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)
Default settings are not very efficient, since they do not even reuse
connections.
And when each query requires connection you can have only ~80 QPS, while
by simply enabling connection reuse (connection_auto_close=false) you
can have ~500 QPS (and by increasing connection_pool_size you can have
better QPS throughput).
So this patch allows to pass through some connection related settings
for the StorageMySQL engine, like:
- connection_pool_size=16
- connection_max_tries=3
- connection_auto_close=true
v2: remove connection_pool_default_size
v3: remove num_tries_on_connection_loss
* Added a new type of Exception
for better recognition of connection failures
* Added more logging for mysql communication
* Retries on connection loss during query.
Make MySQL dictionary source resilient to unexpected loss of connection
during querying MySQL. This behavior is controlled with
".fail_on_connection_loss" config parameter, which defaults to false.
* Stripped some debugging leftover garbage
* Minor followup corrections
* Corrections after PR comments
* Yet more fixes
