________ Cake mailing list Cake@lists.bufferbloat.net https://lists.bufferbloat.net/listinfo/cake
