On Tue, 17 Apr 2018, Vlastimil Babka wrote: > 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. Wasting 37% of memory is still better than the kernel randomly returning -ENOMEM when higher-order allocation fails. > > 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. For example, if you create a cache with 17KB objects, the slab subsystem will pad it up to 32KB. You are wasting almost 1/2 memory, but the allocation is realiable and it won't fail. If you use order higher than 32KB, you get less wasted memory, but you also get random -ENOMEMs (yes, we had a problem in dm-thin that it was randomly failing during initialization due to 64KB allocation). Mikulas
