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Subject: Re: [PATCH v6] add MAP_UNLOCKED mmap flag
From: Peter Zijlstra <peterz@infradead.org>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Avi Kivity <avi@redhat.com>, Gleb Natapov <gleb@redhat.com>,
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Date: Mon, 18 Jan 2010 16:44:34 +0100
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On Mon, 2010-01-18 at 15:41 +0000, Alan Cox wrote:
> On Mon, 18 Jan 2010 16:24:07 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Mon, 2010-01-18 at 17:19 +0200, Avi Kivity wrote:
> > > > I would not advice that, just mlock() the text and data you need for the
> > > > real-time thread. mlockall() is a really blunt instrument.
> > > >    
> > > 
> > > May not be feasible due to libraries. 
> > 
> > Esp for the real-time case I could advise not to use those libraries
> > then, since they're clearly not designed for that use case.
> 
> In "hard" real time cases an awful lot of libraries have things like
> memory allocations in them and don't care about stack growth which can
> cause faults and sleeps. The memory allocator if you are running threaded
> was not real time priority aware either last time I checked so the
> standard libraries are not going to give the behaviour you want unless
> you have a proper RT environment, and even then it may be a bit iffy here
> and there.

I'm quite aware of that, which is why we recommend people to
pre-allocate, mlock() and pre-fault everything in advance and make sure
the RT thread doesn't touch any data/text outside of that and uses a
limited set of system calls.

You can also do that for stacks using pthread_attr_setstack().