On 2020/10/8 04:35, Eric Wheeler wrote: > [+cc coly] > > On Mon, 5 Oct 2020, Eric Wheeler wrote: >> On Sun, 4 Oct 2020, Kai Krakow wrote: >> >>> Hey Nix! >>> >>> Apparently, `git send-email` probably swallowed the patch 0/3 message for you. >>> >>> It was about adding one additional patch which reduced boot time for >>> me with idle mode active by a factor of 2. >>> >>> You can look at it here: >>> https://github.com/kakra/linux/pull/4 >>> >>> It's "bcache: Only skip data request in io_prio bypass mode" just if >>> you're curious. >>> >>> Regards, >>> Kai >>> >>> Am So., 4. Okt. 2020 um 15:19 Uhr schrieb Nix <nix@xxxxxxxxxxxxx>: >>>> >>>> On 3 Oct 2020, Kai Krakow spake thusly: >>>> >>>>> Having idle IOs bypass the cache can increase performance elsewhere >>>>> since you probably don't care about their performance. In addition, >>>>> this prevents idle IOs from promoting into (polluting) your cache and >>>>> evicting blocks that are more important elsewhere. >>>> >>>> FYI, stats from 20 days of uptime with this patch live in a stack with >>>> XFS above it and md/RAID-6 below (20 days being the time since the last >>>> reboot: I've been running this patch for years with older kernels >>>> without incident): >>>> >>>> stats_total/bypassed: 282.2G >>>> stats_total/cache_bypass_hits: 123808 >>>> stats_total/cache_bypass_misses: 400813 >>>> stats_total/cache_hit_ratio: 53 >>>> stats_total/cache_hits: 9284282 >>>> stats_total/cache_miss_collisions: 51582 >>>> stats_total/cache_misses: 8183822 >>>> stats_total/cache_readaheads: 0 >>>> written: 168.6G >>>> >>>> ... so it's still saving a lot of seeking. This is despite having >>>> backups running every three hours (in idle mode), and the usual updatedb >>>> runs, etc, plus, well, actual work which sometimes involves huge greps >>>> etc: I also tend to do big cp -al's of transient stuff like build dirs >>>> in idle mode to suppress caching, because the build dir will be deleted >>>> long before it expires from the page cache. >>>> >>>> The SSD, which is an Intel DC S3510 and is thus read-biased rather than >>>> write-biased (not ideal for this use-case: whoops, I misread the >>>> datasheet), says >>>> >>>> EnduranceAnalyzer : 506.90 years >>>> >>>> despite also housing all the XFS journals. I am... not worried about the >>>> SSD wearing out. It'll outlast everything else at this rate. It'll >>>> probably outlast the machine's case and the floor the machine sits on. >>>> It'll certainly outlast me (or at least last long enough to be discarded >>>> by reason of being totally obsolete). Given that I really really don't >>>> want to ever have to replace it (and no doubt screw up replacing it and >>>> wreck the machine), this is excellent. >>>> >>>> (When I had to run without the ioprio patch, the expected SSD lifetime >>>> and cache hit rate both plunged. It was still years, but enough years >>>> that it could potentially have worn out before the rest of the machine >>>> did. Using ioprio for this might be a bit of an abuse of ioprio, and >>>> really some other mechanism might be better, but in the absence of such >>>> a mechanism, ioprio *is*, at least for me, fairly tightly correlated >>>> with whether I'm going to want to wait for I/O from the same block in >>>> future.) >>> >> From Nix on 10/03 at 5:39 AM PST >>> I suppose. I'm not sure we don't want to skip even that for truly >>> idle-time I/Os, though: booting is one thing, but do you want all the >>> metadata associated with random deep directory trees you access once a >>> year to be stored in your SSD's limited space, pushing out data you >>> might actually use, because the idle-time backup traversed those trees? >>> I know I don't. The whole point of idle-time I/O is that you don't care >>> how fast it returns. If backing it up is speeding things up, I'd be >>> interested in knowing why... what this is really saying is that metadata >>> should be considered important even if the user says it isn't! >>> >>> (I guess this is helping because of metadata that is read by idle I/Os >>> first, but then non-idle ones later, in which case for anyone who runs >>> backups this is just priming the cache with all metadata on the disk. >>> Why not just run a non-idle-time cronjob to do that in the middle of the >>> night if it's beneficial?) >> >> (It did not look like this was being CC'd to the list so I have pasted the >> relevant bits of conversation. Kai, please resend your patch set and CC >> the list linux-bcache@xxxxxxxxxxxxxxx) >> >> I am glad that people are still making effective use of this patch! >> >> It works great unless you are using mq-scsi (or perhaps mq-dm). For the >> multi-queue systems out there, ioprio does not seem to pass down through >> the stack into bcache, probably because it is passed through a worker >> thread for the submission or some other detail that I have not researched. >> >> Long ago others had concerns using ioprio as the mechanism for cache >> hinting, so what does everyone think about implementing cgroup inside of >> bcache? From what I can tell, cgroups have a stronger binding to an IO >> than ioprio hints. >> >> I think there are several per-cgroup tunables that could be useful. Here >> are the ones that I can think of, please chime in if anyone can think of >> others: >> - should_bypass_write >> - should_bypass_read >> - should_bypass_meta >> - should_bypass_read_ahead >> - should_writeback >> - should_writeback_meta >> - should_cache_read >> - sequential_cutoff >> >> Indeed, some of these could be combined into a single multi-valued cgroup >> option such as: >> - should_bypass = read,write,meta > > > Hi Coly, > > Do you have any comments on the best cgroup implementation for bcache? > > What other per-process cgroup parameters might be useful for tuning > bcache behavior to various workloads? Hi Eric, This is much better than the magic numbers to control io prio. I am not familiar with cgroup configuration and implementation, I just wondering because most of I/Os in bcache are done by kworker or kthread, is it possible to do per-process control. Anyway, we may start from the bypass stuffs in your example. If you may help to compose patches and maintain them in long term, I am glad to take them in. Thanks. Coly Li Thanks. Coly Li
