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 sent a patch to change the k210 defconfig to using SLUB. However...

The current default config using SLOB gives about 630 free memory pages
after boot (cat /proc/vmstat). Switching to SLUB, this is down to about
400 free memory pages (CONFIG_SLUB_CPU_PARTIAL is off).

This is with a buildroot kernel 5.19 build including a shell and sd-card
boot. With SLUB, I get clean boots and a shell prompt as expected. But I
definitely see more errors with shell commands failing due to allocation
failures for the shell process fork. So as far as the K210 is concerned,
switching to SLUB is not ideal.

I would not want to hold on kernel mm improvements because of this toy
k210 though, so I am not going to prevent SLOB deprecation. I just wish
SLUB itself used less memory :)


-- 
Damien Le Moal
Western Digital Research