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To: Alan Cox <alc@rice.edu>
Subject: Re: Behavior of madvise(MADV_FREE)
In-reply-to: <50786023.1000206@rice.edu>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
References: <9FEBC10C-C453-41BE-8829-34E830585E90@xcllnt.net>
 <4835.1350062021@critter.freebsd.dk> <50786023.1000206@rice.edu>
Date: Fri, 12 Oct 2012 18:46:42 +0000
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Cc: Tim LaBerge <tlaberge@juniper.net>, Marcel Moolenaar <marcel@xcllnt.net>,
 "freebsd-arch@freebsd.org Arch" <freebsd-arch@freebsd.org>
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--------
In message <50786023.1000206@rice.edu>, Alan Cox writes:

>Given the way that madvise(MADV_FREE) is still used by malloc()/free(), 
>any additional overhead would still be a pessimization most of the 
>time.

That would be my fear too.

>Over the years, I think you and I have both said that what we 
>really need is the ability for the kernel to deliver an event to 
>malloc()/free() when memory is trending toward scarcity.

I still think this would be a good idea, and I can only assume that
the absense of this feature is due to Jason Evans failing in his
malloc-maintainers duty of bugging the VM people for this feature
at every conceiveable opportunity :-)

That being said, my experience with Varnish does indicate that intelligent
use of VM from userland is close to a lost case, in no small part because
of the lack of a good API.

If you can spare a student of sufficient quality, it might be a good assignment
to design a new VM-API, relative to the current reality of things like
malware attacks, "no-execute" bits etc.


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.