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Re: [Maria-discuss] Known limitation with TokuDB in Read Free Replication & parallel replication ?


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 <knielsen@xxxxxxxxxxxxxxx>

> Rich Prohaska <prohaska7@xxxxxxxxx> 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.

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