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82270c8699
The foreground reads metric is derived from the number of live read executors minus the number of background reads. Background reads are counted down when their resolver times out. However, a read executor may still be around for a while, resulting in such reads being accounted as foreground. Usually, the gap in which this happens is short, because executor reference holders timeout quickly as well. It's not always the case though. For instance, local read executor doesn't time out quickly when the target shard has an overloaded CPU, and it takes a while before the request goes through all the queues, even if IO is not involved. Observed in #3628. Fixes #3734. Another problem is that all reads which received CL responses are accounted as background, until all replicas respond, but if such read needs reconciliation, it's still practically a foreground read and should be accounted as such. Found during code review. Fixes #3745. This patch fixes both issues by rearranging accounting to track foreground reads instead of background reads, and considering all reads as foreground until the resulting promise is resolved. Message-Id: <1535999620-25784-1-git-send-email-tgrabiec@scylladb.com>