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Re: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests

To: "Mr. Teo En Ming (Zhang Enming)" <space.time.universe@xxxxxxxxx>, "Pasi Kärkkäinen" <pasik@xxxxxx>
Subject: Re: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests
From: "Nick Couchman" <Nick.Couchman@xxxxxxxxx>
Date: Mon, 09 Nov 2009 08:06:54 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, xen-users@xxxxxxxxxxxxxxxxxxx, Robert Dunkley <Robert@xxxxxxxxx>
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>>> On 2009/11/09 at 05:05, Pasi Kärkkäinen<pasik@xxxxxx> wrote: 
> On Mon, Nov 09, 2009 at 08:01:00PM +0800, Mr. Teo En Ming (Zhang
Enming) 
> wrote:
>>    No, I didn't limit dom0 memory in grub.conf.
>> 
> 
> You should. 

Really?  I thought current conventional wisdom was to allow Xen to
self-manage memory in both dom0 and domUs, and not to manually adjust
this?  I run several Xen systems with anywhere from 8 to 24 GB of RAM
and 20 to 30 domUs on some of these systems and have *never* specified
the dom0 memory at boot time - the Xen ballooning has always functioned
perfectly fine, and never crashed my dom0.  Furthermore, while I'm not
Linux developer and so not familiar with how Linux calculates buffering
and caching, I do know that my Linux systems dynamically manage buffers
and caches, and when memory is reduced or some application requires a
larger amount of physical memory, Linux reduces the amount of data in
buffers and caches.

Of course, a lot of this depends on what you're doing in dom0 - on my
Xen servers, my dom0 is strictly for Xen management - I'm not running
anything else in dom0 that would require large amounts of memory, memory
buffers and caches, etc.

-Nick

> 
> If dom0 has all the memory at boot time, you need to balloon down
dom0 
> memory every time you create a new guest - this can (and will) cause

> problems with the dom0 linux kernel.
> 
> Linux calculates some internal parameters/buffers/values based on
the
> _boot time_ amount of memory. And when the amount of memory goes down
to
> only a small fraction of that while creating new guests bad things
can
> happen..
> 
> It still shouldn't crash though.. I bet your problem will get fixed
when
> you limit the dom0 memory to say dom0_mem=512M and reboot.
> 
> -- Pasi
> 
>>    Here's my xm info output after I have shutdown all the virtual
machines.
>> 
>>    [root@fedora11-x86-64-host ~]# xm list
>>    Name                                        ID   Mem VCPUs     
State
>>    Time(s)
>>    Domain-0                                     0  2812     2    
r-----
>>    3242.5
>>    [root@fedora11-x86-64-host ~]# xm info
>>    host                   : fedora11-x86-64-host
>>    release                : 2.6.30-rc3-enming.teo-tip
>>    version                : #1 SMP Wed Aug 19 23:14:15 SGT 2009
>>    machine                : x86_64
>>    nr_cpus                : 2
>>    nr_nodes               : 1
>>    cores_per_socket       : 2
>>    threads_per_core       : 1
>>    cpu_mhz                : 2800
>>    hw_caps                :
>>   
bfebfbff:20100800:00000000:00000140:0400e3bd:00000000:00000001:00000000
>>    virt_caps              : hvm hvm_directio
>>    total_memory           : 6039
>>    free_memory            : 3124
>>    node_to_cpu            : node0:0-1
>>    node_to_memory         : node0:3124
>>    xen_major              : 3
>>    xen_minor              : 5
>>    xen_extra              : -unstable
>>    xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p
hvm-3.0-x86_32
>>    hvm-3.0-x86_32p hvm-3.0-x86_64
>>    xen_scheduler          : credit
>>    xen_pagesize           : 4096
>>    platform_params        : virt_start=0xffff800000000000
>>    xen_changeset          : Tue Sep 01 11:34:31 2009 +0100 
> 20143:a7de5bd776ca
>>    xen_commandline        : iommu=1
>>    cc_compiler            : gcc version 4.4.1 20090725 (Red Hat
4.4.1-2)
>>    (GCC)
>>    cc_compile_by          : root
>>    cc_compile_domain      : (none)
>>    cc_compile_date        : Thu Sep 10 07:01:13 SGT 2009
>>    xend_config_format     : 4
>> 
>>    --
>>    Mr. Teo En Ming (Zhang Enming) Dip(Mechatronics)
BEng(Hons)(Mechanical
>>    Engineering)
>>    Alma Maters:
>>    (1) Singapore Polytechnic
>>    (2) National University of Singapore
>>    My Primary Blog:
[1]http://teo-en-ming-aka-zhang-enming.blogspot.com
>>    My Secondary Blog: [2]http://enmingteo.wordpress.com
>>    My Youtube videos: [3]http://www.youtube.com/user/enmingteo
>>    Email: [4]space.time.universe@xxxxxxxxx
>>    Mobile Phone (Starhub Prepaid): +65-8369-2618
>>    Street: Bedok Reservoir Road
>>    Country: Singapore
>> 
>>    On Mon, Nov 9, 2009 at 7:54 PM, Pasi Kärkkäinen <[5]pasik@xxxxxx>
wrote:
>> 
>>      On Mon, Nov 09, 2009 at 06:52:37PM +0800, Mr. Teo En Ming
(Zhang 
> Enming)
>>      wrote:
>>      >    Hi,
>>      >
>>      >    Please watch this 4-minute video at
>>      >    [1][6]http://www.youtube.com/watch?v=LbLaPpwNAx4
>>      >
>>      >    I have only started 3 HVM Linux guests with 1 GB ram each.
I can't
>>      start
>>      >    the 4th HVM guest. If I attempt to start the 4th instance,
it will
>>      crash
>>      >    dom0.
>>      >
>>      >    Are there anything in the xm dmesg output that could
explain the
>>      low limit
>>      >    to the number of VMs that I could start before dom0
becomes
>>      unresponsive?
>>      >
>> 
>>      Have you limited dom0 memory (by specifying dom0_mem=XMB option
in
>>      grub.conf for xen.gz) ?
>> 
>>      What does "xm info" say about free memory before starting any
guests?
>>      -- Pasi
>> 
>> References
>> 
>>    Visible links
>>    1. http://teo-en-ming-aka-zhang-enming.blogspot.com/
>>    2. http://enmingteo.wordpress.com/
>>    3. http://www.youtube.com/user/enmingteo
>>    4. mailto:space.time.universe@xxxxxxxxx
>>    5. mailto:pasik@xxxxxx
>>    6. http://www.youtube.com/watch?v=LbLaPpwNAx4



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