Mike B asked:
I’ve got a CentOS 5.10 (32-bit) server running on VMWare. It’s allocated 4 GB of RAM.
If I run
dmidecode -t 17 | grep Size | grep MB I see:
Size: 4096 MB
Yet when I run
free, I see:
total used free shared buffers cached Mem: 3107140 1239244 1867896 0 332 400464 -/+ buffers/cache: 838448 2268692 Swap: 2096472 0 2096472
Why is there a discrepancy between the total amount of memory
free reports and the
The kernel I’m running is:
2.6.18-371.4.1.el5 #1 SMP Thu Jan 30 06:09:24 EST 2014 i686 i686 i386 GNU/Linux
Admittedly, the kernel is not running
PAE but I thought that was only necessary for memory exceeding 4 GB.
I know I’m missing something simple – can someone please elaborate?
It definitely looks like my kernel is reserving a bunch of memory for other stuff. Here’s what I see in
Linux version 2.6.18-371.4.1.el5 ([email protected]) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-54)) #1 SMP Thu Jan 30 06:09:24 EST 2014 BIOS-provided physical RAM map: BIOS-e820: 0000000000010000 - 000000000009f800 (usable) BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved) BIOS-e820: 00000000000ca000 - 00000000000cc000 (reserved) BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved) BIOS-e820: 0000000000100000 - 00000000bfef0000 (usable) BIOS-e820: 00000000bfef0000 - 00000000bfeff000 (ACPI data) BIOS-e820: 00000000bfeff000 - 00000000bff00000 (ACPI NVS) BIOS-e820: 00000000bff00000 - 00000000c0000000 (usable) BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved) BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved) BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved) BIOS-e820: 00000000fffe0000 - 0000000100000000 (reserved) BIOS-e820: 0000000100000000 - 0000000140000000 (usable) Warning only 4GB will be used. Use a PAE enabled kernel. 3200MB HIGHMEM available. 896MB LOWMEM available. found SMP MP-table at 000f6bf0 Memory for crash kernel (0x0 to 0x0) notwithin permissible range
With a 32-bit kernel, you only have 4GB of available address space. Some of this address space has to be used by the (virtual or physical) hardware in the system, such as video cards, NICs, etc., for their own purposes. This usage is usually between 256MB-1GB depending on how much address space the particular hardware needs.
Since that address space is used by hardware, the corresponding RAM is generally inaccessible to a 32-bit system.
You have a couple of options:
- The preferred option is to run a 64-bit operating system. This dramatically expands the address space, so there is plenty of room for all the RAM and hardware. It also breaks the 2GB/3GB 32-bit limit on applications while maintaining the ability to run 32-bit programs. In general, any system with 2GB of more of RAM should run a 64-bit OS to avoid these issues.
- Another option is to run a 32-bit kernel with PAE enabled. This will unhide the RAM, but each process will still be limited to 2GB/3GB of address space, depending on the particulars of the kernel build. Since 64-bit OSes will run 32-bit applications perfectly well, this has no advantage and many disadvantages (such as a lack of an upgrade path).
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