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
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