On Fri, 12 Oct 2018 14:24:57 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> wrote: > 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. Confused. Higher-order slab pages never go through the pcp lists, do they? I'd have thought that by tending to increase the amount of order-0 pages which are used by slab, such stranding would be *increased*? > 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. > > mm/slab.c | 15 +++++++++++++++ Do slub and slob also suffer from this effect?
