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Any suggestions? Adding mem=4G to grub doesn`t help.

             total       used       free     shared    buffers     cached
Mem:       2538496     299684    2238812          0       7380      34328
-/+ buffers/cache:     257976    2280520
Swap:      9896000          0    9896000

uname -a
Linux 2.6.26-1-xen-amd64 #1 SMP Fri Mar 13 21:39:38 UTC 2009 x86_64 GNU/Linux

grub> displaymem
 EISA Memory BIOS Interface is present
 Address Map BIOS Interface is present
 Lower memory: 640K, Upper memory (to first chipset hole): 3072K
 [Address Range Descriptor entries immediately follow (values are 64-bit)]
   Usable RAM:  Base Address:  0x0 X 4GB + 0x0,
      Length:   0x0 X 4GB + 0xa0000 bytes
   Reserved:  Base Address:  0x0 X 4GB + 0xa0000,
      Length:   0x0 X 4GB + 0x60000 bytes
   Usable RAM:  Base Address:  0x0 X 4GB + 0x100000,
      Length:   0x0 X 4GB + 0x300000 bytes
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migrated from Sep 22 '10 at 16:01

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Your kernel must be 32bit without PAE, enable PAE or switch to a 64 bit kernel.

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It could also be that your xen domU servers are using 2GB and a dom0 only shows you usable memory excluding what is allocated to a domU.

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