The slab allocator has a heuristic that checks whether the internal fragmentation is satisfactory and, if not, increases cachep->gfporder to try to improve this.
If the amount of waste is the same at higher cachep->gfporder values, there is no significant benefit to allocating higher order memory. There will be fewer calls to the page allocator, but each call will require zone->lock and finding the page of best fit from the per-zone free areas. Instead, it is better to allocate order-0 memory if possible so that pages can be returned from the per-cpu pagesets (pcp). There are two reasons to prefer this over allocating high order memory: - allocating from the pcp lists does not require a per-zone lock, and - this reduces stranding of MIGRATE_UNMOVABLE pageblocks on pcp lists that increases slab fragmentation across a zone. We are particularly interested in the second point to eliminate cases where all other pages on a pageblock are movable (or free) and fallback to pageblocks of other migratetypes from the per-zone free areas causes high-order slab memory to be allocated from them rather than from free MIGRATE_UNMOVABLE pages on the pcp. Signed-off-by: David Rientjes <[email protected]> --- mm/slab.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/mm/slab.c b/mm/slab.c --- a/mm/slab.c +++ b/mm/slab.c @@ -1748,6 +1748,7 @@ static size_t calculate_slab_order(struct kmem_cache *cachep, for (gfporder = 0; gfporder <= KMALLOC_MAX_ORDER; gfporder++) { unsigned int num; size_t remainder; + int order; num = cache_estimate(gfporder, size, flags, &remainder); if (!num) @@ -1803,6 +1804,20 @@ static size_t calculate_slab_order(struct kmem_cache *cachep, */ if (left_over * 8 <= (PAGE_SIZE << gfporder)) break; + + /* + * If a higher gfporder would not reduce internal fragmentation, + * no need to continue. The preference is to keep gfporder as + * small as possible so slab allocations can be served from + * MIGRATE_UNMOVABLE pcp lists to avoid stranding. + */ + for (order = gfporder + 1; order <= slab_max_order; order++) { + cache_estimate(order, size, flags, &remainder); + if (remainder < left_over) + break; + } + if (order > slab_max_order) + break; } return left_over; }
