io_queue: Maintain per-shard-class stall threshold #1609
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This value is known by the time disk_config is prepared and can be assigned per-shard inside allocate_io_queues callback. Signed-off-by: Pavel Emelyanov <[email protected]>
IO queues expect reactor to poll them at least once every "IO latency goal". If reactor fails to do it, the disk queues may starve for concurrency which is not what scheduler expects. The new metrics would show how may times queues were spend without being polled _above_ the expected latency goal. Signed-off-by: Pavel Emelyanov <[email protected]>
Since it's global metrics, it will come in separate section Signed-off-by: Pavel Emelyanov <[email protected]>
The stall threshold is the duration a request is expected to get dispatched and executed within. In current model it's expected to be io-latency-goal value, but it's better to have it configurable. When the threshold is broken, the report helper is called and the threshold is doubled. Once in a while the threshold is lowered to allow for more reports in the future. Reported is the number of queued requests in the whole queue and per-class as well as the number of currently executing requests also both kinds -- in the whole queue and per-class. fixes: scylladb#1311 Signed-off-by: Pavel Emelyanov <[email protected]>
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This PR is based on top of #1492 and carries its patches on board. Once (if) the latter gets merged, I'll rebase this one. The essence of this PR is the last patch in the series