Messages in this thread | | | Subject | Re: [PATCH] ipc/msg.c: wake up senders until there is a queue empty capacity | From | Manfred Spraul <> | Date | Mon, 1 Jun 2020 16:20:56 +0200 |
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Hi Artus,
On 6/1/20 4:02 PM, Artur Barsegyan wrote: > Hi, Manfred. > > Did you get my last message?
Yes, I'm just too busy right now.
My plan/backlog is:
- the xarray patch from Matthew
- improve finding max_id in ipc_rmid(). Perhaps even remove max_id entirely and instead calculate it on demand.
- your patch to avoid waking up too many tasks, including my bugfix.
> On Wed, May 27, 2020 at 02:22:57PM +0300, Artur Barsegyan wrote: >> [sorry for the duplicates — I have changed my email client] >> >> About your case: >> >> The new receiver puts at the end of the receivers list. >> pipelined_send() starts from the beginning of the list and iterates until the end. >> >> If our queue is always full, each receiver should get a message because new receivers appends at the end. >> In my vision: we waste some time in that loop but in general should increase the throughout. But it should be tested. >> >> Yes, I'm gonna implement it and make a benchmark. But maybe it should be done in another patch thread?
My biggest problem is always realistic benchmarks:
Do we optimize for code size/small amount of branches, or add special cases for things that we think could be common?
Avoiding thundering herds is always good, avoiding schedule() is always good.
Thus I would start with pipelined_receive, and then we would need feedback from apps that use sysv msg.
(old fakeroot is what I remember as test app)
>> On Wed, May 27, 2020 at 08:03:17AM +0200, Manfred Spraul wrote: >>> Hello Artur, >>> >>> On 5/26/20 9:56 AM, Artur Barsegyan wrote: >>>> 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. >>> I don't get your concern, or perhaps this is a feature that I had always >>> assumed as "normal": >>> >>> "msg_fits_inqueue(msq, msgsz)" is in the loop, this ensures progress. >>> >>> The rational is a design decision: >>> >>> The check for pipeline sending is only done if there would be space to store >>> the message in the queue. >>> >>> I was afraid that performing the pipeline send immediately, without checking >>> queue availability, could break apps: >>> >>> Some messages would arrive immediately (if there is a waiting receiver), >>> other messages are stuck forever (since the queue is full). >>> >>> Initial patch: https://lkml.org/lkml/1999/10/3/5 (without any remarks about >>> the design decision) >>> >>> The risk that I had seen was theoretical, I do not have any real bug >>> reports. So we could change it. >>> >>> Perhaps: Go in the same direction as it was done for POSIX mqueue: implement >>> pipelined receive. >>> >>>> 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 <a.barsegyan96@gmail.com> >>>>>> --- >>>>>> 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 <manfred@colorfullife.com> >>>>> 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 <manfred@colorfullife.com> >>>>> --- >>>>> 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 >>>>>
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