4GB RAM -> Missing a Gig?!

[Cyker]

I just nabbed a couple of 1GB DDR sticks which a friend was throwing out (), so now I have 4GB! \o/

Now, previously, my server had 3GB - On swapping out the 2x512's for 2x1GBs, the usual only-3.2GB-or-so-visible thing kicked in, so I went to the BIOS, changed a few things around (MTRR: Continuous->Discrete, S/W MemHole Remapping->Enabled) and the BIOS could now see the whole lot!

Booted Linux, checked dmesg:

3200MB HIGHMEM available. (Used to be 2175MB HIGHMEM available.)
896MB LOWMEM available.

Woo!

But then when it finishes booting up, I find that actually I only have 3290M available

Any ideas where the rest went, and how/if I can get it back?

And before anyone says 64-bit, yes yes that is coming along but I can't even start it until I have a big stretch of time (Like... Xmas??! ), but in the *meantime*, how can I go about recovering this space?

The server has no video card aside from an old PCI 4MB ATI Mach64V, so it's not the V-RAM stealing the space, which is the usual culprit!

[NeddySeagoon]

Cyker,

You have various memory mapped hardware items mapped into the top of the 4G address space, e.g. The BIOS, the PCI space which you cannot map out, well, not if you want them to use them anyway.

The RAM you just fitted cannot be addressed here - you get your BIOS/Hardware instead.
Motherboards do two things - throw the excess RAM away by hiding it from you, often startting at a fairly arbitaray sounding address like 3.2G.
64 bit systems map it into the the address space above 4G and can use it there. As you have a 32 bit install, that does not apply to you.

You can try to turn on 64G himem in the kernel but this comes at a performance price because the kernel uses Physical Address space Extension to page memory into the 4G space you can use, a little like Expanded Memory on 286 and older systems.
Its not worth bothering with until you can go 64 bit
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[Cyker]

Awww...

Thanks for such a quick reply; I was hoping I could shrink that reserved area a bit, since this thing has virtually no peripherals.

I even removed anything that used mmapio in the kernel! O.o

I might try the 64GB mode just to see what the difference is, but on reflection I think I might be better off putttng the RAM in my desktop system :/

(Or find a cheap cool quiet motherboard that can take 8+GB of DDR1 >:P)

[Cyker]

Curiously, setting the BIOS back to MTRR Continuous and disabling the memory hole remap gives me 3408MB of usable memory vs the 3290MB, despite the BIOS saying 3440MB available, vs ~3980MB. Weird!

Haven't tried PAE yet 'tho; Having the BIOS set to Discrete + Remap enabled seems to be causing weird stability issues... smells like the A8N32 w/ remapped hole 4GB is causing some weird data corruption?!

[eccerr0r]

I've found I have to use 64GB PAE to access all 4GB RAM I have in my Foxconn G9657MA (Core2 E6700 CPU) motherboard. I don't think it's motherboard dependent, if it has only 4GB of address space to work with, you can't fit in any io address space or deal with shared/kernel memory.

I've not noticed a large penalty to using 64G PAE - but the main reason for this is that I do not typically run two processes that are big enough to require page flipping between them. There is a penalty to get to kernel code, and thus there is a small penalty that I do pay for 64G PAE but I can't really tell it's there (my E6700 is still faster than my Q6600 in single-thread apps, and maybe I should swap motherboards for these machines as the Q6600's m/b only supports 2GB...).
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Intel Core i7 2700K/Radeon R7 250/24GB DDR3/256GB SSD
What am I supposed watching?

[Cyker]