RE: [Xen-devel] linux kernel 2.6.37

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Vasiliy G Tolstov [mailto:v.tolstov@xxxxxxxxx]
> Sent: Tuesday, January 25, 2011 10:35 AM
> To: Konrad Rzeszutek Wilk
> Cc: Don Brearley; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] linux kernel 2.6.37
> 
> On Tue, 2011-01-25 at 09:45 -0500, Konrad Rzeszutek Wilk wrote:
> >  I think I know why this is happening = the
> > kernel allocates pagetables as if you have 4GB of memory and that
> eats much of low memory.
> > If you decrease the 'maxmem' you should see a smaller amount being
> used.
> >
> 
> And why xenlinux kernel from centos (2.6.18-194.26.1.el5xen) displays
> all memory as available:
> 
> cat /proc/meminfo under 2.6.37 (iommu=off acpi=off):
> MemTotal:         447220 kB
> 
> cat /proc/meminfo under 2.6.18-194.26.1.el5xen:
> MemTotal:       524288 kB
> 
> ?

FYI, I've seen a similar problem with 2.6.37 but noticed
it when manually ballooning a PV guest.  For example:

# echo $((256*1024)) > /sys/devices/system/xen_memory/xen_memory0/target_kb

should reduce memory to 256M (or at least somewhere close)
but instead /proc/meminfo shows 217812 kB, or about 15%
lower than expected.  This looks like approximately the
same percentage of memory loss that Vasiliy is reporting
on boot.

/me wonders if it might be related to this thread:

http://lists.xensource.com/archives/html/xen-devel/2010-09/msg00987.html 

assuming the related patch got upstream.  Jeremy cc'ed just in case.

I do NOT see this issue with 2.6.35.4.  In fact, I now see
a MemTotal boot-time difference when I boot both with mem=512
(in vm.cfg): 2.6.35.4 kernel shows 506864, 2.6.37 shows 495456.
Not as much as Vasiliy reports, but still a big mystery.

If this problem is real and widespread, I hope nobody is comparing
KVM vs Xen (or Xen vs native) on 2.6.37 as that much memory loss
could result in a significant performance loss for a Xen guest.

Dan

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