Azat Khuzhin ed70ed6f71 Introduce connection_no_wait setting for MySQL engine. ... This will allow to avoid superfluous sleep during query execution, since this not only not desired behavoiur, but also may hang the server, since if you will execute enough queries that will use MySQL database but will not allow enough connections (or your MySQL server is too slow) then you may run out of threads in the global thread pool. Also note that right now it is possible to get deadlock when the mysql pool is full, consider the following scenario: - you have m1 and m2 mysql tables - you have q1 and q2 queries, bot queries join m1 and m2 - q1 allocated connection for m1 but cannot allocate connection for m2 - q2 allocated connection for m2 but cannot allocate connection for m1 - but to resolve the lock one should give up on the locking while it is not possible right now... And then you got no free threads and this: # grep -h ^202 /proc/$(pgrep clickhouse-serv)/task/*/syscall | cut -d' ' -f2 | sort | uniq -c | sort -nr | head 1554 0x7ffb60b92fe8 # mutex in mysqlxx::PoolWithFailover::get 1375 0x7ffb9f1c4748 # mutex in ::PoolEntryHelper::~PoolEntryHelper from DB::MultiplexedConnections::invalidateReplica 1160 0x7ffb612918b8 # mutex in mysqlxx::PoolWithFailover::get 42 0x7ffb9f057984 # mutex in ThreadPoolImpl<std::__1::thread>::worker *NOTE: 202 is a `futex` with WAIT* (Went with `syscall` because debugging 10k+ threads is not easy, and eventually it may TRAP)		2021-09-02 22:23:37 +03:00
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MySQLSettings.cpp
MySQLSettings.h	Introduce connection_no_wait setting for MySQL engine.	2021-09-02 22:23:37 +03:00