distinguish between Maxmem, realmem, physmem
Jung-uk Kim
jkim at FreeBSD.org
Thu Jun 28 16:27:33 UTC 2012
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On 2012-06-28 11:18:45 -0400, Andrew Boyer wrote:
> On Jun 28, 2012, at 10:46 AM, Steve Rikli wrote:
>
>> On Thu, Jun 28, 2012 at 09:58:13AM -0400, John Baldwin wrote:
>>> On Tuesday, June 26, 2012 3:41:10 pm Ping Chen wrote:
>>>>
>>>> I am a bit confused with all these variables defined in
>>>> freebsd(especially in freebsd 6.1): Which one of this
>>>> represents the real memory of a system? Say we bought a
>>>> system with 4G ram, which one tells me the RAM is 4G?
>>>>
>>>> Accordign to source code: Maxmem ==> the highest page of
>>>> phisycal address page : if I understand correctly, this is
>>>> the highest page number of physical memory that is usable?
>>>>
>>>> realMem --> somehow get assigned by realmem = Maxmem: this
>>>> is confuing, if they are the same, why bother a realmem
>>>> variables
>>>
>>> I think realMem is legacy.
>>>
>>>> physmem --> the number of usage pages : this seems the right
>>>> one extract the memory info, however, it seems system
>>>> allocate portion of memory to messge buffer which makes this
>>>> physmem < 4G (assume RAM is 4G)
>>>
>>> Correct. Note that the firmware can also take up part of RAM
>>> as well (e.g. to hold ACPI tables).
>>
>> Given that, are the values for real & avail memory from 'dmesg'
>> presented anywhere like sysctl? E.g. one of my small servers has
>> these from dmesg:
>>
>> real memory = 1073741824 (1024 MB) avail memory = 1023852544
>> (976 MB)
>>
>> but the presumably equivalent sysctl values are different:
>>
>> hw.realmem: 1065287680 hw.physmem: 1047953408
>>
>> For system hardware inventory purposes (I assume OP is asking for
>> reasons along those lines, as am I) I'd want the value which
>> reflects 1024MB of RAM in this case, understanding that it may
>> not be precisely the amount available for system/user/etc.
>> usage.
>>
>> Is dmesg the (only?) place to get that number?
>>
>> Cheers, sr.
>
> Here I use hw.physmem and round up to the nearest 1GB.
>
> You can also check the output of 'dmidecode -t 17' and total up the
> listed size of each DIMM. It gets its info from the BIOS, which
> probed the SPD on each EEPROM at boot time. dmidecode is in
> ports/sysutils/dmidecode.
You don't have to use dmidecode for that. The same information is
already available from loader(8) for very long time:
% kenv smbios.memory.enabled
4194304
% dmesg | grep 'real memory'
real memory = 4294967296 (4096 MB)
http://svnweb.freebsd.org/base?view=revision&revision=190599
> Sometimes a flaky motherboard won't find all of the memory, so it's
> useful to make sure they match.
Correct. In fact, the "real memory" from dmesg is derived from the
same SMBIOS data unless it is less than "avail memory".
http://svnweb.freebsd.org/base?view=revision&revision=196412
Jung-uk Kim
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=rbey
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