CentOS 6.2 32bit does not recognize 24GB memory ?

geocrger · Post by **geocrger** » 2012/01/04 08:19:17

I installed CentOS 6.2 on a HP Z800 workstation with dual X5550 and 4*6=24GB memory , for historical reason I choose the 32bit version of CentOS6.2.
But the system monitor report the memory is 15.2GB ,not 24GB.

It has been documented that the PAE is build-in in CentOS6, does this mean the build-in PAE does not work as well as CentOS5.5 with PAE kernel, which can recognize all of the 24GB memory ?

the output of free:[code]
total used free shared buffers cached
Mem: 15901416 1140080 14761336 0 84424 582148
-/+ buffers/cache: 473508 15427908
Swap: 0 0 0
[/code]

and all the memory work well seeing by dmidecode
[code]
/usr/sbin/dmidecode |grep -A 16 "Memory Device"|grep MB
Size: 4096 MB
Size: 4096 MB
Size: 4096 MB
Size: 4096 MB
Size: 4096 MB
Size: 4096 MB
Range Size: 2 MB[/code][Moderator edit: Added [i]code[/i] tags to preserve formatting.]

Post by **toracat** » 2012/01/04 14:40:25

I was rather surprised to hear CentOS 5 could see all 24GB of RAM. You can see various hardware limits at the upstream's [url=http://www.redhat.com/rhel/compare/]comparison table[/url].

If I have that much or memory, I would definitely go for 64-bit.

pschaff · Post by **pschaff** » 2012/01/04 19:46:59

According to [b]free[/b] 8GB seems to be missing, as might be expected from the upstream table.

CaViCcHi · Post by **CaViCcHi** » 2015/03/06 02:26:46

Hi Guys,

I didn't want to open a new topic altogether since this is exactly the issue I have.

I have a machine 32bit with 24GB of ram (8x3)

With the kernel 2.6.18-274.18.1.el5PAE I see 24GB of ram usable
With the kernel 2.6.32-279.el6.i686 I only see 16GB.

From the documentation I see that both CentOS5 and 6 should support UP to 16gb,

why is that the PAE supports more? And why doesn't the el6 kernel (since it's builtin with PAE)?

thank you very much, and again apologies for the 3-yr bump, but this was EXACTLY the same issue...

avij · Post by **avij** » 2015/03/06 03:49:33

The issue is exactly the same, and the recommendation is exactly the same: switch to 64 bits.

I have not looked this up from the documentation, but the documentation might state that memory configurations up to 16GB are supported. Anything above that is unsupported. Note that this wording does not specify the maximum amount of memory that might work in some specific circumstances. If CentOS 5 recognizes more than 16GB memory, well, good for you, but if it doesn't, sorry, 16GB is the max supported amount of memory.

In the case of CentOS (and RHEL) 6, RHEL kernel developers apparently needed to set a hard limit for the maximum amount of memory for some reason. I'm afraid there is no way around that limitation, apart from switching to 64 bits. Please note that a 64 bit CentOS can run 32 bit programs just fine.

CentOS