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Fresh Debian Squeeze (6) Installation - Very High Ram Usage

We have a Dell Poweredge R210 server with 8GB of installed RAM. This is a fresh install of Debian Squeeze, with pretty much only build-essentials and Xen 4.0.1 installed through aptitude. The kernel version is 2.6.32-5-xen-amd64.

1GB of our memory is "absent", and is not available and the output of dmesg | grep Memory is:

[    2.069861] Memory: 6842924k/8132468k available (3147k kernel code, 1058596k absent, 230948k reserved, 1908k data, 600k init)

Here is the output of free -m:

             total       used       free     shared    buffers     cached
Mem:          7195       1349       5846          0          3         28
-/+ buffers/cache:       1316       5878
Swap:         3813          0       3813

Whereas, if I use the original kernel (2.6.32-5-amd64), I have access to the full 8GB, despite 1GB still being absent - note the 9.5GB(!) apparently available:

[    0.000000] Memory: 8177956k/9437184k available (3079k kernel code, 1058596k absent, 200632k reserved, 1892k data, 592k init)

And free -m:

             total       used       free     shared    buffers     cached
Mem:          7995         91       7904          0          2         22
-/+ buffers/cache:         66       7929
Swap:         3813          0       3813

I am aware of the situation of absent memory being basically memory holes in the memory map https://bugzilla.redhat.com/show_bug.cgi?id=598290 - but why would a different kernel on the same hardware show such different behaviour?

So, my questions are:

1) Is this likely to be a kernel bug, and therefore should I report it?

2) Is there any hope for recovering the memory, or find out how / why the Xen kernel has taken it and fix it?

Thanks for any help!

  1. No
  2. No. Because Xen is allocating this memory for private use and also reserves s small fixed portion of every virtual address space.


  • But an eighth of the memory? Losing a full GB of RAM seems like some pretty heavy overhead for virtualization. I'm also pretty sure that our previous box took nothing like this much memory away - given, it was Ubuntu 8.04 and Xen 3.2. – James Mar 21 '11 at 11:43
  • Put xm list content here. with xentop you see all the memory ? – Sacx Mar 21 '11 at 12:08
  • Domain-0 0 7195 8 r----- 5.8 we only have dom0 at the moment. And output from xentop: Domain-0 -----r 5 0.2 6614132 78.9 no limit – James Mar 21 '11 at 12:43

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