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scylladb/utils
Avi Kivity 020ccbd76a Merge 'utils: cached_file: Mark permit as awaiting on page miss' from Tomasz Grabiec
Otherwise, the read will be considered as on-cpu during promoted index
search, which will severely underutlize the disk because by default
on-cpu concurrency is 1.

I verified this patch on the worst case scenario, where the workload
reads missing rows from a large partition. So partition index is
cached (no IO) and there is no data file IO (relies on https://github.com/scylladb/scylladb/pull/20522).
But there is IO during promoted index search (via cached_file).

Before the patch this workload was doing 4k req/s, after the patch it does 30k req/s.

The problem is much less pronounced if there is data file or partition index IO involved
because that IO will signal read concurrency semaphore to invite more concurrency.

Fixes #21325

Closes scylladb/scylladb#21323

* github.com:scylladb/scylladb:
  utils: cached_file: Mark permit as awaiting on page miss
  utils: cached_file: Push resource_unit management down to cached_file
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