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Date: Fri, 22 Dec 2000 13:25:41 -0700
From: "Jeff V. Merkey" <jmerkey@vger.timpanogas.org>
To: Andi Kleen <ak@suse.de>
Cc: Pauline Middelink <middelin@polyware.nl>, linux-kernel@vger.kernel.org,
        jmerkey@timpanogas.org
Subject: Re: bigphysarea support in 2.2.19 and 2.4.0 kernels
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In-Reply-To: <20001222202137.A27844@gruyere.muc.suse.de>; from ak@suse.de on Fri, Dec 22, 2000 at 08:21:37PM +0100
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On Fri, Dec 22, 2000 at 08:21:37PM +0100, Andi Kleen wrote:
> On Fri, Dec 22, 2000 at 11:35:30AM -0700, Jeff V. Merkey wrote:
> > The real question is how to guarantee that these pages will be contiguous
> > in memory.  The slab allocator may also work, but I think there are size
> > constraints on how much I can get in one pass.
> 
> You cannot guarantee it after the system has left bootup stage. That's the
> whole reason why bigphysarea exists.
> 
> -Andi

I am wondering why the drivers need such a big contiguous chunk of memory.
For message passing operatings, they should not.  Some of 
the user space libraries appear to need this support.  I am going through 
this code today attempting to determine if there's a way to reduce this 
requirement or map the memory differently.   I am not using these cards
for a ccNUMA implementation, although they have versions of these 
adapters that can provide this capability, but for message passing with 
small windows of coherence between machines with push/pull DMA-style
behavior for high speed data transfers.  99.9% of the clustering 
stuff on Linux uses this model, so this requirement perhaps can be
restructured to be a better fit for Linux.

Just having the patch in the kernel for bigphysarea support would solve
this issue if it could be structured into a form Alan finds acceptable.
Absent this, we need a workaround that's more tailored for the 
requirments for Linux apps.

Jeff

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