suse 11 SP1 x86_64

do you have the output of gdb thread apply all bt ? Also error log, and my.cnf would be helpful too. what were the threadpool related parameters?

Here is the my.cnf parameters about threadpool:

# # thread pool

#plugin-load=thread_pool.so

thread_handling=pool-of-threads

thread_pool_size=36

#thread_pool_stall_limit=6

thread_pool_oversubscribe=5

max_connections = 50000

thread_cache_size = 18

thread_concurrency = 64

thread_stack = 512K

////////////////////////////////////////////////////////////////////////
And the MariaDB's threads stack are similar:
There are 551 threads, except some other io_handler_thread 、monitor thread 、purge threads
500 threads hang at:

#0  0x00007fb079c6a879 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0

#1  0x0000000000675e9c in MDL_wait::timed_wait(MDL_context_owner*, timespec*, bool, PSI_stage_info_v1 const*) ()

#2  0x000000000067869b in MDL_context::acquire_lock(MDL_request*, unsigned long) ()

#3  0x000000000055524e in open_table_get_mdl_lock(THD*, Open_table_context*, MDL_request*, unsigned int, MDL_ticket**) ()

#4  0x0000000000556f22 in open_table(THD*, TABLE_LIST*, st_mem_root*, Open_table_context*) ()

#5  0x0000000000558a7a in open_tables(THD*, TABLE_LIST**, unsigned int*, unsigned int, Prelocking_strategy*) ()

#6  0x0000000000558dcb in open_and_lock_tables(THD*, TABLE_LIST*, bool, unsigned int, Prelocking_strategy*) ()

And MariaDB Connector/C client hang at:

#0  0x00007f2f9c748896 in poll () from /lib64/libc.so.6

#1  0x00007f2f562a09ef in pvio_socket_wait_io_or_timeout (pvio=<optimized out>, is_read=<optimized out>, timeout=-1) at /home/sal/mariadb/mariadb/plugins/pvio/pvio_socket.c:510

#2  0x00007f2f562a0a68 in pvio_socket_read (pvio=0x7f2f5d9bb730, buffer=0x7f2f043e55a0 "\f", length=16384) at /home/sal/mariadb/mariadb/plugins/pvio/pvio_socket.c:278

#3  0x00007f2f562acced in ma_pvio_read (pvio=0x7f2f04441cc0, buffer=0x7f2f043e55a0 "\f", length=16384) at /home/sal/mariadb/mariadb/libmariadb/ma_pvio.c:251

#4  0x00007f2f562acead in ma_pvio_cache_read (pvio=0x7f2f5d9bb730, buffer=0x7f2f043e95b0 "\n", length=4) at /home/sal/mariadb/mariadb/libmariadb/ma_pvio.c:291

#5  0x00007f2f562a4267 in ma_real_read (net=0x7f2f044415b0, complen=0x7f2f5d9bb860) at /home/sal/mariadb/mariadb/libmariadb/ma_net.c:463

#6  0x00007f2f562a44f5 in ma_net_read (net=0x7f2f5d9bb730) at /home/sal/mariadb/mariadb/libmariadb/ma_net.c:517

#7  0x00007f2f562aa95c in ma_net_safe_read (mysql=0x7f2f044415b0) at /home/sal/mariadb/mariadb/libmariadb/mariadb_lib.c:188

even a simple SQL like "select 1 from dual" was hanged.

////////////////////////////////////////////////////////////////////////
And MariaDB's error log:

161102 11:34:10 [ERROR] Threadpool could not create additional thread to handle queries, because the number of allowed threads was reached. Increasing 'thread_pool_max_threads' parameter can help in this situation.

 If 'extra_port' parameter is set, you can still connect to the database with superuser account (it must be TCP connection using extra_port as TCP port) and troubleshoot the situation. A likely cause of pool blocks are clients that lock resources for long time. 'show processlist' or 'show engine innodb status' can give additional hints.

161102 11:34:10 [Note] Threadpool has been blocked for 30 seconds

////////////////////////////////////////////////////////////////////////
We just test high concurrency, every connection start transaction and execute Insert/Update/Select, if finish then commit , or else rollback.

very confused..

I hoped the message in error log was helpful.

thread_pool_max_threads is the hard limit of number of threads in the threadpool. It might be 500 or so as default.

If this is less than max_connections, it can deadlock on a global lock. It won't ever deadlock if you size it as max_connections + thread_pool_size.

Also, the message about extra_port is accurate. If you had enabled it, you could connect as superuser and resolve the deadlock by setting thread_pool_max_threads higher.

That client hangs is because the server does not answer. The server does not answer because it is stalled, because the thread_pool_max_threads had reached.

So I suggest to set thread_pool_max_threads=60000 or something big enough for you to never see this message again.

which global lock ? And I'm confused why threads were blocked at open tables procedure ? ..

It does not matter which lock, for the purpose of explaining threadpool. In this case it is MDL lock. It is possible to google to find out what an MDL lock does.

It can theoretically be a table lock, user lock, "flush table with read lock" lock. It can also be a row lock in Innodb as well, if everyone is waiting for a specific row.

catofpei IS there anything else needing clarification? mdl lock is probably a result of LOCK TABLES or FLUSH TABLES WITH READ LOCK. you can know it better than we.

Very sorry for my delayed reply.

I tried to increase the threadpool_max_threads value as you advised. And the test result was OK!
No more threads were hanged in mariadb server, and the client was OK, too.

In our test code, there were scenarios in which some threads tried to update the same row ，
and some threads would threw the exception if lock wait timeout : Lock wait timeout exceeded, try restarting transaction.

But we have no operations like "FLUSH TABLES" 、"LOCK TABLES" 、 DDL operations. Maybe as you wrote, "it can also be a row lock in Innodb" ..