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Date:   Mon, 27 Sep 2021 19:03:19 +0200
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To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
References: <20210927090347.GA2533@linux.asia-northeast3-a.c.our-ratio-313919.internal>
From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [QUESTION] is SLAB considered legacy and deprecated?
In-Reply-To: <20210927090347.GA2533@linux.asia-northeast3-a.c.our-ratio-313919.internal>
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On 9/27/21 11:03, Hyeonggon Yoo wrote:
> Hello there,
> 
> I've been working on adding 'lockless cache' on sl[au]b for a while.
> But what it actually does is actually adding 'queuing' on slub.

Yeah, I pointed out those threads from 2011 that called it exactly that...
was there any conclusion why that was not ultimately merged?

> So there is a fundamental question coming into my mind:
> 	'is SLAB considered legacy and deprecated?'

To some extend, but not yet in a sense where we would have a deadline to get
rid of it. In some contexts it's still being preferred, AFAIK. But it's okay
if new optional features require just SLUB - for example PREEMPT_RT.

> It seems there are little development on SLAB and people think that
> SLAB is legacy and deprecated, so CONFIG_SLUB is used by default.
> 
> But I think both has pros and cons for their own:
> 	SLAB: more temporal locality (cache friendly)
> 	but high usage of memory, and less spatial locality (TLB misses) than SLUB.
> 
> 	SLUB: less temporal locality (less cache friendly) than SLAB
> 	but more spatial locality (TLB hit), and low usage of memory
> 	and good debugging feature.

I'm not so sure about the usage of memory, SLUB can easily use more I
believe. Instead of caching some arrays of objects it will have one or more
private slabs per cpu, and the slabs are larger-order pages.

> Why do people say SLAB is deprecated/legacy?

Do they?

> Thanks,
> Hyeonggon
>