Hello Kristian, See attached snapshot of slave threads and tokudb locks. Thread 16 is waiting for a tokudb lock held by thread 16, which is waiting for a tokudb lock held by thread 14. Thread 14 is waiting for a prior transaction to complete, presumably either thread 15 or 16. So, we have a deadlock that tokudb can not detect because the ordering constraint is not available to tokudb. I assume that the optimistic scheduler killed thread 16, but since tokudb does not implement the kill_query function, the deadlock is only resolved when the tokudb lock timer pops.
On Mon, Aug 15, 2016 at 8:16 AM, Rich Prohaska <[email protected]> wrote: > Hello Kristian, > The simplest kill_query implementation for tokudb would just signal all of > the pending lock request's condition variables. This would cause the > killed callback to be called. A performance refinement, if necessary, > would allow thread A (executing the kill_query function) to identify and > signal a condition variable for a blocked thread B. > > On Mon, Aug 15, 2016 at 5:42 AM, Kristian Nielsen < > [email protected]> wrote: > >> Rich Prohaska <[email protected]> writes: >> >> > tokudb lock timeouts are resolving the replication stall. >> unfortunately, >> > the tokudb lock timeout is 4 seconds, so the throughput is almost zero. >> >> Yes. Sorry for not making it clear that my proof-of-concept patch was >> incomplete... >> >> >> > I suspect that the poor slave replication performance for optimistic >> >> > replication occurs because TokuDB does not implement the kill_query >> >> > handlerton function. kill_handlerton gets called to resolve lock >> wait >> >> >> Possibly, but I'm not sure it's that important. The kill will be >> effective >> >> as soon as the wait is over. >> >> No, you're absolutely right, after testing (and thinking) some more, I >> realise that indeed the kill_query functionality is important. >> >> A possible scenario is, given transactions T1, T2, and T3 in that order: >> >> T3 acquires a lock on row R3, T2 similarly acquires R2. >> Now T3 tries to acquire R2, but has to wait for T2 to release it. >> Later T1 tries to acquire R3, also has to wait. >> >> At this point, we kill T3, since it is holding a lock (R3) needed by an >> earlier transaction T1. However, T3 will not notice the kill until its own >> wait (on R2 held by T2) times out. T2 cannot release the lock because it >> is >> waiting for T1 to commit first. So we have a deadlock :-/ >> >> With InnoDB, the kill causes T3 to wake up immediately and roll back, so >> that T1 can proceed without much delay. >> >> Ok, so something more is needed here. I see there is a killed_callback() >> which seems to check for the kill, so I'm hoping that can be used with a >> suitable wakeup of the offending lock_request (or all requests, >> perhaps). But as I'm completely new to TokuDB, I still need some more time >> to read the code and try to understand how everything fits together... >> >> > TokuFT implements pessimistic locking and 2 phase locking algorithms. >> This >> > wiki describes locking and concurrency in a little more detail: >> > https://github.com/percona/tokudb-engine/wiki/Transactions- >> and-Concurrency. >> >> Thanks, this was quite helpful. >> >> > Yes, I think they are false positives since the thd_report_wait_for API >> is >> > called but it does NOT call the THD::awake function. >> >> Ah. Then it's probably normal, caused by the group-commit optimisation. In >> conservative mode, if two transactions T1 and T2 did not group commit on >> the >> master, then cannot be started in parallel on the slave. But T2 can start >> as >> soon as T1 has reached COMMIT. Thus, if T2 happens to conflict with T1, >> there is a small window where T2 can need to wait on T1 until T1 has >> completed its commit. >> >> Thanks, >> >> - Kristian. >> > >
Id User Host db Command Time State Info Progress 13 system user NULL Connect 62 Waiting for master to send event NULL 0.000 14 system user NULL Connect 15 Waiting for prior transaction to commit NULL 0.000 15 system user NULL Connect 15 Delete_rows_log_event::find_row(-1) NULL 0.000 16 system user NULL Killed 15 Delete_rows_log_event::find_row(-1) NULL 0.000 17 system user NULL Connect 62 Waiting for room in worker thread event queue NULL 0.000 21 root localhost information_schema Query 0 init show processlist 0.000 trx_id trx_mysql_thread_id trx_time 41844148 15 16 41844151 14 16 41844154 16 16 locks_trx_id locks_mysql_thread_id locks_dname locks_key_left locks_key_right locks_table_schema locks_table_name locks_table_dictionary_name 41844148 15 ./sbtest/sbtest1-main 00f1010000 00f1010000 sbtest sbtest1 main 41844148 15 ./sbtest/sbtest1-main 00f1010000 00f1010000 sbtest sbtest1 main 41844148 15 ./sbtest/sbtest1-main 00f1010000 00f1010000 sbtest sbtest1 main 41844148 15 ./sbtest/sbtest1-main 00f7010000 00f7010000 sbtest sbtest1 main 41844148 15 ./sbtest/sbtest1-main 00f7010000 00f7010000 sbtest sbtest1 main 41844148 15 ./sbtest/sbtest1-key-k_1 00f2010000f1010000 00f2010000f1010000 sbtest sbtest1 key-k_1 41844148 15 ./sbtest/sbtest1-key-k_1 00f3010000f1010000 00f3010000f1010000 sbtest sbtest1 key-k_1 41844151 14 ./sbtest/sbtest1-main 00f8010000 00f8010000 sbtest sbtest1 main 41844151 14 ./sbtest/sbtest1-main 00f8010000 00f8010000 sbtest sbtest1 main 41844151 14 ./sbtest/sbtest1-main 00f0010000 00f0010000 sbtest sbtest1 main 41844151 14 ./sbtest/sbtest1-main 00f0010000 00f0010000 sbtest sbtest1 main 41844151 14 ./sbtest/sbtest1-key-k_1 00df010000f8010000 00df010000f8010000 sbtest sbtest1 key-k_1 41844151 14 ./sbtest/sbtest1-key-k_1 00e0010000f8010000 00e0010000f8010000 sbtest sbtest1 key-k_1 41844154 16 ./sbtest/sbtest1-main 00f3010000 00f3010000 sbtest sbtest1 main 41844154 16 ./sbtest/sbtest1-main 00f3010000 00f3010000 sbtest sbtest1 main 41844154 16 ./sbtest/sbtest1-main 00f9010000 00f9010000 sbtest sbtest1 main 41844154 16 ./sbtest/sbtest1-main 00f9010000 00f9010000 sbtest sbtest1 main 41844154 16 ./sbtest/sbtest1-key-k_1 00f5010000f3010000 00f5010000f3010000 sbtest sbtest1 key-k_1 41844154 16 ./sbtest/sbtest1-key-k_1 00f6010000f3010000 00f6010000f3010000 sbtest sbtest1 key-k_1 41844154 16 ./sbtest/sbtest1-key-k_1 00f2010000f9010000 00f2010000f9010000 sbtest sbtest1 key-k_1 41844154 16 ./sbtest/sbtest1-key-k_1 00f3010000f9010000 00f3010000f9010000 sbtest sbtest1 key-k_1 requesting_trx_id blocking_trx_id lock_waits_dname lock_waits_key_left lock_waits_key_right lock_waits_start_time lock_waits_table_schema lock_waits_table_name lock_waits_table_dictionary_name 41844148 41844154 ./sbtest/sbtest1-main 00f9010000 00f9010000 1471275642868 sbtest sbtest1 main 41844154 41844151 ./sbtest/sbtest1-main 00f0010000 00f0010000 1471275642868 sbtest sbtest1 main
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