Deprecating and removing SLOB

[Hyeonggon Yoo]

On Mon, Nov 14, 2022 at 10:36:31AM +0100, 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?
> 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.

IIRC overhead of s->min_partial (between 5 and 10) was pretty big because SLUB
caches at most (s->min_partial) * (nr of caches) * (size of slab) bytes of
unused memory.

Passing slub_max_order=0 also may help a little bit.

> The extra overhead could be also in percpu allocations, code etc.

SLUB do not use large amount of percpu allocator I think, less than
30kB on such a small machine.

Maybe also it would help reducing code size to disable CONFIG_MEMCG and CONFIG_TRACING,
CONFIG_SLUB_DEBUG and CONFIG_SYSFS.

I started from tinyconfig and enabled only necessary configs when testing in [1]
(it's a bit laborious cuz pure tinyconfig does not even boot...).

> >> 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)
> > * 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.
> > 
> > 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.
> 
> [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/

-- 
Thanks,
Hyeonggon