On 04/17/2018 04:45 PM, Christopher Lameter wrote: > On Mon, 16 Apr 2018, Mikulas Patocka wrote: > >> This patch introduces a flag SLAB_MINIMIZE_WASTE for slab and slub. This >> flag causes allocation of larger slab caches in order to minimize wasted >> space. >> >> This is needed because we want to use dm-bufio for deduplication index and >> there are existing installations with non-power-of-two block sizes (such >> as 640KB). The performance of the whole solution depends on efficient >> memory use, so we must waste as little memory as possible. > > Hmmm. Can we come up with a generic solution instead? Yes please. > This may mean relaxing the enforcement of the allocation max order a bit > so that we can get dense allocation through higher order allocs. > > But then higher order allocs are generally seen as problematic. I think in this case they are better than wasting/fragmenting 384kB for 640kB object. > Note that SLUB will fall back to smallest order already if a failure > occurs so increasing slub_max_order may not be that much of an issue. > > Maybe drop the max order limit completely and use MAX_ORDER instead? For packing, sure. For performance, please no (i.e. don't try to allocate MAX_ORDER for each and every cache). > That > means that callers need to be able to tolerate failures. Is it any different from now? I suppose there would still be smallest-order fallback involved in sl*b itself? And if your allocation is so large it can fail even with the fallback (i.e. >= costly order), you need to tolerate failures anyway? One corner case I see is if there is anyone who would rather use their own fallback instead of the space-wasting smallest-order fallback. Maybe we could map some GFP flag to indicate that. >
