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Re: MDEV-4956 - Reduce usage of LOCK_open: TABLE_SHARE::tdc.used_tables


Hi, Sergey!

On Sep 13, Sergey Vojtovich wrote:
> Hi Sergei,
> comments inline and a question: 10.0 throughput is twice lower than 5.6
> in a specific case. It is known to be caused by tc_acquire_table() and
> tc_release_table(). Do we want to fix it? If yes - how?

How is it caused by tc_acquire_table/tc_release_table?
In what specific case?

> > > > Why per-share lists are updated under the global mutex?
> > > Alas, it doesn't solve CPU cache coherence problem.
> > It doesn't solve CPU cache coherence problem, yes.
> > And it doesn't help if you have only one hot table.
> > But it certainly helps if many threads access many tables.
> Ok, let's agree to agree: it will help in certain cases. Most probably it
> won't improve situation much if all threads access single table.

Of course.

> We could try to ensure that per-share mutex is on the same cache line as
> free_tables and used_tables list heads. In this case I guess
> mysql_mutex_lock(&share->tdc.LOCK_table_share) will load list heads into
> CPU cache along with mutex structure. OTOH we still have to read per-TABLE
> prev/next pointers. And in 5.6 per-partition mutex should less frequently
> jump out of CPU cache than our per-share mutex. Worth trying?

Did you benchmark that these cache misses are a problem?
What is the main problem that impacts the performance?


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