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…ack hints Add sysfs entries to support to hint for bypass/writeback by the ioprio assigned to the bio. If the bio is unassigned, use current's io-context ioprio for cache writeback or bypass (configured per-process with `ionice`). 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. If you really nead the performance at the expense of SSD wearout, then configure ioprio_writeback and set your `ionice` appropriately. For example: echo 2,7 > /sys/block/bcache0/bcache/ioprio_bypass echo 2,0 > /sys/block/bcache0/bcache/ioprio_writeback See the documentation commit for details. v2: Removed calls to get_task_io_context() v3: bio_prio() macro has been deleted Co-authored-by: Eric Wheeler <bcache@linux.ewheeler.net> Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net> Acked-by: Kent Overstreet <kent.overstreet@gmail.com> Tested-by: Kai Krakow <kai@kaishome.de> Cc: nix@esperi.org.uk Signed-off-by: Kai Krakow <kai@kaishome.de>
v2: get_task_io_context() no longer exists in the kernel Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net> Signed-off-by: Kai Krakow <kai@kaishome.de>
Even if ioprio_bypass hints bypassing the request, we still allow it for REQ_META|REQ_PRIO bio. Cc: Eric Wheeler <bcache@linux.ewheeler.net> Signed-off-by: Kai Krakow <kai@kaishome.de>
It seems to queue a lot of work, and I've seen messages like this, suggesting to switch to WQ_UNBOUND: > kernel: workqueue: cache_lookup hogged CPU for >10000us 11 times, consider switching to WQ_UNBOUND Short observation shows, that this might also improve perceived desktop responsiveness. But this needs more testing. Signed-off-by: Kai Krakow <kai@kaishome.de>
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Added a commit to prevent the following message: Early perceived result is improved desktop responsiveness. |
bcache: check ioprio_bypass, not ioprio_writeback, for the bypass hint check_should_bypass() only honoured the ioprio_bypass hint if ioprio_writeback was valid. ioprio_writeback defaults to (0,0), i.e. IOPRIO_CLASS_NONE, so with the default writeback hint the bypass hint never triggered, no matter what ioprio_bypass was set to. Low-priority IO (e.g. ionice -c3 dedup crawlers like bees) was promoted into the cache and evicted the working set, which is exactly what the hint is meant to prevent. The condition was copied from should_writeback() when the hints were introduced in commit a8b349f ("bcache: introduce bcache sysfs entries for ioprio-based bypass/writeback hints"). Check the validity of ioprio_bypass instead, so both hints can be enabled independently. Observed with ioprio_bypass=2,7, ioprio_writeback=0,0 and bees running with ionice idle: the bcache_read tracepoint showed zero bypassed reads on writearound devices while bees read cold extents. Signed-off-by: Kai Krakow <kai@kaishome.de>
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Export patch series: https://github.com/kakra/linux/pull/41.patch