Re: [PATCH 1/3] bcache: introduce bcache sysfs entries for ioprio-based bypass/writeback hints

[Eric Wheeler <bcache@lists.ewheeler.net>]

[+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@esperi.org.uk>:
> > >
> > > 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@vger.kernel.org)
> 
> 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?

-Eric

--
Eric Wheeler