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

[mad Engineer <themadengin33r@xxxxxxxxx>]

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

Thanks,

Hi,

  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) "

dmesg:Âhttp://pastebin.com/FmVRniLJ

Question1:

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

dmesg:Âhttp://pastebin.com/jgNfcxc5

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

Question2:

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.

ThanksÂ

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