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Date:	Fri, 13 Apr 2018 21:17:34 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: More oddities with multiple processor groups
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On Apr 13 12:29, Brian Inglis wrote:
> On 2018-04-13 08:12, L A Walsh wrote:
> > Achim Gratz wrote:
> >> The problem here is that on Linux you don't need to do anything extra =
to=20
> >> use any of the advertised logical processors from a single application=
,=20
> >> while on Windows you need to first create a thread and set it's affini=
ty to
> >> a different group than where your process was started in, then assign =
each
> >> new thread an affinity to one of the available groups.  If you don't do
> >> that, all threads will be restricted to the original group.
> > Not exactly true.  They are not *restricted* -- it's a *feature* of the
> > Windows scheduler, in that future procs/threads inherit the cpu of the
> > parent.  Linux's scheduler is more advanced as well as being replaceabl=
e.  MS
> > doesn't want you to do that
> >> there might need to be some option to restrict Cygwin to a single proc=
essor
> >> group for some applications to work (correctly).
> > There is.  Start them all on a single cpu & set the cpu mask.  Pretty m=
uch=20
> > the same way you restrict procs on linux -- you can run them with a spe=
cific
> > cpu mask, and most programs will keep running w/that mask.
> > Unfortunately, AFAIK, I don't think POSIX specifies a way to set affini=
ties,
> > so I'm not sure how cygwin would do it.
>=20
> Glibc adds {pthread,sched}_...affinity... functions.
> Linux uses namespaces, control groups (cgroups), cpusets, sysfs/kernfs:

We would only be able to support it partially.  Windows doesn't
allow to set thread affinity across CPU groups.  There's no simple
function to set process CPU group affinity, only a function to set
process affinity within a single group.  And it has a restriction
in that all current threads have to run in the same CPU group.
The API is pretty tricky.


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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