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