Messages in this thread | | | Date | Mon, 15 Oct 2018 22:41:03 +0000 | From | Christopher Lameter <> | Subject | Re: [patch] mm, slab: avoid high-order slab pages when it does not reduce waste |
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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?
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