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@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? > 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@xxxxxxxxxxxxxxxxxxx-northeast-1.compute.internal/ > [2] https://lore.kernel.org/all/eebc9dc8-6a45-c099-61da-230d06cb3157@xxxxxxx/ -- Thanks, Hyeonggon
