Messages in this thread | | | Subject | Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE | From | Vlastimil Babka <> | Date | Tue, 17 Apr 2018 18:16:13 +0200 |
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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.
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