Re: [PATCH 00/12] Introduce CONFIG_SLUB_TINY and deprecate SLOB

[Vlastimil Babka]

On 11/22/22 17:33, Arnd Bergmann wrote:
> On Mon, Nov 21, 2022, at 18:11, Vlastimil Babka wrote:
>>
>> this continues the discussion from [1]. Reasons to remove SLOB are
>> outlined there and no-one has objected so far. The last patch of this
>> series therefore deprecates CONFIG_SLOB and updates all the defconfigs
>> using CONFIG_SLOB=y in the tree.
>>
>> There is a k210 board with 8MB RAM where switching to SLUB caused issues
>> [2] and the lkp bot wasn't also happy about code bloat [3]. To address
>> both, this series introduces CONFIG_SLUB_TINY to perform some rather
>> low-hanging fruit modifications to SLUB to reduce its memory overhead.
>> This seems to have been successful at least in the k210 case [4]. I
>> consider this as an acceptable tradeoff for getting rid of SLOB.
> 
> I agree that this is a great success for replacing SLOB on the
> smallest machines that have 32MB or less and have to run a
> a highly customized kernel, and this is probably enough to
> have a drop-in replacement without making any currently working
> system worse.
> 
> On the other hand, I have the feeling that we may want something
> a bit less aggressive than this for machines that are slightly
> less constrained, in particular when a single kernel needs to
> scale from 64MB to 512MB, which can happen e.g. on OpenWRT.
> I have seen a number of reports over the years that suggest
> that new kernels handle fragmentation and low memory worse than
> old ones, and it would be great to improve that again.

I see. That would need to study such reports and see if the problem there is
actually SLUB, or the page allocator or something else entirely.

> I can imagine those machines wanting to use sysfs in general
> but not for the slab caches, so having a separate knob to
> configure out the sysfs stuff could be useful without having
> to go all the way to SLUB_TINY.

Right, but AFAIK that wouldn't save much except some text size and kobjects,
so probably negligible for >32MB?

> For the options that trade off performance against lower
> fragmentation (MIN/MAX_PARTIAL, KMALLOC_RECLAIM, percpu
> slabs), I wonder if it's possible to have a boot time
> default based on the amount of RAM per CPU to have a better
> tuned system on most cases, rather than having to go
> to one extreme or the other at compile time.

Possible for some of these things, but for others that brings us back to the
question what are the actual observed issues. If it's low memory in absolute
number of pages, these can help, but if it's fragmentation (and the kind if
RAM sizes should have page grouping by mobility enabled), ditching e.g. the
KMALLOC_RECLAIM could make it worse. Unfortunately some of these tradeoffs
can be rather unpredictable.

Thanks,
Vlastimil

>     Arnd
> 
> https://openwrt.org/toh/views/toh_standard_all?datasrt=target&dataflt%5B0%5D=availability_%3DAvailable%202021