linux-kernel - Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE

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Date:   Tue, 17 Apr 2018 18:16:13 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christopher Lameter <cl@...ux.com>,
        Mikulas Patocka <mpatocka@...hat.com>
Cc:     Mike Snitzer <snitzer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pekka Enberg <penberg@...nel.org>, linux-mm@...ck.org,
        dm-devel@...hat.com, David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE

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.

>