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Subject: Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE
To:     Mikulas Patocka <mpatocka@redhat.com>
Cc:     Christopher Lameter <cl@linux.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>,
        linux-kernel@vger.kernel.org
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From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <8ab1e75f-9cf9-2a99-d071-c8c7a3554b95@suse.cz>
Date:   Tue, 17 Apr 2018 21:13:53 +0200
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On 04/17/2018 07:26 PM, Mikulas Patocka wrote:
> 
> 
> On Tue, 17 Apr 2018, Vlastimil Babka wrote:
> 
>> 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.
> 
> Wasting 37% of memory is still better than the kernel randomly returning 
> -ENOMEM when higher-order allocation fails.

Of course, see below.

>>> 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

^ There: "I suppose there would still be smallest-order fallback
involved in sl*b itself?"

If SLAB doesn't currently support fallback to different order, it either
learns to do that, or keeps wasting memory and more people will migrate
to SLUB. Simple.