Re: [rfc 08/45] cpu alloc: x86 support

["H. Peter Anvin" <hpa@zytor.com>]

Christoph Lameter wrote:
> 
> For the UP and SMP case map the area using 4k ptes. Typical use of per cpu
> data is around 16k for UP and SMP configurations. It goes up to 45k when the
> per cpu area is managed by cpu_alloc (see special x86_64 patchset).
> Allocating in 2M segments would be overkill.
> 
> For NUMA map the area using 2M PMDs. A large NUMA system may use
> lots of cpu data for the page allocator data alone. We typically
> have large amounts of memory around on those size. Using a 2M page size
> reduces TLB pressure for that case.
> 
> Some numbers for envisioned maximum configurations of NUMA systems:
> 
> 4k cpu configurations with 1k nodes:
> 
> 	4096 * 16MB = 64TB of virtual space.
> 
> Maximum theoretical configuration 16384 processors 1k nodes:
> 
> 	16384 * 16MB = 256TB of virtual space.
> 
> Both fit within the established limits established.
> 

You're making the assumption here that NUMA = large number of CPUs. 
This assumption is flat-out wrong.

On x86-64, most two-socket systems are still NUMA, and I would expect 
that most distro kernels probably compile in NUMA.  However,
burning megabytes of memory on a two-socket dual-core system when we're 
talking about tens of kilobytes used would be more than a wee bit insane.

I do like the concept, overall, but the above distinction needs to be fixed.

	-hpa