linux-kernel - Re: Ques: [kernel/time/*] Is there any disadvantage in using sleep

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Message-ID: <CADYu30_bOMwDUc=gvX7O_y9Hw0-MF2a_=0P_HROncmbWZuf8JA@mail.gmail.com>
Date:	Mon, 21 Dec 2015 22:52:58 +0530
From:	Aniroop Mathur <aniroop.mathur@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Clemens Ladisch <clemens@...isch.de>,
	John Stultz <john.stultz@...aro.org>, a.mathur@...sung.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Ques: [kernel/time/*] Is there any disadvantage in using
 sleep_range for more than 20ms delay ?

On Tue, Dec 15, 2015 at 1:23 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 15 Dec 2015, Aniroop Mathur wrote:
>> On Sun, Dec 13, 2015 at 3:17 PM, Clemens Ladisch <clemens@...isch.de> wrote:
>> > Aniroop Mathur wrote:
>> >>> 2. If we use msleep(40), is it possible that process could wake up after
>> >>> 60 ms or 70 ms or 100 ms or more ?
>> >
>> > Yes, if lots of other processes compete for the CPU.
>>
>> You mean to say "yes" for process competing for changing state
>> from "waiting" to "ready" or "ready" to "running" ?
>
> There is no state ready. We change from [un]interruptible to running,
> but that's just the wakeup by the timer callback. When the task gets
> on the CPU is a different question.
>
>> Regarding above, could you please help to confirm below things?
>> 1. Using msleep(40) with HZ=1000 (1ms), process can still be in
>> "waiting" state and will not move to "ready" state even after 42,45 or 50 ms.
>> Right ?
>
> kernel/time/timer.c:apply_slack()
>
>> 2. Using usleep_range(40000, 41000), it is guaranteed that process will
>> change its state from waiting to ready state before or at 41 ms.
>> Right ?
>
> 1) It is supposed to be woken up by the timer in that range, but there
>    is no guarantee. Depends also on your kernel configuration and
>    hardware capabilities.
>
> 2) The state changes from [un]interruptible to running. And again
>    that does not tell you when it gets on the CPU.
>
>> Regarding usleep_range disadvatage:
>> 1. Usleep_range has a disadvantage that it does not allow the system to
>> do optimal timer batching for power savings. Except that, Is there any
>> other disadvantage?
>
> Higher CPU usage.
>

Really appreciate your input, Mr. Thomas.
Suddenly today, I thought again and realised than Less Power Saving and
High CPU Usage are disadvantages of usleep_range as a whole and not only
when used for delays more than 20ms.
I hope my understanding is right.
If yes, Could you please help to confirm my above understanding ?
and let me know if there is any disadvantage "specifically for large delays" ?

Thanks again for your support on this.

Regards,
Aniroop Mathur

>> 2. Is the impact of optimal timer batching in systems like android or ubuntu
>> with moderate cpu speed significant or it can be negligible also?
>> To understand the impact better, it would be really appreciating if you could
>> kindly explain in detail with some case and data input.
>
>   http://bfy.tw/3HaV
>   http://bfy.tw/3Han
>   ....
>
> Thanks,
>
>         tglx
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