RE: [Xen-users] RE: RE: RE: RE: can XEN be extended ?

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of anant
> Sent: 30 January 2007 10:55
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] RE: RE: RE: RE: can XEN be extended ?
> 
> 
> 
> 
> >Where are you running openWBEM? In Dom0 it should give you 
> real hardware
> >information. 
> 
> Yeah openWBEM is runnning in dom0
> 
> >In Dom0 it should give you real hardware
> >information. 
> 
> I am bit confused here. I was looking for a proof whether it 
> gives real
> value or no...
> 
> Well i was monitoring the memory value......i.e.
> 
> Actual Memory = 1024000 KB RAM approx.
> 
> after entering command cat /proc/meminfo over terminal.. i got
> 
> Total Memory = 953344 KB [loss of 70656KB memory, that means this much
> amount of memory is consumed by running OS] 

70 MB used by the OS seems about right. 

> Free Memory = 560828 KB  
> 
> Now started openWBEM 
> 
> here Total memory = 560828 KB 
>        Free memory = 444608 KB [loss of 116220 KB memory, 
> that means this
> much amount of  memory took my openWBEM to get running]

Not necessarily - see below. 

> 
> Now closed openWBEM and started Virtual OS on top of XEN 
> Memory allocated to Virtual OS = 400MB
> 
> now again checking memory
> 
> Terminal -> 
> Total Memory = 606208 KB
> Free Memory = 15628 KB 
> 
> OpenWBEM ->
> Total Memory = 606208 KB
> Free Memory = 8256KB [This time there is loss of 7372KB 
> memory, that means
> this time openWBEM tool this much memory to get running]
> 
> Why is the difference in this free memory all the 
> time....Does it makes any
> sence to you...?
> Do you think the value is of real hardware...or no....?

"Memory" is not something that you can "measure" realisticly from within
even Dom0, as it's hidden from even Dom0 how much memory there REALLY is
in the machine (using the dom0_mem=xxx will make Dom0 believe that the
actual amount of memory is xxx). Only the hypervisor knows how much
memory there REALLY is in the system. "xm info" will tell you that. 

Next comes "free" memory. That's very much like measuring the length of
a rubber-band. It depends on what you're doing with the machine, how
much memory is free. And the fact that there is NO free memory doesn't
necessarily mean that you can't use more memory either - because there
is swap to store "not currently needed" data/code in. 

The OS will deal with free memory in various ways, including using it
for buffers for disk-caching and freeing memory in reserve for new
applications in case of "low memory" situations. Also, loading an
application will drag in shared libraries (.so files in Linux, .dll in
Windows). These may be kept resident in memory if there's no demand for
more memory (this is why starting large apps a second time is often
quicker than the first time - the shared libraries have been loaded, and
don't need to be loaded again). Of course, if you have just loaded 100MB
of shared libraries into memory, and there's no demand for memory at the
present time, it's going to show 100MB less than it did before. If you
then load something that requires a large amount of memory but not using
much of the shared libraries, some unused libraries may be unloaded and
when you quit the large-amount-of-memory app, the amount of free memory
is larger than it was before you started the app (because the shared
libraries that are still not needed are no longer in memory). 

And just to add complexity, the xend, xenstore and other xen components
may well use some amount of memory within Dom0 for each domain that you
create, so the amount of free memory is definitely expected to reduce
there, even if all other uses of memory stay constant.

--
Mats
> 
> Thanks
> 
> 
> -- 
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> 
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