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

[ananthan <ananthannair935@xxxxxxxxx>]

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) "

dmesg: http://pastebin.com/FmVRniLJ

Question1:

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

dmesg: http://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 have something to do with with pvops kernel, or is it because of changes in Linux 3.0+ Memory Management?

Regards,

Ananthan

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