On Fri, 12 Oct 2018, Andrew Morton wrote: > > 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. There is a benefit because the management overhead is halved. > > Instead, it is better to allocate order-0 memory if possible so that pages > > can be returned from the per-cpu pagesets (pcp). Have a benchmark that shows this? > > > 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. The slab allocators generally buffer pages from the page allocator to avoid this effect given the slowness of page allocator operations anyways. > 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*? Potentially. > > 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. Well does this actually do some good?
