Deprecating and removing SLOB

[Damien Le Moal]

On 11/14/22 18:36, Vlastimil Babka wrote:
> On 11/14/22 06:48, Damien Le Moal wrote:
>> On 11/14/22 10:55, Damien Le Moal wrote:
>>> 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...
> 
> Thanks!
> 
>>> 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).
> 
> Thanks for the testing! How much RAM does the system have btw? I found 8MB
> somewhere, is that correct?

Yep, 8MB, that's it.

> So 230 pages that's a ~920 kB difference. Last time we saw less  dramatic
> difference [1]. But that was looking at Slab pages, not free pages. The
> extra overhead could be also in percpu allocations, code etc.
> 
>>> 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 :)
>>
>> Did further tests with kernel 6.0.1:
>> * SLOB: 630 free pages after boot, shell working (occasional shell fork
>> failure happen though)
>> * SLAB: getting memory allocation for order 7 failures on boot already
>> (init process). Shell barely working (high frequency of shell command fork
>> failures)

I forgot to add here that the system was down to about 500 free pages
after boot (again from the shell with "cat /proc/vmstat").

>> * SLUB: getting memory allocation for order 7 failures on boot. I do get a
>> shell prompt but cannot run any shell command that involves forking a new
>> process.

For both slab and slub, I had cpu partial off, debug off and slab merge
on, as I suspected that would lead to less memory overhead.
I suspected memory fragmentation may be an issue but doing

echo 3 > /proc/sys/vm/drop_caches

before trying a shell command did not help much at all (it usually does on
that board with SLOB). Note that this is all with buildroot, so this echo
& redirect always works as it does not cause a shell fork.

>>
>> So if we want to keep the k210 support functional with a shell, we need
>> slob. If we reduce that board support to only one application started as
>> the init process, then I guess anything is OK.
> 
> In [1] it was possible to save some more memory with more tuning. Some of
> that required boot parameters and other code changes. In another reply [2] I
> considered adding something like SLUB_TINY to take care of all that, so
> looks like it would make sense to proceed with that.

If you want me to test something, let me know.

> 
> [1]
> https://lore.kernel.org/all/Yg9xSWEaTZLA+hYt@ip-172-31-19-208.ap-northeast-1.compute.internal/
> [2] https://lore.kernel.org/all/eebc9dc8-6a45-c099-61da-230d06cb3157@suse.cz/

-- 
Damien Le Moal
Western Digital Research