RE: [Xen-users] [XCP] Strange bug this avaible memory for guests

[Dave Scott <Dave.Scott@xxxxxxxxxxxxx>]

Hi George,

I think I can explain this one, comments inline:
> I got really strange situation with memory on host:
> 
> 
> xe host-list params=memory-free
> memory-free ( RO)    : 10305536
> (10Mb)

I bet this is the same value as "free_pages" (suitably converted) as shown by
  /opt/xensource/debug/xenops physinfo

This represents the actual free memory on the host at this point in time. I bet 
you've configured VM.dynamic_min and VM.dynamic_max so that most memory is 
being shared between the VMs. The ballooning daemon (squeezed) always keeps 
9-10MiB free on the host because (IIRC)
* only free memory with physical addresses < 4 GiB (IIRC) can be used for 
certain xen structures
* xen allocates memory from the highest physical address downwards
=> therefore the 10MiB we keep free will be 10MiB with low physical addresses 
which can be used for certain xen structures

I think before we kept this memory free we got into bad states where there was 
lots of memory free but you couldn't create a domain. If we had something like 
memory defrag then we could let this number drop to 0, which would be ideal 
because it would represent maximum memory utilization.

> 
> xe host-compute-free-memory
> 3147059200
> (3Gb)

I had to look into the code to understand this one :) This function is 
calculating, "the maximum amount of memory we *could* free if we ballooned 
every domain down to its VM.dynamic_min". This is the amount of memory which is 
considered when deciding whether we could in theory start another VM.

Does that help explain what's going on or are there still discrepancies?

Cheers,
Dave
> 
> A huge difference! And xentop show different numbers too!
> 
> I will keep this host online "as is" and I am ready to provide
> information for debbugging or perform needed actions.
> 
> --
> wBR,George.
> 
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