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Date:      Sat, 10 Nov 2012 11:48:01 -0500
From:      Eitan Adler <eadler@freebsd.org>
To:        Alfred Perlstein <bright@mu.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:  <CAF6rxgnfm4HURYp=O4MY8rB6H1tGiqJ3rdPx0rZ8Swko5mAOZg@mail.gmail.com>
In-Reply-To: <509E847E.30509@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> <509E847E.30509@mu.org>

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On 10 November 2012 11:44, Alfred Perlstein <bright@mu.org> wrote:
> 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.

I understand this. What I was confused by is where the number "384"
comes from? Was this empirically tested? Mathematically deduced?
Arbitrarily chosen?

I think the log messages you pasted mostly answered this.


-- 
Eitan Adler
Source, Ports, Doc committer
Bugmeister, Ports Security teams



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