Re: bigphysarea support in 2.2.19 and 2.4.0 kernels

Jeff V. Merkey
2000-12-22T20:01:27Z

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