[Xen-devel] Balloon Driver Internals

["Gaurav Dhiman" <dimanuec@xxxxxxxxx>]

Hi All,

I am quite new to Xen and am trying to understand the way balloon driver works in XenoLinux. I have the following observations and questions; please correct me if I am wrong:

1. Balloon driver adds pages between xen_start_info->nr_pages and max_pfn to the ballooned list. While nr_pages is derived from the 'memory' parameter specified while creating the domain, how is the value of max_pfn evaluated/specified?

2. The memory/target in the xenstore on domain0 indicates the target number of pages for each domain. If the value changes, watch_target function of the balloon driver gets invoked. Right? Accordingly the balloon driver increases/decreases the number of pages (using increase_reservation/decrease_reservation).

3.  It seems that increasing/decreasing reservation happens manually by modifying memory/target in xenstore. What all needs to be done to have a dynamic scenario where the domains can increase/decrease their reservation according to their requirements? Would this require modification to memory manager of guest OS?
Consider the following scenario:

• we just have 2 domains (say domain0 and domain1) each with 200MB of allocated memory (total physical memory is 1GB).
  
• we do not modify memory/target in xenstore.
• if domain1 uses up it's share (200MB), pages will get swapped out to disk even though a lot of physical memory is free.
• cant we have a case where domain1 requests increase in reservation to make use of surplus physical memory?
• or does it already happen? if yes, then how?
  

Kindly provide your views, it would be really helpful!! 

Sorry for the long mail.

Thanks in advance,
Gaurav

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