From: Vlastimil Babka <vbabka@suse.cz>
To: Damien Le Moal <damien.lemoal@opensource.wdc.com>,
Conor Dooley <conor@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>,
Christoph Lameter <cl@linux.com>,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Pekka Enberg <penberg@kernel.org>,
Hyeonggon Yoo <42.hyeyoo@gmail.com>,
Matthew Wilcox <willy@infradead.org>,
Roman Gushchin <roman.gushchin@linux.dev>,
Linus Torvalds <torvalds@linux-foundation.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Rustam Kovhaev <rkovhaev@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Josh Triplett <josh@joshtriplett.org>,
Arnd Bergmann <arnd@arndb.de>,
Russell King <linux@armlinux.org.uk>,
Alexander Shiyan <shc_work@mail.ru>,
Aaro Koskinen <aaro.koskinen@iki.fi>,
Janusz Krzysztofik <jmkrzyszt@gmail.com>,
Tony Lindgren <tony@atomide.com>,
Yoshinori Sato <ysato@users.sourceforge.jp>,
Rich Felker <dalias@libc.org>, Jonas Bonn <jonas@southpole.se>,
Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
Stafford Horne <shorne@gmail.com>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
openrisc@lists.librecores.org, linux-riscv@lists.infradead.org,
linux-sh@vger.kernel.org,
Geert Uytterhoeven <geert@linux-m68k.org>,
Conor.Dooley@microchip.com, Paul Cercueil <paul@crapouillou.net>
Subject: Re: Deprecating and removing SLOB
Date: Mon, 14 Nov 2022 10:36:31 +0100 [thread overview]
Message-ID: <44da078c-b630-a249-bf50-67df83cd8347@suse.cz> (raw)
In-Reply-To: <93079aba-362e-5d1e-e9b4-dfe3a84da750@opensource.wdc.com>
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@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@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. 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)
> * 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/
next prev parent reply other threads:[~2022-11-14 9:36 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <b35c3f82-f67b-2103-7d82-7a7ba7521439@suse.cz>
[not found] ` <CA+CK2bD-uVGJ0=9uc7Lt5zwY+2PM2RTcfOhxEd65S7TvTrJULA@mail.gmail.com>
2022-11-09 9:00 ` Deprecating and removing SLOB Vlastimil Babka
2022-11-09 15:50 ` Aaro Koskinen
2022-11-09 16:45 ` Geert Uytterhoeven
2022-11-09 17:45 ` Mike Rapoport
2022-11-09 21:16 ` Janusz Krzysztofik
2022-11-11 10:33 ` Vlastimil Babka
2022-11-11 20:46 ` Conor Dooley
2022-11-12 1:40 ` Damien Le Moal
2022-11-14 1:55 ` Damien Le Moal
2022-11-14 5:48 ` Damien Le Moal
2022-11-14 9:36 ` Vlastimil Babka [this message]
2022-11-14 11:35 ` Damien Le Moal
2022-11-14 14:47 ` Hyeonggon Yoo
2022-11-15 4:24 ` Damien Le Moal
2022-11-15 4:28 ` Damien Le Moal
2022-11-16 7:57 ` Matthew Wilcox
2022-11-16 8:02 ` Damien Le Moal
2022-11-16 17:51 ` Vlastimil Babka
2022-11-17 0:22 ` Damien Le Moal
2022-11-21 4:30 ` Damien Le Moal
2022-11-21 17:02 ` Vlastimil Babka
2022-11-14 11:50 ` Hyeonggon Yoo
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