svn commit: r242847 - in head/sys: i386/include kern

[Peter Wemm]

On Sat, Nov 10, 2012 at 11:18 AM, Andre Oppermann <andre at freebsd.org> wrote:
> On 10.11.2012 19:04, Peter Wemm wrote:
>>
>> On Sat, Nov 10, 2012 at 9:48 AM, Eitan Adler <eadler at freebsd.org> wrote:
>>>
>>> On 10 November 2012 12:45, Peter Wemm <peter at wemm.org> wrote:
>>>>
>>>> On Sat, Nov 10, 2012 at 9:33 AM, Eitan Adler <eadler at freebsd.org> wrote:
>>>>>
>>>>> On 10 November 2012 12:04, Alfred Perlstein <bright at mu.org> wrote:
>>>>>>
>>>>>> Sure, if you'd like you can help me craft that comment now?
>>>>>
>>>>>
>>>>> I think this is short and clear:
>>>>> ===
>>>>> Limit the amount of kernel address space used to a fixed cap.
>>>>> 384 is an arbitrarily chosen value that leaves 270 MB of KVA available
>>>>> of the 2 MB total. On systems with large amount of memory reduce the
>>>>> the slope of the function in order to avoiding exhausting KVA.
>>>>> ===
>>>>
>>>>
>>>> That's actually completely 100% incorrect...
>>>
>>>
>>> okay. I'm going by the log messages posted so far. I have no idea how
>>> this works. Can you explain it better?
>>
>>
>> That's exactly my point..
>>
>> You get 1 maxuser per 2MB of physical ram.
>> If you get more than 384 maxusers (ie: 192GB of ram) we scale it
>> differently for the part past 192GB.  I have no idea how the hell to
>
>
> Rather past 768MB of RAM.

You're right. It is 768M ram before the slope of the cap changes.

maxusers = physpages / (2 * 1024 * 1024 / PAGE_SIZE);

This is simple math.
physpages is units of pages.
(2 * 1024 * 1024 / PAGE_SIZE) = 2MB, converted to pages.
So maxusers = 1 per 2MB of physical ram.

20MB of ram calculates maxusers = 10.
768M ram gives maxusers = 384.
Above 768M, one maxuser is allocated per 2 * 8 = 16MB of ram.
4G gives maxusers = 592.
16G gives maxusers = 1360.

>> calculate that.
>> You get an unlimited number of regular mbufs.
>> You get 64 clusters per maxuser (128k)
>> Unless I fubared the numbers, this currently works out to be 6%, or 1/16.
>>
>> Each MD backend gets to provide a cap for maxusers, which is in units
>> of 2MB.  For an i386 PAE machine you have a finite amount of KVA space
>> (1GB, but this is adjustable.. you can easily configure it for 3GB kva
>> with one compile option for the kernel).  The backends where the
>> nmbclusters comes out of KVA should calculate the number of 2MB units
>> to avoid running out of KVA.
>>
>> amd64 does a mixture of direct map and kva allocations. eg: mbufs and
>> clusters come from direct map, the jumbo clusters come from kva.

I'm aware of this.  Behavior of how to set a "Don't die" exhaustion is MD.

-- 
Peter Wemm - peter at wemm.org; peter at FreeBSD.org; peter at yahoo-inc.com; KI6FJV
"All of this is for nothing if we don't go to the stars" - JMS/B5
"If Java had true garbage collection, most programs would delete
themselves upon execution." -- Robert Sewell