Re: Deprecating and removing SLOB

[Damien Le Moal]

On 11/17/22 02:51, Vlastimil Babka wrote:
> On 11/15/22 05:24, Damien Le Moal wrote:
>> On 11/14/22 23:47, Hyeonggon Yoo wrote:
>>> On Mon, Nov 14, 2022 at 08:35:31PM +0900, Damien Le Moal wrote:
>>>> 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@xxxxxxx> 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@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.
>>>
>>> Would you try this please?
>>>
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index a24b71041b26..1c36c4b9aaa0 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -4367,9 +4367,7 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
>>>  	 * The larger the object size is, the more slabs we want on the partial
>>>  	 * list to avoid pounding the page allocator excessively.
>>>  	 */
>>> -	s->min_partial = min_t(unsigned long, MAX_PARTIAL, ilog2(s->size) / 2);
>>> -	s->min_partial = max_t(unsigned long, MIN_PARTIAL, s->min_partial);
>>> -
>>> +	s->min_partial = 0;
>>>  	set_cpu_partial(s);
>>>  
>>>  #ifdef CONFIG_NUMA
>>>
>>>
>>> and booting with and without boot parameter slub_max_order=0?
>>
>> Test notes: I used Linus 6.1-rc5 as the base. That is the only thing I
>> changed in buildroot default config for the sipeed maix bit card, booting
>> with SD card. The test is: booting and run "cat /proc/vmstat" and register
>> the nr_free_pages value. I repeated the boot + cat 3 to 4 times for each case.
>>
>> Here are the results:
>>
>> 6.1-rc5, SLOB:
>>     - 623 free pages
>>     - 629 free pages
>>     - 629 free pages
>> 6.1-rc5, SLUB:
>>     - 448 free pages
>>     - 448 free pages
>>     - 429 free pages
>> 6.1-rc5, SLUB + slub_max_order=0:
>>     - Init error, shell prompt but no shell command working
>>     - Init error, no shell prompt
>>     - 508 free pages
>>     - Init error, shell prompt but no shell command working
>> 6.1-rc5, SLUB + patch:
>>     - Init error, shell prompt but no shell command working
>>     - 433 free pages
>>     - 448 free pages
>>     - 423 free pages
>> 6.1-rc5, SLUB + slub_max_order=0 + patch:
>>     - Init error, no shell prompt
>>     - Init error, shell prompt, 499 free pages
>>     - Init error, shell prompt but no shell command working
>>     - Init error, no shell prompt
>>
>> No changes for SLOB results, expected.
>>
>> For default SLUB, I did get all clean boots this time and could run the
>> cat command. But I do see shell fork failures if I keep running commands.
>>
>> For SLUB + slub_max_order=0, I only got one clean boot with 508 free
>> pages. Remaining runs failed to give a shell prompt or allow running cat
>> command. For the clean boot, I do see higher number of free pages.
>>
>> SLUB with the patch was nearly identical to SLUB without the patch.
>>
>> And SLUB+patch+slub_max_order=0 gave again a lot of errors/bad boot. I
>> could run the cat command only once, giving 499 free pages, so better than
>> regular SLUB. But it seems that the memory is more fragmented as
>> allocations fail more often.
>>
>> Hope this helps. Let me know if you want to test something else.
> 
> Could you please try this branch with CONFIG_SLUB_TINY=y?
> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-tiny-v1r0
> 
> Seeing your results I didn't modify default slub_max_order by this new
> CONFIG (yet?) so maybe after trying the default, trying then also with
> manual slub_max_order=0 and slub_max_order=1 would be useful too. Otherwise
> it should be all changes to lower SLUB memory footprint. Hopefully it will
> be visible in the number of free pages. But if fragmentation is an issue, it
> might not be enough. BTW, during boot there should be a line "Built X
> zonelists, mobility grouping ..." can you grep for it and provide please, I
> wonder if mobility grouping ends up being off or on on that system.

I ran your branch with CONFIG_SLUB_TINY=y. Here are the results with 3-4
runs per config:

* tiny slub with default slub_max_order:
	- Clean boot, 579 free pages
	- Clean boot, 575 free pages
	- Clean boot, 579 free pages

* tiny slub with slub_max_order=0 as boot argument:
        - Init error, shell prompt but no shell command working
	- Init error, shell prompt, 592 free pages
	- Init error, shell prompt, 591 free pages
	- Init error, shell prompt, 591 free pages

* tiny slub with slub_max_order=1 as boot argument:
	- Clean boot, 601 free pages
	- Clean boot, 601 free pages
	- Clean boot, 591 free pages
	- Clean boot, 601 free pages

For all cases, mobility grouping was reported as off:

[    0.000000] Built 1 zonelists, mobility grouping off.  Total pages: 2020

So it looks like your tiny slub branch with slub_max_order=1 puts us
almost on par with slob and that slub_max_order=0 seems to be generating
more fragmentation leading to unreliable boot. I also tried
slub_max_order=2, which gives clean boot and around 582 free pages, almost
the same as the default.

With this branch applied, I have no issues with having slob deprecated :)
Thanks !


> 
> Thanks!
> 
>> Cheers.
>>
> 

-- 
Damien Le Moal
Western Digital Research