Hello Kristian, I have a prototype of the TokuFT code that will cause ALL lock waiters to call their killed callback here: https://github.com/prohaska7/tokuft/tree/kill_lockers
On Mon, Aug 15, 2016 at 11:51 AM, Rich Prohaska <[email protected]> wrote: > 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-a >>> nd-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. >>> >> >> >
_______________________________________________ Mailing list: https://launchpad.net/~maria-developers Post to : [email protected] Unsubscribe : https://launchpad.net/~maria-developers More help : https://help.launchpad.net/ListHelp
