makedumpfile memory usage grows with system memory size

[Michael Holzheu]

Hello Kumagai,

On Fri, 2012-04-06 at 17:09 +0900, Atsushi Kumagai wrote:
> Hello Michael,
> 
> On Mon, 02 Apr 2012 19:15:33 +0200
> Michael Holzheu <holzheu at linux.vnet.ibm.com> wrote:
> 
> > Hello Ken'ichi,
> > 
> > On Thu, 2012-03-29 at 17:09 +0900, Ken'ichi Ohmichi wrote:
> > > On Wed, 28 Mar 2012 17:22:04 -0400
> > > makedumpfile uses the system memory of 2nd-kernel for a bitmap if RHEL.
> > > The bitmap represents each page of 1st-kernel is excluded or not.
> > > So the bitmap size depends on 1st-kernel's system memory.
> > 
> > Does this mean that makedumpfile's memory demand linearly grows with 1
> > bit per page of 1-st kernel's memory?
> 
> Yes, you are right. (Precisely, 2 bit per page.)
> 
> > Is that the exact factor, if /tmp is in memory? Or is there any other
> > memory allocation that is not constant regarding the 1-st kernel memory
> > size?
> 
> bitmap file is main cause of memory consuming if 2nd kernel uses initramfs
> only. There are other parts where the size of allocated memory varies based
> on 1-st kernel memory size, but they don't have big influence.

Thanks for the explanation. 

I ask because I want to exactly calculate the required size for the
crashkernel parameter. On s390 the kdump kernel memory consumption is
fix and not dependent on the 1st kernel memory size. So based on your
explanation I, will use:

crashkernel=<base size> + <variable size>

where

<variable size> = <pages of 1st kernel> * (2 + x) / 8

where "x" is the variable makedumpfile memory allocation that is on top
of the bitmap allocation. What would be a good value for "x"?

Michael