Re: [Xen-users] Dom0 seeing 2Gb, but not 4GB ram

[Tom Brown <tbrown@xxxxxxxxxxxxx>]

>
> CONFIG_HIGHMEM4G won't fix your problem: access to 4Gigs of RAM isn't enough
> on your system.  See below...
>
> > Nov  9 19:02:03 localhost kernel:  BIOS-e820:
> > 0000000000100000 - 0000000080000000 (usable)
>
> OK, that's about 2 gig of your memory, mapped into a sane place in the
> physical address space...
>
> > Nov  9 19:02:03 localhost kernel:  BIOS-e820:
> > 00000000ff780000 - 0000000100000000 (reserved)
>
> And this is an enormous hole, taking up the next 2 gigs of your physical
> address space.
>
> Even with 4G himem, your CPU will only be able to access this far: 2Gigs of
> your real memory, 2Gigs that your BIOS (for some reason) has reserved for IO
> regions.
>
> This means that your other 2Gig is mapped too high in the physical address
> space: with 4G himem there is simply no way to address it.
>
> > Nov  9 19:02:03 localhost kernel:  BIOS-e820:
> > 0000000100000000 - 0000000180000000 (usable)
>
> And here's the other 2Gig.  This is mapped from 4Gig-6Gig in your
physical
> address space, so isn't mappable without PAE support.  With PAE you can
> easily address this range, so the memory becomes usable for you.
>
> I've never *seen* a memory hole 2gigs in size.  Maybe there's a reason for it
> or maybe I just had a sheltered upbringing.


I just booted a box like this :-)


   Linux version 2.4.29 (root@xxxxxxxxxxxxxxxxxxxx) (gcc version 2.96
   20000731 (Red Hat Linux 7.1 2.96-81)) #3 Wed Oct 12 15:11:08 PDT 2005
   BIOS-provided physical RAM map:
    BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
    BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
    BIOS-e820: 00000000000e8000 - 0000000000100000 (reserved)
    BIOS-e820: 0000000000100000 - 000000007fff0000 (usable)
    BIOS-e820: 000000007fff0000 - 000000007ffff000 (ACPI data)
    BIOS-e820: 000000007ffff000 - 0000000080000000 (ACPI NVS)
    BIOS-e820: 00000000ff780000 - 0000000100000000 (reserved)
    BIOS-e820: 0000000100000000 - 0000000180000000 (usable)
   Warning only 4GB will be used.
   Use a PAE enabled kernel.
   3200MB HIGHMEM available.
   896MB LOWMEM available.
   On node 0 totalpages: 1048576
<snip>
   Memory: 2044840k/4194304k available (1518k kernel code, 51860k reserved,
   487k data, 284k init, 1179584k highmem)
<snip>

so, (for the reasons Mark outlined) the box is only showing 2 gig
of memory on a kernel compiled for 4 gig...  However it _did_ say
there was an issue, and what one solution is.

Now, if you look at the motherboard manual for this dual opteron
system, you will see that half the memory is connected to one
processor, and the other half of the memory is connected to the
other processor. Since I only installed the memory a few hours
ago, the image is fairly fresh in my mind :)

Anyhow, I thought I would bring it up.
The motherboard is described here:

  http://www.supermicro.com/Aplus/motherboard/Opteron/8132/H8DAR-T.cfm

manual:
  http://www.supermicro.com/manuals/motherboard/A_8132/MNL-H8DAR-TE.pdf

I haven't had a chance to do much more than boot it, but while I can say
that the Opteron supermicro products aren't quite up to the standard of
their P4/Xeon mainstream products (they just aren't as polished -- the
fans are loose, there was no system manual, and I'm ticked that heatsinks
seem hard to get), I expect that we (baremetal.com) will focus on them...

  http://www.supermicro.com/Aplus/system/1U/1020/AS-1020A-T.cfm

I have a pair of 270's and 4 gig of memory (hopefully 8 before I
go into production) in this box. Will report if we have any
trouble (I don't expect to).

I believe that we can "just" run a 64 bit XEN and 64 bit kernel
(e.g. amd64) with our normal 32 bit Linux distribution? I will
know soon enough.

-Tom



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