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[Xen-users] Allocated Memory difference in xen kernel and pvops kernel on DomU

I asked this question in many forums and IRC,but didnt get proper explanation. 

Can someone explain to me why there is a memory difference in Guests with the same configuration?

  • Hypervisor: xen 4.2 64 bit
  • VM1: Centos 5.7 with 2.6.18-274.el5xen

Assigned memory from xen is exactly the same as memory inside guest, 524288k (512m):

free -k
         total       used       free     shared    buffers     cached
Mem:        524288     445004      79284          0      18040     182652
-/+ buffers/cache:     244312     279976
Swap:       524280          0     524280

despite showing:

" Memory: 500984k/524288k available (2533k kernel code, 23212k reserved, 1746k data, 196k init) "



Why does free show exact memory, while dmesg clearly showed 2.5MB for kernel code and 23MB as reserved?

  • VM2: Ubuntu 12.04 ,3.2.0-48-

For the same resources Ubuntu 12.04 with pvopskernel is showing 501236k.

free -k
         total       used       free     shared    buffers     cached
Mem:        501236     147188     354048          0      12588      87668
-/+ buffers/cache:      46932     454304
Swap:            0          0          0


Memory: 481820k/524288k available (6553k kernel code, 448k absent, 42020k reserved, 6642k data, 924k init)


What might be the reason for this behavior, can this have something to do with with pvops kernel, or is it because of changes in Linux 3.0+ Memory Management?



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