linux-kernel - Re: Deprecating and removing SLOB

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Message-ID: <1823fa9f-f67d-9581-0e6f-55f0c050fcc6@suse.cz>
Date:   Wed, 9 Nov 2022 10:09:42 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yosry Ahmed <yosryahmed@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Rustam Kovhaev <rkovhaev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Deprecating and removing SLOB

On 11/8/22 21:13, Yosry Ahmed wrote:
> On Tue, Nov 8, 2022 at 10:47 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>>
>> On Tue, Nov 08, 2022 at 04:55:29PM +0100, Vlastimil Babka wrote:
>> > Hi,
>> >
>> > as we all know, we currently have three slab allocators. As we discussed at
>> > LPC [1], it is my hope that one of these allocators has a future, and two of
>> > them do not.
>> >
>> > The unsurprising reasons include code maintenance burden, other features
>> > compatible with only a subset of allocators (or more effort spent on the
>> > features), blocking API improvements (more on that below), and my inability
>> > to pronounce SLAB and SLUB in a properly distinguishable way, without
>> > resorting to spelling out the letters.
>> >
>> > I think (but may be proven wrong) that SLOB is the easier target of the two
>> > to be removed, so I'd like to focus on it first.
>>
>> Great!
>>
>> SLOB is not supported by the kernel memory accounting code, so if we'll
>> deprecate SLOB, we can remove all those annoying ifndefs.
>>
>> But I wonder if we can deprecate SLAB too? Or at least use the moment to
>> ask every non-SLUB user on why they can't/don't want to use SLUB.
>> Are there any known advantages of SLAB over SLUB?

Yeah it was my plan to inquire about SLAB next, once SLOB's fate is settled,
as I did expect greater resistance there. My hope is that if there are still
workloads that benefit from SLAB's percpu arrays, we could e.g. add
(per-cache opt-in) percpu arrays to SLUB, as that would be still better than
having two different complete implementations.

> We use SLAB at Google, but I am not the right person to answer the
> question of why we can't/don't use SLUB. Adding Greg here who recently
> looked into this and might have answers. I see David is already
> tagged, he might have a good answer as well.

Yeah, Google folks were indeed against removing SLAB due to such workloads
in the past discussions.

>>
>> Also, for memory-constrained users we might want to add some guide on how
>> to configure SLUB to minimize the memory footprint.
>>
>> Thank you!
>>
>> Roman
>>