Hello, Manfred! Thank you, for your review. I've reviewed your patch.
I forgot about the case with different message types. At now with your patch, a sender will force message consuming if that doesn't hold own capacity. I have measured queue throughput and have pushed the results to: https://github.com/artur-barsegyan/systemv_queue_research But I'm confused about the next thought: in general loop in the do_msgsnd() function, we doesn't check pipeline sending availability. Your case will be optimized if we check the pipeline sending inside the loop. On Sun, May 24, 2020 at 03:21:31PM +0200, Manfred Spraul wrote: > 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 <[email protected]> > > --- > > 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? > > -- > > Manfred > > > > From fe2f257b1950a19bf5c6f67e71aa25c2f13bcdc3 Mon Sep 17 00:00:00 2001 > From: Manfred Spraul <[email protected]> > Date: Sun, 24 May 2020 14:47:31 +0200 > Subject: [PATCH 2/2] ipc/msg.c: Handle case of senders not enqueuing the > message > > The patch "ipc/msg.c: wake up senders until there is a queue empty > capacity" avoids the thundering herd problem by wakeing up > only as many potential senders as there is free space in the queue. > > This patch is a fix: If one of the senders doesn't enqueue its message, > then a search for further potential senders must be performed. > > Signed-off-by: Manfred Spraul <[email protected]> > --- > ipc/msg.c | 21 +++++++++++++++++++++ > 1 file changed, 21 insertions(+) > > diff --git a/ipc/msg.c b/ipc/msg.c > index 52d634b0a65a..f6d5188db38a 100644 > --- a/ipc/msg.c > +++ b/ipc/msg.c > @@ -208,6 +208,12 @@ static inline void ss_del(struct msg_sender *mss) > list_del(&mss->list); > } > > +/* > + * ss_wakeup() assumes that the stored senders will enqueue the pending > message. > + * Thus: If a woken up task doesn't send the enqueued message for whatever > + * reason, then that task must call ss_wakeup() again, to ensure that no > + * wakeup is lost. > + */ > static void ss_wakeup(struct msg_queue *msq, > struct wake_q_head *wake_q, bool kill) > { > @@ -843,6 +849,7 @@ static long do_msgsnd(int msqid, long mtype, void __user > *mtext, > struct msg_queue *msq; > struct msg_msg *msg; > int err; > + bool need_wakeup; > struct ipc_namespace *ns; > DEFINE_WAKE_Q(wake_q); > > @@ -869,6 +876,7 @@ static long do_msgsnd(int msqid, long mtype, void __user > *mtext, > > ipc_lock_object(&msq->q_perm); > > + need_wakeup = false; > for (;;) { > struct msg_sender s; > > @@ -898,6 +906,13 @@ static long do_msgsnd(int msqid, long mtype, void __user > *mtext, > /* enqueue the sender and prepare to block */ > ss_add(msq, &s, msgsz); > > + /* Enqueuing a sender is actually an obligation: > + * The sender must either enqueue the message, or call > + * ss_wakeup(). Thus track that we have added our message > + * to the candidates for the message queue. > + */ > + need_wakeup = true; > + > if (!ipc_rcu_getref(&msq->q_perm)) { > err = -EIDRM; > goto out_unlock0; > @@ -935,12 +950,18 @@ static long do_msgsnd(int msqid, long mtype, void > __user *mtext, > msq->q_qnum++; > atomic_add(msgsz, &ns->msg_bytes); > atomic_inc(&ns->msg_hdrs); > + > + /* we have fulfilled our obligation, no need for wakeup */ > + need_wakeup = false; > } > > err = 0; > msg = NULL; > > out_unlock0: > + if (need_wakeup) > + ss_wakeup(msq, &wake_q, false); > + > ipc_unlock_object(&msq->q_perm); > wake_up_q(&wake_q); > out_unlock1: > -- > 2.26.2 >
