Re: [PATCH] ipc/msg.c: wake up senders until there is a queue empty capacity

[Manfred Spraul]

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Hello Artur,

On 5/23/20 10:34 PM, Artur Barsegyan wrote:
> Take into account the total size of the already enqueued messages of
> previously handled senders before another one.
>
> Otherwise, we have serious degradation of receiver throughput for
> case with multiple senders because another sender wakes up,
> checks the queue capacity and falls into sleep again.
>
> Each round-trip wastes CPU time a lot and leads to perceptible
> throughput degradation.
>
> Source code of:
> 	- sender/receiver
> 	- benchmark script
> 	- ready graphics of before/after results
>
> is located here: https://github.com/artur-barsegyan/systemv_queue_research

Thanks for analyzing the issue!

> Signed-off-by: Artur Barsegyan <a.barsegyan96@xxxxxxxxx>
> ---
>   ipc/msg.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/ipc/msg.c b/ipc/msg.c
> index caca67368cb5..52d634b0a65a 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -214,6 +214,7 @@ static void ss_wakeup(struct msg_queue *msq,
>   	struct msg_sender *mss, *t;
>   	struct task_struct *stop_tsk = NULL;
>   	struct list_head *h = &msq->q_senders;
> +	size_t msq_quota_used = 0;
>   
>   	list_for_each_entry_safe(mss, t, h, list) {
>   		if (kill)
> @@ -233,7 +234,7 @@ static void ss_wakeup(struct msg_queue *msq,
>   		 * move the sender to the tail on behalf of the
>   		 * blocked task.
>   		 */
> -		else if (!msg_fits_inqueue(msq, mss->msgsz)) {
> +		else if (!msg_fits_inqueue(msq, msq_quota_used + mss->msgsz)) {
>   			if (!stop_tsk)
>   				stop_tsk = mss->tsk;
>   
> @@ -241,6 +242,7 @@ static void ss_wakeup(struct msg_queue *msq,
>   			continue;
>   		}
>   
> +		msq_quota_used += mss->msgsz;
>   		wake_q_add(wake_q, mss->tsk);

You have missed the case of a do_msgsnd() that doesn't enqueue the message:

Situation:

- 2 messages of type 1 in the queue (2x8192 bytes, queue full)

- 6 senders waiting to send messages of type 2

- 6 receivers waiting to get messages of type 2.

If now a receiver reads one message of type 1, then all 6 senders can send.

WIth your patch applied, only one sender sends the message to one 
receiver, and the remaining 10 tasks continue to sleep.


Could you please check and (assuming that you agree) run your benchmarks 
with the patch applied?

--

ÂÂÂ Manfred