[Cake] [PATCH net-next] net: sched: cake: Optimize number of calls to cake_heapify()

[Cake] [PATCH net-next] net: sched: cake: Optimize number of calls to cake_heapify()

[Kuan-Wei Chiu]

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.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 net/sched/sch_cake.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index edee926ccde8..2eabc4dc5b79 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1512,7 +1512,7 @@ static unsigned int cake_drop(struct Qdisc *sch, struct sk_buff **to_free)
 	if (!q->overflow_timeout) {
 		int i;
 		/* Build fresh max-heap */
-		for (i = CAKE_MAX_TINS * CAKE_QUEUES / 2; i >= 0; i--)
+		for (i = CAKE_MAX_TINS * CAKE_QUEUES / 2 - 1; i >= 0; i--)
 			cake_heapify(q, i);
 	}
 	q->overflow_timeout = 65535;
-- 
2.34.1

Re: [Cake] [PATCH net-next] net: sched: cake: Optimize number of calls to cake_heapify()

[Toke Høiland-Jørgensen]

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

Re: [Cake] [PATCH net-next] net: sched: cake: Optimize number of calls to cake_heapify()

[Kuan-Wei Chiu]

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

Re: [Cake] [PATCH net-next] net: sched: cake: Optimize number of calls to cake_heapify()

[Toke Høiland-Jørgensen]

Kuan-Wei Chiu <visitorckw@gmail.com> writes:

> 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.

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