Xen users - Jul 2010 - [XCP] Strange bug this avaible memory for guests

I got really strange situation with memory on host:


xe host-list params=memory-free
memory-free ( RO)    : 10305536
(10Mb)

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

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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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.
> 
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
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Thank you.

I think, I found an source of problem:


name-label ( RW)            : one
           power-state ( RO): running
         memory-actual ( RO): 8589934592
    memory-dynamic-max ( RW): 11739856896
    memory-dynamic-min ( RW): 11739856896
     memory-static-min ( RW): 8589934592

(11739856896 - 8589934592 is my lost 3Gb of memory)

I do not understand one thing: how memory-actual can be actualy lower,
than memory-dynamic-min?

How I do this:

1) I boot machine with memory-dynamic-max/min about 11 Gb. 
2) I found I have no memory.
3) I decide to reduce memory to lower values:

xe vm-memory-dynamic-range-set vm=one max=8GiB min=8GiB
(no errors).

After few retries I got reverse situation:

name-label ( RW)            : one
           power-state ( RO): running
         memory-actual ( RO): 11739856896
    memory-dynamic-max ( RW): 9663676416
    memory-dynamic-min ( RW): 9663676416
     memory-static-min ( RW): 8589934592

And I think this is REALLY bug, because memory-actual shall not be
higher, than memory-dynamic-max.


... and I think (I''m not sure) this related to second bug I wrote few
days ago about impossibility to raize dynamic-max over value, set up
prior to VM statup. I reconfirm it at XCP 0.11, XCP 0.5RC3 and XCP0.5,
so I''ll repost it in separate letter.

В Срд, 07/07/2010 в 20:40 +0100, Dave Scott пишет:
> 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.
> > 
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@lists.xensource.com
> > http://lists.xensource.com/xen-users

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