linux-kernel - Re: Re: [PATCH v6] io_uring: Statistics of the true utilization of sq threads.

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Date: Wed,  3 Jan 2024 13:49:37 +0800
From: Xiaobing Li <xiaobing.li@...sung.com>
To: axboe@...nel.dk
Cc: asml.silence@...il.com, linux-kernel@...r.kernel.org,
	io-uring@...r.kernel.org, kun.dou@...sung.com, peiwei.li@...sung.com,
	joshi.k@...sung.com, kundan.kumar@...sung.com, wenwen.chen@...sung.com,
	ruyi.zhang@...sung.com, xiaobing.li@...sung.com
Subject: Re: Re: [PATCH v6] io_uring: Statistics of the true utilization of
 sq threads.

On 12/30/23 9:27 AM, Pavel Begunkov wrote:
> Why it uses jiffies instead of some task run time?
> Consequently, why it's fine to account irq time and other
> preemption? (hint, it's not)
> 
> Why it can't be done with userspace and/or bpf? Why
> can't it be estimated by checking and tracking
> IORING_SQ_NEED_WAKEUP in userspace?
> 
> What's the use case in particular? Considering that
> one of the previous revisions was uapi-less, something
> is really fishy here. Again, it's a procfs file nobody
> but a few would want to parse to use the feature.
> 
> Why it just keeps aggregating stats for the whole
> life time of the ring? If the workload changes,
> that would either totally screw the stats or would make
> it too inert to be useful. That's especially relevant
> for long running (days) processes. There should be a
> way to reset it so it starts counting anew.

Hi, Jens and Pavel,
I carefully read the questions you raised.
First of all, as to why I use jiffies to statistics time, it
is because I have done some performance tests and found that
using jiffies has a relatively smaller loss of performance 
than using task run time. Of course, using task run time is 
indeed more accurate.  But in fact, our requirements for 
accuracy are not particularly high, so after comprehensive 
consideration, we finally chose to use jiffies.
Of course, if you think that a little more performance loss 
here has no impact, I can use task run time instead, but in 
this case, does the way of calculating sqpoll thread timeout
also need to be changed, because it is also calculated through
jiffies.
Then there’s how to use this metric.
We are studying some optimization methods for io-uring, including
performance and CPU utilization, but we found that there is
currently no tool that can observe the CPU ratio of sqthread's 
actual processing IO part, so we want to merge this method  that
can observe this value so that we can more easily observe the 
optimization effects.

Best regards,
--
Xiaobing Li