Deprecating and removing SLOB

[Damien Le Moal]

On 11/12/22 05:46, Conor Dooley wrote:
> On Fri, Nov 11, 2022 at 11:33:30AM +0100, Vlastimil Babka wrote:
>> On 11/8/22 22:44, Pasha Tatashin wrote:
>>> On Tue, Nov 8, 2022 at 10:55 AM Vlastimil Babka <vbabka at suse.cz> 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.
>>>>
>>>> I believe SLOB can be removed because:
>>>>
>>>> - AFAIK nobody really uses it? It strives for minimal memory footprint
>>>> by putting all objects together, which has its CPU performance costs
>>>> (locking, lack of percpu caching, searching for free space...). I'm not
>>>> aware of any "tiny linux" deployment that opts for this. For example,
>>>> OpenWRT seems to use SLUB and the devices these days have e.g. 128MB
>>>> RAM, not up to 16 MB anymore. I've heard anecdotes that the performance
>>>> SLOB impact is too much for those who tried. Googling for
>>>> "CONFIG_SLOB=y" yielded nothing useful.
>>>
>>> I am all for removing SLOB.
>>>
>>> There are some devices with configs where SLOB is enabled by default.
>>> Perhaps, the owners/maintainers of those devices/configs should be
>>> included into this thread:
>>>
>>> tatashin at soleen:~/x/linux$ git grep SLOB=y
> 
>>> arch/riscv/configs/nommu_k210_defconfig:CONFIG_SLOB=y
>>> arch/riscv/configs/nommu_k210_sdcard_defconfig:CONFIG_SLOB=y
>>> arch/riscv/configs/nommu_virt_defconfig:CONFIG_SLOB=y
> 
>>
>> Turns out that since SLOB depends on EXPERT, many of those lack it so
>> running make defconfig ends up with SLUB anyway, unless I miss something.
>> Only a subset has both SLOB and EXPERT:
>>
>>> git grep CONFIG_EXPERT `git grep -l "CONFIG_SLOB=y"`
> 
>> arch/riscv/configs/nommu_virt_defconfig:CONFIG_EXPERT=y
> 
> I suppose there's not really a concern with the virt defconfig, but I
> did check the output of `make nommu_k210_defconfig" and despite not
> having expert it seems to end up CONFIG_SLOB=y in the generated .config.
> 
> I do have a board with a k210 so I checked with s/SLOB/SLUB and it still
> boots etc, but I have no workloads or w/e to run on it.

I will try with SLUB over the weekend.

-- 
Damien Le Moal
Western Digital Research