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Date:      Mon, 24 Mar 2003 22:10:32 -0800
From:      Wes Peters <wes@softweyr.com>
To:        Garance A Drosihn <drosih@rpi.edu>, John Baldwin <jhb@FreeBSD.ORG>
Cc:        freebsd-arch@FreeBSD.ORG
Subject:   Re: Patch to protect process from pageout killing
Message-ID:  <200303242210.32055.wes@softweyr.com>
In-Reply-To: <p05200f43baa58a1eb364@[128.113.24.47]>
References:  <XFMail.20030324140902.jhb@FreeBSD.org> <200303241752.40245.wes@softweyr.com> <p05200f43baa58a1eb364@[128.113.24.47]>

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On Monday 24 March 2003 20:53, Garance A Drosihn wrote:
> At 5:52 PM -0800 3/24/03, Wes Peters wrote:
> >
> >It's not clear to me the SIGDANGER interface allows me to say
> >"go elsewhere bub, I'm really important."  In this case, that
> >is essential.  I think even in the general FreeBSD case you can
> >make a point for a setting like this in, say, named.
>
> Please check out the descriptions I posted previously.  SIGDANGER
> (as implemented by AIX) explicitly provides two things.  The process
> gets to decide which one they (the process) wants:
>
>     1) signal me at the first sign of trouble, and I'll free
>        up some virtual memory (possibly by exit()-ing).
>     2) do not ever kill me to free up memory.

The current situation, leave me alone until you're really hurting, then 
just kill me quickly, should not only be an option but be the default.  
Is that covered?  As the default?  I.e. if I don't specify any handling 
of SIGDANGER at all, does it continue to work as now?

I guess my biggest worry about SIGDANGER is that minds much brighter than 
yours or mine share my worries about it.  Relying on signal delivery is 
just not in my nature.

> I think that we could improve upon the AIX implementation if we
> wanted to, but I think people are so used to having problems with
> AIX that they hate the idea of SIGDANGER as soon as they see the
> letters AIX. 

Yeah, that's pretty much my knee-jerk reaction.  I haven't really used AIX 
since about 3.2.5, but it was just fugly in those days.

> Having used AIX for more than ten years now, I can
> sympathize with that, but in the specific case of SIGDANGER there
> is an idea that can work quite well.
>
> (reference on sigdanger was at:
> http://nscp.upenn.edu/aix4.3html/aixbman/baseadmn/pag_space_under.htm
> )
>
> >The SIGDANGER interface worries me in general, partly because it's
> >a signal and partly because it complicates the design of EVERYTHING
> >just  to handle it.  I guess a lot depends on the implementation
> >details of how SIGDANGER and the default handlers are designed,
> >but nothing I saw  last week gave me a warm fuzzy about that.
>
> I don't know enough about the lower-level implementation details,
> but I did think the recent discussion on the src-committers list
> did include a number of good ideas.  I am horribly over-committed
> with things that I've promised to do (including stuff for my real-
> world job...), so I can't look into SIGDANGER ideas right now, but
> I'm more than happy to try to explain how it should work.

Some of the explanations were reasonable enough to erase all of my 
objections EXCEPT "it's a signal."  Do we have signal delivery to 
multi-threaded processes worked out enough to rely on SIGDANGER for such 
a critical function?  If so, it's news to me, but that doesn't mean it's 
not done...

-- 

        Where am I, and what am I doing in this handbasket?

Wes Peters                                               wes@softweyr.com


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