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Re: [Xen-devel] failed to start centos 5 domU with maxmem=30000

To: v.tolstov@xxxxxxxxx
Subject: Re: [Xen-devel] failed to start centos 5 domU with maxmem=30000
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 12 Aug 2010 11:34:10 -0700
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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 On 08/12/2010 05:24 AM, Vasiliy G Tolstov wrote:
В Чтв, 12/08/2010 в 05:17 -0700, Dan Magenheimer пишет:
From: Vasiliy G Tolstov [mailto:v.tolstov@xxxxxxxxx]
Sent: Thursday, August 12, 2010 5:38 AM
To: Pasi Kärkkäinen
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] failed to start centos 5 domU with

В Чтв, 12/08/2010 в 14:37 +0300, Pasi Kärkkäinen пишет:
On Thu, Aug 12, 2010 at 02:24:05PM +0400, Vasiliy G Tolstov wrote:
Hello. I'm try to start centos domU pvm under sles 11 sp1 xen
with memory=512 and maxmem=30000. Centos doing boot and freeze.
If i'm try to use maxmem=15000 all works.

Can You tell me what's wrong?

Is the centos PV domU 32bit or 64bit ?
64 bit kernel-xen.

I found this in xen sources:

/* Simple continuous piecewiese linear function:
          *  max MiB ->  min MiB  gradient
          *       0         0
          *      16        16
          *      32        24
          *     128        72    (1/2)
          *     512       168    (1/4)
          *    2048       360    (1/8)
          *    8192       552    (1/32)
          *   32768      1320
          *  131072      4392

Why this is not provided in documentation or on web site?
Hi Vasily --

This function limits how far memory can be reduced when
ballooning a guest (including dom0).  It is only a heuristic
but is intended to take into account the various overheads
a guest Linux kernel requires to manage memory to avoid
out-of-memory conditions.

But I think you are correct... the same (or similar)
function should be published as it also serves as a
guideline for the ratio between memory= and maxmem=
parameters when creating a guest:  If the ratio
of maxmem divided by memory is too high, the guest
will not even boot.

Is that possible to use memory=32 and maxmem=60000 ?

A factor of ~2000 between smallest and largest size won't work. The problem is that lots of kernel structures are scaled with the amount of boot-time memory, and they'll simply be overloaded when you add more memory. For example, the page structure array for 60GB needs ~15.3 million entries, which at 64 bytes per entry will take ~980MB...

I am curious as to why you would specify memory= so
much smaller than maxmem=.  Are you trying to overcommit
memory for guests that are often idle but sometimes use
a very large amount of memory?
We want to provide ability to use small as possible memory if the guest
is idle. And much as possible when the guest under heavy load.

This is exactly the use-case for the hotplug memory stuff Daniel is working on - which I guess is why you've been testing it.


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