[dm-devel] slab: introduce the flag SLAB_MINIMIZE_WASTE

[Christopher Lameter]

On Mon, 16 Apr 2018, Vlastimil Babka wrote:

> >> Its not a senseless increase. The more objects you fit into a slab page
> >> the higher the performance of the allocator.
>
> It's not universally without a cost. It might increase internal
> fragmentation of the slabs, if you end up with lots of 4MB pages
> containing just few objects. Thus, waste of memory. You also consume
> high-order pages that could be used elsewhere. If you fail to allocate
> 4MB, then what's the fallback, order-0? I doubt it's "the highest
> available order". Thus, a more conservative choice e.g. order-3 will
> might succeed more in allocating order-3, while a choice of 4MB will
> have many order-0 fallbacks.

Obviously there is a cost. But here the subsystem has a fallback.

> I think it should be possible without a new flag. The slub allocator
> could just balance priorities (performance vs memory efficiency) better.
> Currently I get the impression that "slub_max_order" is a performance
> tunable. Let's add another criteria for selecting an order, that would
> try to pick an order to minimize wasted space below e.g. 10% with some
> different kind of max order. Pick good defaults, add tunables if you must.

There is also slub_min_objects.

> I mean, anyone who's creating a cache for 640KB objects most likely
> doesn't want to waste another 384KB by each such object. They shouldn't
> have to add a flag to let the slub allocator figure out that using 2MB
> pages is the right thing to do here.

I agree if we do this then preferably without a flag.