[Solved]3GB limit even when using 64-bit Arch?

nomorewindows · 2014-01-13 15:31:09

I have decided to install memory to the specified limit of 4GB on a machine that starts with 2GB. I can expect XP 32-bit to not see all of it as seen over and over again. The BIOS doesn't have a remap function (which degrades performance). But I PXE boot it into 32-bit and 64-bit Arch and it still only uses the same 3GB. Maybe this is all it can do.

Last edited by nomorewindows (2014-05-08 19:26:40)

henk · 2014-01-13 16:54:04

Have you checked for "hardware reserved memory"?
http://ubuntuforums.org/showthread.php?t=2177690

mich41 · 2014-01-13 18:26:12

Without memory hole remapping you obviously lose the part of physical memory shadowed by PCI devices.

nomorewindows · 2014-01-13 19:24:27

There was somebody who was recompiling the 64-bit kernel to adjust the limit upto 64G instead of 4G, but then it would just move those hardware reserved memory elsewhere? I would think that the stock kernel should be able to do this anyway.

Last edited by nomorewindows (2014-01-13 20:18:43)

mich41 · 2014-01-13 23:05:51

This memory isn't reserved, it's inaccessible to the CPU.

There are only 4GB of sub-4GB physical addresses (obviously) and about 1GB of them is devoted to memory-mapped PCI devices. This leaves 3GB of sub-4GB addresses available for bytes in memory.

If you want to use more memory, you have to divide it and map one part (at most 3GB) to some addresses below 4GB and the remaining part to addresses above 4GB - a process known as "remapping around memory hole", "memory hole remapping" or "memory hoisting".

If your hardware and/or BIOS don't support this, you are out of luck.

If running dmesg |grep e820 doesn't show memory ranges above 0x100000000, you really are out of luck. Unless you are wrong about your motherboard not supporting memory hole remapping.

Last edited by mich41 (2014-01-13 23:12:57)

nomorewindows · 2014-01-13 23:48:41

I have some machines that don't go above 0x100000000, and they won't go past 2GB.

nomorewindows · 2014-05-06 21:01:12

Someone gave me some of their old memory, with 512MB instead of 1GB sticks and the CAS all match, so well see what happens. May just be the mixed CAS. The same 3GB.

Last edited by nomorewindows (2014-05-06 21:01:58)

nomorewindows · 2014-05-08 19:26:22

Now that the memory all matches, it looks as if it is working.

nomorewindows · 2014-11-03 21:43:01

Tried some of the memory on another machine, and yes it appears that some machines have the hard limit below 4G. Running 4G on a machine that supports more than that according to dmidecode, and it gives no trouble. So it wasn't the memory, but the number of memory lines were limited.

Arch Linux

#1 2014-01-13 15:31:09

[Solved]3GB limit even when using 64-bit Arch?

#2 2014-01-13 16:54:04

Re: [Solved]3GB limit even when using 64-bit Arch?

#3 2014-01-13 18:26:12

Re: [Solved]3GB limit even when using 64-bit Arch?

#4 2014-01-13 19:24:27

Re: [Solved]3GB limit even when using 64-bit Arch?

#5 2014-01-13 23:05:51

Re: [Solved]3GB limit even when using 64-bit Arch?

#6 2014-01-13 23:48:41

Re: [Solved]3GB limit even when using 64-bit Arch?

#7 2014-05-06 21:01:12

Re: [Solved]3GB limit even when using 64-bit Arch?

#8 2014-05-08 19:26:22

Re: [Solved]3GB limit even when using 64-bit Arch?

#9 2014-11-03 21:43:01

Re: [Solved]3GB limit even when using 64-bit Arch?

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