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Date:      Sat, 10 Nov 2012 08:44:46 -0800
From:      Alfred Perlstein <bright@mu.org>
To:        Eitan Adler <eadler@freebsd.org>
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org, Alfred Perlstein <alfred@freebsd.org>, src-committers@freebsd.org
Subject:   Re: svn commit: r242847 - in head/sys: i386/include kern
Message-ID:  <509E847E.30509@mu.org>
In-Reply-To: <509E830D.5080006@mu.org>
References:  <201211100208.qAA28e0v004842@svn.freebsd.org> <CAF6rxg=HPmQS1T-LFsZ=DuKEqH30iJFpkz%2BJGhLr4OBL8nohjg@mail.gmail.com> <509DC25E.5030306@mu.org> <509E3162.5020702@FreeBSD.org> <509E7E7C.9000104@mu.org> <CAF6rxgmV8dx-gsQceQKuMQEsJ%2BGkExcKYxEvQ3kY%2B5_nSjvA3w@mail.gmail.com> <509E830D.5080006@mu.org>

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On 11/10/12 8:38 AM, Alfred Perlstein wrote:
> On 11/10/12 8:25 AM, Eitan Adler wrote:
>> On 10 November 2012 11:19, Alfred Perlstein <bright@mu.org> wrote:
>>> Please consult the svn log for this file, it's relatively clear just 
>>> in the
>>> commit logs/comments.  Grep for 384/512 and look around.
>> Can this reasoning be added as a comment? I did grep for 384 in the 
>> log, but
>> a) I didn't find the answer
>> b) one shouldn't have to.
>>
>>
> It probably could be added, but then a bunch of other people would 
> complain about the comment being too wordy or "not in English".
>
> I will paste the relevant log messages which do explain it.  If you 
> would like to add a comment or work on a comment that won't be 
> criticized for being "too wordy" then we can do that together.
>
> r89769 | dillon | 2002-01-24 17:54:16 -0800 (Thu, 24 Jan 2002) | 9 lines
>
> Make the 'maxusers 0' auto-sizing code slightly more conservative. Change
> from 1 megabyte of ram per user to 2 megabytes of ram per user, and
> reduce the cap from 512 to 384.  512 leaves around 240 MB of KVM 
> available
> while 384 leaves 270 MB of KVM available.  Available KVM is important
> in order to deal with zalloc and kernel malloc area growth.
>
> Reviewed by:    mckusick
> MFC: either before 4.5 if re's agree, or after 4.5
>
>
> r87546 | dillon | 2001-12-08 17:57:09 -0800 (Sat, 08 Dec 2001) | 6 lines
>
> Allow maxusers to be specified as 0 in the kernel config, which will
> cause the system to auto-size to between 32 and 512 depending on the
> amount of memory.
>
> MFC after:      1 week
>
Let me add a bit of commentary on these logs...

Effectively what happens is that as "maxusers" goes up, the amount of 
kernel memory needed for the structures grows.  Unfortunately at a 
certain point the slope of this function is too steep and makes it too 
easy to exhaust kernel address space (which kills the crab (and the 
kernel)) what was decided at the time was just to cap it hard.

What was really needed was for the slope to be relaxed and for 
architectures to be allowed to override/cap it if needed.

This is now done.  Hopefully it's done correctly and will last us some time.

-Alfred






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