Memory on new computer

[jdow]

On 2012/07/16 04:55, Paweł Brodacki wrote:
> 2012/7/16 Martín Marqués <martin.marques at gmail.com>:
>> I had a problem with my main board and CPU and had to buy a new one.
>> Also memory, as the old computer had DDR2 memory.
>>
>> Th problem is that I see only 3.5Gb of memory, while I had a 4Gb bank
>> put in the Motherboard (the motherboard says it can handle up to 32Gb
>> or ram).
>>
>> I saw this in dmesg:
>>
>> [    0.000000] Memory: 3491896k/4702208k available (6177k kernel code,
>> 1053240k absent, 157072k reserved, 7004k data, 1004k init)
>>
>> Why do I get this:
>>
>> $ free
>>               total       used       free     shared    buffers     cached
>> Mem:       3513412    2591976     921436          0       8332     914584
>> -/+ buffers/cache:    1669060    1844352
>> Swap:            0          0          0
>>
>
> There are several possible factors:
> 1) Integrated video card, which uses system memory.
> 2) Peripheral devices I/O regions are mapped below 4 GB, shadowing the
> physical ram.
>
> Here is (old, but still valid) explanation given by IBM for their
> servers: https://www-947.ibm.com/support/entry/portal/docdisplay?lndocid=MIGR-4E4RRF
> and here's the same issue addressed by Microsoft:
> http://support.microsoft.com/kb/978610
> I did not manage to find Red Hat's take on the issue.
>
> In short: Works as designed. You may try to disable devices you do not
> use to salvage some address space, but you won't reach 4096 MB
> available.

This is well known issue. The missing space is occupied by the page table
index. When pages are 4k in size you need a lot of table entries to describe
a full 4 gigabytes, like a million of them. Linux seems to have optimized
this a little. Windows loses a bit more memory, if I recall correctly.

{^_^}