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From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@toke.dk>
To: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: jhs@mojatatu.com, xiyou.wangcong@gmail.com, jiri@resnulli.us,
 davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
 pabeni@redhat.com, jserv@ccns.ncku.edu.tw, cake@lists.bufferbloat.net,
 netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Kuan-Wei Chiu
 <visitorckw@gmail.com>
Subject: Re: [PATCH net-next v2] net: sched: cake: Optimize the number of
 function calls and branches in heap construction
In-Reply-To: <20240408174716.751069-1-visitorckw@gmail.com>
References: <20240408174716.751069-1-visitorckw@gmail.com>
Date: Tue, 09 Apr 2024 11:45:03 +0200
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Kuan-Wei Chiu <visitorckw@gmail.com> writes:

> When constructing a heap, heapify operations are required on all
> non-leaf nodes. Thus, determining the index of the first non-leaf node
> is crucial. In a heap, the left child's index of node i is 2 * i + 1
> and the right child's index is 2 * i + 2. Node CAKE_MAX_TINS *
> CAKE_QUEUES / 2 has its left and right children at indexes
> CAKE_MAX_TINS * CAKE_QUEUES + 1 and CAKE_MAX_TINS * CAKE_QUEUES + 2,
> respectively, which are beyond the heap's range, indicating it as a
> leaf node. Conversely, node CAKE_MAX_TINS * CAKE_QUEUES / 2 - 1 has a
> left child at index CAKE_MAX_TINS * CAKE_QUEUES - 1, confirming its
> non-leaf status. The loop should start from it since it's not a leaf
> node.
>
> By starting the loop from CAKE_MAX_TINS * CAKE_QUEUES / 2 - 1, we
> minimize function calls and branch condition evaluations. This
> adjustment theoretically reduces two function calls (one for
> cake_heapify() and another for cake_heap_get_backlog()) and five branch
> evaluations (one for iterating all non-leaf nodes, one within
> cake_heapify()'s while loop, and three more within the while loop
> with if conditions).
>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>

Acked-by: Toke H=C3=B8iland-J=C3=B8rgensen <toke@toke.dk>