Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Mon, Feb 3, 2020 at 8:03 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Jun 26, 2018 at 12:47 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Fri, Apr 27, 2018 at 4:25 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > > On Thu, Apr 26, 2018 at 3:10 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > >>> I think the real question is whether the scenario is common enough to
> > >>> worry about. In practice, you'd have to be extremely unlucky to be
> > >>> doing many bulk loads at the same time that all happened to hash to
> > >>> the same bucket.
> > >>
> > >> With a bunch of parallel bulkloads into partitioned tables that really
> > >> doesn't seem that unlikely?
> > >
> > > It increases the likelihood of collisions, but probably decreases the
> > > number of cases where the contention gets really bad.
> > >
> > > For example, suppose each table has 100 partitions and you are
> > > bulk-loading 10 of them at a time. It's virtually certain that you
> > > will have some collisions, but the amount of contention within each
> > > bucket will remain fairly low because each backend spends only 1% of
> > > its time in the bucket corresponding to any given partition.
> > >
> >
> > I share another result of performance evaluation between current HEAD
> > and current HEAD with v13 patch(N_RELEXTLOCK_ENTS = 1024).
> >
> > Type of table: normal table, unlogged table
> > Number of child tables : 16, 64 (all tables are located on the same tablespace)
> > Number of clients : 32
> > Number of trials : 100
> > Duration: 180 seconds for each trials
> >
> > The hardware spec of server is Intel Xeon 2.4GHz (HT 160cores), 256GB
> > RAM, NVMe SSD 1.5TB.
> > Each clients load 10kB random data across all partitioned tables.
> >
> > Here is the result.
> >
> > childs | type | target | avg_tps | diff with HEAD
> > --------+----------+---------+------------+------------------
> > 16 | normal | HEAD | 1643.833 |
> > 16 | normal | Patched | 1619.5404 | 0.985222
> > 16 | unlogged | HEAD | 9069.3543 |
> > 16 | unlogged | Patched | 9368.0263 | 1.032932
> > 64 | normal | HEAD | 1598.698 |
> > 64 | normal | Patched | 1587.5906 | 0.993052
> > 64 | unlogged | HEAD | 9629.7315 |
> > 64 | unlogged | Patched | 10208.2196 | 1.060073
> > (8 rows)
> >
> > For normal tables, loading tps decreased 1% ~ 2% with this patch
> > whereas it increased 3% ~ 6% for unlogged tables. There were
> > collisions at 0 ~ 5 relation extension lock slots between 2 relations
> > in the 64 child tables case but it didn't seem to affect the tps.
> >
>
> AFAIU, this resembles the workload that Andres was worried about. I
> think we should once run this test in a different environment, but
> considering this to be correct and repeatable, where do we go with
> this patch especially when we know it improves many workloads [1] as
> well. We know that on a pathological case constructed by Mithun [2],
> this causes regression as well. I am not sure if the test done by
> Mithun really mimics any real-world workload as he has tested by
> making N_RELEXTLOCK_ENTS = 1 to hit the worst case.
>
> Sawada-San, if you have a script or data for the test done by you,
> then please share it so that others can also try to reproduce it.

Unfortunately the environment I used for performance verification is
no longer available.

I agree to run this test in a different environment. I've attached the
rebased version patch. I'm measuring the performance with/without
patch, so will share the results.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
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