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Date: Tue, 17 Apr 2018 09:49:28 -0500 (CDT)
From: Christopher Lameter <cl@linux.com>
Subject: Re: slab: introduce the flag SLAB_MINIMIZE_WASTE
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To: Vlastimil Babka <vbabka@suse.cz>
Cc: Mikulas Patocka <mpatocka@redhat.com>, Mike Snitzer <snitzer@redhat.com>, Matthew Wilcox <willy@infradead.org>, Pekka Enberg <penberg@kernel.org>, linux-mm@kvack.org, dm-devel@redhat.com, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>

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.