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List: linux-api
Subject: Re: [SCHED_DEADLINE man pages 2/2] sched(7) SCHED_DEADLINE
From: "Michael Kerrisk (man-pages)" <mtk.manpages () gmail ! com>
Date: 2014-05-13 17:54:42
Message-ID: 53725C62.1060809 () gmail ! com
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Hello Juri,
On 05/13/2014 05:52 PM, Juri Lelli wrote:
> Hello,
>
> On Tue, 13 May 2014 17:00:57 +0200
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> wrote:
>
>> Hello Peter et al.
>>
>> Here is the section of the sched(7) page that describes SCHED_DEADLINE,
>> as rendered text, with the (entire) raw page source attached. Please
>> carefully review.
>>
>> Cheers,
>>
>> Michael
[...]
>> The following diagram clarifies these terms:
>>
>> arrival/wakeup absolute deadline
>> | start time |
>> | | |
>> v v v
>> -----x--------xooooooooooooooooo-------x--------x---
>> |<- comp. time ->|
>> |<------- relative deadline ----->|
>> |<-------------- period ------------------>|
>>
>> When setting a SCHED_DEADLINE policy for a thread using
>> sched_setattr(2), one can specify three parameters: Runtime,
>> Deadline, and Period. These parameters do not necessarily
>> correspond to the aforementioned terms: usual practice is to
>> set Runtime to something bigger than the average computation
>> time (or worst-case execution time for hard real-time tasks),
>> Deadline to the relative deadline, and Period to the period
>> of the task. Thus, for SCHED_DEADLINE scheduling, we have:
>>
>> arrival/wakeup absolute deadline
>> | start time |
>> | | |
>> v v v
>> -----x--------xooooooooooooooooo-------x--------x---
>> |<-- Runtime --->|
>
> |<-- Runtime --->|
>
> I originally drew this slightly bigger than comp. time above because we
> usually don't want tasks to be throttled in the average case. It's just
> a rule of thumb.
Ahh -- yes, I see now that I messed this up when I tweaked the
ASCII art. Thanks for catching that. Fixed now.
>> |<----------- Deadline ---------->|
>> |<-------------- Period ------------------>|
>>
>> The three deadline-scheduling parameters correspond to the
>> sched_runtime, sched_deadline, and sched_period fields of the
>> sched_attr structure; see sched_setattr(2). These fields
>> express value in nanoseconds. If sched_period is specified
>> as 0, then it is made the same as sched_deadline.
>>
>> The kernel requires that:
>>
>> sched_runtime <= sched_deadline <= sched_period
>>
>> In addition, under the current implementation, all of the
>> parameter values must be at least 1024 (i.e., just over one
>> microsecond, which is the resolution of the implementation).
>
> And below 2^63, as per the last bug fix we discussed.
Fixed.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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