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Re: soft real-time on linux

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On 02/09/2017 03:16 AM, Jack wrote:
> Il giorno giovedì 9 febbraio 2017 03:17:31 UTC+1, Phil Hobbs ha
> scritto:
>>
>> The Linux thread scheduler is horribly broken, or was last time I
>> used it (a while ago, admittedly).  The info pages will tell you
>> all about how to specify priorities, scheduling algorithms, and so
>> forth, but leave out two vital items:
>>
>> 1. You can only hack those things if you're running as root, and
>
> the priority of your processes can be modified with "nice". If you
> want to change some kernel config of course you need to be root (same
> thing in any other OS)

Only process-by-process.
>
>> 2. You can't have both real-time and normal threads in the same
>> process.
>
> since threads are created/destroyed dinamically, and the OS can't
> know what the fuck is going on in the thread, it's pretty normal that
> the priority is process based and not thread based.

Not true in Windows, for instance.  It's a serious handicap in my case, 
where I have compute threads and comms threads in a clusterized 
simulator: another box can totally stall waiting for a comms thread on 
the next box to finish its job.  It trashes the scaling--Windows boxes 
scale nearly linearly up to 25 hosts or so, whereas Linux ones start 
getting less efficient at around 8 hosts.

On a single box, the thread scheduler has a global view of what's going 
on, so it isn't as big a deal.

>
> If you want to prioritize a particular thread in a process, you need
> to do it at programming time.
>
> good old stackoverflow:
> http://stackoverflow.com/questions/3649281/how-to-increase-thread-priority-in-pthreads
>
Only works as root, and if you want any thread to be real-time, you have 
to put the whole compute-bound process in the RT class, which brings the 
system to its knees.  What I need is a RT comms thread that is usually 
blocked on a semaphore and then runs furiously about 1% of the time.
>
>>
>> Both (1) and (2) are easy in Windows (or the late lamented OS/2,
>> which
>
> the (1) is "easy" in win because a normal user is automatically
> Administrator, if aou use a user without admin privileges, well you
> can change the priority of your processes, but not others.

But not the relative priority of threads within the process, whereas you 
can in Windows.  As I said originally, I'd be quite happy to be able 
just to _reduce_ the priority of the compute threads vs. the comms 
threads,  but _noooooooo_.  Broken as designed.

Cheers

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

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soft real-time on linux Randy Yates <yates@digitalsignallabs.com> - 2016-11-12 08:53 -0500
  Re: soft real-time on linux Grant Edwards <invalid@invalid.invalid> - 2016-11-12 17:16 +0000
  Re: soft real-time on linux Mel Wilson <mwilson@the-wire.com> - 2016-11-12 17:33 +0000
  Re: soft real-time on linux upsidedown@downunder.com - 2016-11-12 20:34 +0200
  Re: soft real-time on linux Don Y <blockedofcourse@foo.invalid> - 2016-11-12 13:53 -0700
    Re: soft real-time on linux sbattazzo@gmail.com - 2017-02-09 11:06 -0800
  Re: soft real-time on linux Paul Rubin <no.email@nospam.invalid> - 2016-11-12 14:12 -0800
  Re: soft real-time on linux Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-08 21:17 -0500
    Re: soft real-time on linux Jack <jack4747@gmail.com> - 2017-02-09 00:16 -0800
      Re: soft real-time on linux David Brown <david.brown@hesbynett.no> - 2017-02-09 10:00 +0100
      Re: soft real-time on linux Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-10 11:12 -0500
    Re: soft real-time on linux George Neuner <gneuner2@comcast.net> - 2017-02-11 01:33 -0500
      Re: soft real-time on linux Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-10 22:10 -0500

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