linux-kernel - Re: [PATCH 0/8] Create ZONE_MOVABLE to partition memory between movable and non-movable pages

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Message-Id: <20070129143654.27fcd4a4.akpm@osdl.org>
Date:	Mon, 29 Jan 2007 14:36:54 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Christoph Lameter <clameter@...r.sgi.com>
Subject: Re: [PATCH 0/8] Create ZONE_MOVABLE to partition memory between
 movable and non-movable pages

On Mon, 29 Jan 2007 13:54:38 -0800 (PST)
Christoph Lameter <clameter@....com> wrote:

> On Fri, 26 Jan 2007, Andrew Morton wrote:
> 
> > > The main benefit is a significant simplification of the VM, leading to 
> > > robust and reliable operations and a reduction of the maintenance 
> > > headaches coming with the additional zones.
> > > 
> > > If we would introduce the ability of allocating from a range of 
> > > physical addresses then the need for DMA zones would go away allowing 
> > > flexibility for device driver DMA allocations and at the same time we get 
> > > rid of special casing in the VM.
> > 
> > None of this is valid.  The great majority of machines out there will
> > continue to have the same number of zones.  Nothing changes.
> 
> All 64 bit machine will only have a single zone if we have such a range 
> alloc mechanism. The 32bit ones with HIGHMEM wont be able to avoid it, 
> true. But all arches that do not need gymnastics to access their memory 
> will be able run with a single zone.

What is "such a range alloc mechanism"?

> > That's all a real cost, so we need to see *good* benefits to outweigh that
> > cost.  Thus far I don't think we've seen that.
> 
> The real savings is the simplicity of VM design, robustness and 
> efficiency. We loose on all these fronts if we keep or add useless zones. 
> 
> The main reason for the recent problems with dirty handling seem to be due 
> to exactly such a multizone balancing issues involving ZONE_NORMAL and 
> HIGHMEM. Those problems cannot occur on single ZONE arches (this means 
> right now on a series of embedded arches, UML and IA64). 
> 
> Multiple ZONES are a recipie for VM fragility and result in complexity 
> that is difficult to manage.

Why do I have to keep repeating myself?  90% of known FC6-running machines
are x86-32.  90% of vendor-shipped kernels need all three zones.  And the
remaining 10% ship with multiple nodes as well.

So please stop telling me what a wonderful world it is to not have multiple
zones.  It just isn't going to happen for a long long time.  The
multiple-zone kernel is the case we need to care about most by a very large
margin indeed.  Single-zone is an infinitesimal corner-case.



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