[+cc:bcache and blocklist] 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 -- Eric Wheeler
