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

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: RE: [Xen-devel] failed to start centos 5 domU with maxmem=30000
From: Vasiliy G Tolstov <v.tolstov@xxxxxxxxx>
Date: Fri, 13 Aug 2010 09:56:11 +0400
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Daniel Kiper <dkiper@xxxxxxxxxxxx>
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В Чтв, 12/08/2010 в 11:00 -0700, Dan Magenheimer пишет:
> > > > 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 ?
> I think the answer is no.  I believe there is a kernel
> data structure for each 4K page in physical memory
> I don't remember the size of this data structure, but
> assume it is 4 bytes.  That means that for 1GB of
> physical memory (as specified by maxmem) this data
> structure requires 1MB of physical memory just to track
> the 1GB.  So for your maxmem=60000, memory=60 would
> only be enough for this one kernel data structure (and
> the kernel requires many other data structures to
> be functional).

Ok. Thank's.

> Daniel Kiper is developing virtual hotplug memory for Xen
> guests.  This may be a good use case for it.

I'm try for my systems. Thank's

> > > 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 a much harder problem than it seems.  You may want
> to look at some of the presentations on Transcendent Memory
> that explain why it is hard.  (Most can be found at
> http://oss.oracle.com/projects/tmem )

Need to support in domU... this is bad. But i'm try it. 

Vasiliy G Tolstov <v.tolstov@xxxxxxxxx>

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