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[Xen-devel] PV Memory allocated difference in kernel 2.6.18 and 3.2

This is a very old doubt i am having and hope some one will clarify it



  This is an old question i asked in serverfault but no one could answer it properly.

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

  • Hypervisor: Xen 4.2Â
  • 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 doesÂfreeÂshow exact memory, whileÂdmesgÂclearly shows 2.5MB for kernel code and 23MB as reserved?

  • VM2:ÂUbuntu 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 has anything to do with with pvops kernel or is it because of changes in Linux 3.0+ Memory Management?

No memory wastage was one of the reason i liked and promoted ÂPV guests now i wish i could explain the reason to others.


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