On Mon, Apr 13, 2020 at 10:53:03PM +1000, Nicholas Piggin wrote: > For platforms that define HAVE_ARCH_HUGE_VMAP and support PMD vmap mappings, > have vmalloc attempt to allocate PMD-sized pages first, before falling back > to small pages. Allocations which use something other than PAGE_KERNEL > protections are not permitted to use huge pages yet, not all callers expect > this (e.g., module allocations vs strict module rwx). > > This gives a 6x reduction in dTLB misses for a `git diff` (of linux), from > 45600 to 6500 and a 2.2% reduction in cycles on a 2-node POWER9. > > This can result in more internal fragmentation and memory overhead for a > given allocation. It can also cause greater NUMA unbalance on hashdist > allocations. > > There may be other callers that expect small pages under vmalloc but use > PAGE_KERNEL, I'm not sure if it's feasible to catch them all. An > alternative would be a new function or flag which enables large mappings, > and use that in callers. Why do we even use vmalloc in this case rather than just doing a huge page allocation? What callers are you intersted in?
