Re: [PATCH 4/7] ppc64 - Specify amount of kernel memory at boot time

[Mel Gorman <mel@csn.ul.ie>]

On Thu, 23 Feb 2006, Dave Hansen wrote:

> On Thu, 2006-02-23 at 18:01 +0000, Mel Gorman wrote:
> > On Thu, 23 Feb 2006, Dave Hansen wrote:
> > > OK, back to the hapless system admin using kernelcore. They have a
> > > 4-node system with 2GB of RAM in each node for 8GB total.  They use
> > > kernelcore=1GB.  They end up with 4x1GB ZONE_DMA and 4x1GB
> > > ZONE_EASYRCLM.  Perfect.  You can safely remove 4GB of RAM.
> > >
> > > Now, imagine that the machine has been heavily used for a while, there
> > > is only 1 node's memory available, but CPUs are available in the same
> > > places as before.  So, you start up your partition again have 8GB of
> > > memory in one node.  Same kernelcore=1GB option.  You get 1x7GB ZONE_DMA
> > > and 1x1GB ZONE_EASYRCLM.  I'd argue this is going to be a bit of a
> > > surprise to the poor admin.
> >
> > That sort of surprise is totally unacceptable but the behaviour of
> > kernelcore needs to be consistent on both the x86 and the ppc (any any
> > other ar. How about;
> >
> > 1. kernelcore=X determines the total amount of memory for !ZONE_EASYRCLM
> >     (be it ZONE_DMA, ZONE_NORMAL or ZONE_HIGHMEM)
>
> Sounds reasonable.  But, if you're going to do that, should we just make
> it the opposite and explicitly be easy_reclaim_mem=?  Do we want the
> limit to be set as "I need this much kernel memory", or "I want this
> much removable memory".  I dunno.

I think we should keep it at kernelcore=. If you have too little easyrclm 
memory, then hot-remove and hugetlb availability is impaired. If you 
have too little kernel memory, you have a really bad day.

> > 2. For every node that can have ZONE_EASYRCLM, split the kernelcore across
> >     the nodes as a percentage of the node size
> >
> >     Example: 4 nodes, 1 GiB each, kernelcore=512MB
> >  		node 0 ZONE_DMA = 128MB
> >  		node 1 ZONE_DMA = 128MB
> >  		node 2 ZONE_DMA = 128MB
> >  		node 3 ZONE_DMA = 128MB
> >
> >  	    2 nodes, 3GiB and 1GIB, kernelcore=512MB
> >  		node 0 ZONE_DMA = 384
> >  		node 1 ZONE_DMA = 128
> >
> > It gets a bit more complex on NUMA for x86 because ZONE_NORMAL is
> > involved but the idea would essentially be the same.
>
> Yes, chopping it up seems like the right thing (or as close as we can
> get) to me.

Ok, will rework the code to make it happen.

--
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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