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630d599c34
All schema changes made to the node locally are serialized on a
semaphore which lives on shard 0. For historical reasons, they don't
queue but rather try to take the lock without blocking and retry on
failure with a random delay from the range [0, 100 us]. Contenders
which do not originate on shard 0 will have an extra disadvantage as
each lock attempt will be longer by the across-shard round trip
latency. If there is constant contention on shard 0, contenders
originating from other shards may keep loosing to take the lock.
Schema merge executed on behalf of a DDL statement may originate on
any shard. Same for the schema merge which is coming from a push
notification. Schema merge executed as part of the background schema
pull will originate on shard 0 only, where the application state
change listeners run. So if there are constant schema pulls, DDL
statements may take a long time to get through.
The fix is to serialize merge requests fairly, by using the blocking
semaphore::wait(), which is fair.
We don't have to back-off any more, since submit_to() no longer has a
global concurrency limit.
Fixes #4436.
Message-Id: <1555349915-27703-1-git-send-email-tgrabiec@scylladb.com>
(cherry picked from commit 3fd82021b1)