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

On Thu, 6 Feb 2020 at 09:44, Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Thu, Feb 6, 2020 at 1:57 AM Mahendra Singh Thalor <mahi6run@gmail.com> wrote:
> >
> > On Wed, 5 Feb 2020 at 12:07, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > On Mon, Feb 3, 2020 at 8:03 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > On Tue, Jun 26, 2018 at 12:47 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > > >
> > > > > On Fri, Apr 27, 2018 at 4:25 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> > > > > > On Thu, Apr 26, 2018 at 3:10 PM, Andres Freund <andres@anarazel.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.
> > >
> >
> > Thanks Sawada-san for patch.
> >
> > From last few days, I was reading this thread and was reviewing v13 patch.  To debug and test, I did re-base of v13 patch. I compared my re-based patch and v14 patch. I think,  ordering of header files is not alphabetically in v14 patch. (I haven't reviewed v14 patch fully because before review, I wanted to test false sharing).  While debugging, I didn't noticed any hang or lock related issue.
> >
> > I did some testing to test false sharing(bulk insert, COPY data, bulk insert into partitions tables).  Below is the testing summary.
> >
> > Test setup(Bulk insert into partition tables):
> > autovacuum=off
> > shared_buffers=512MB -c max_wal_size=20GB -c checkpoint_timeout=12min
> >
> > Basically, I created a table with 13 partitions. Using pgbench, I inserted bulk data. I used below pgbench command:
> > ./pgbench -c $threads -j $threads -T 180 -f insert1.sql@1 -f insert2.sql@1 -f insert3.sql@1 -f insert4.sql@1 postgres
> >
> > I took scripts from previews mails and modified. For reference, I am attaching test scripts.  I tested with default 1024 slots(N_RELEXTLOCK_ENTS = 1024).
> >
> > Clients          HEAD (tps)                     With v14 patch (tps)      %change      (time: 180s)
> > 1                    92.979796                        100.877446                     +8.49 %
> > 32                   392.881863                      388.470622                    -1.12 %
> > 56                   551.753235                       528.018852                   -4.30 %
> > 60                   648.273767                       653.251507                   +0.76 %
> > 64                   645.975124                       671.322140                   +3.92 %
> > 66                   662.728010                       673.399762                   +1.61 %
> > 70                   647.103183                       660.694914                   +2.10 %
> > 74                   648.824027                       676.487622                  +4.26 %
> >
> > From above results, we can see that in most cases, TPS is slightly increased with v14 patch. I am still testing and will post my results.
> >
>
> The number at 56 and 74 client count seem slightly suspicious.   Can
> you please repeat those tests?  Basically, I am not able to come up
> with a theory why at 56 clients the performance with the patch is a
> bit lower and then at 74 it is higher.

Okay. I will repeat test.

>
> > I want to test extension lock by blocking use of fsm(use_fsm=false in code).  I think, if we block use of fsm, then load will increase into extension lock.  Is this correct way to test?
> >
>
> Hmm, I think instead of directly hacking the code, you might want to
> use the operation (probably cluster or vacuum full) where we set
> HEAP_INSERT_SKIP_FSM.  I think along with this you can try with
> unlogged tables because that might stress the extension lock.

Okay. I will test.

>
> In the above test, you might want to test with a higher number of
> partitions (say up to 100) as well.  Also, see if you want to use the
> Copy command.

Okay. I will test.

>
> > Please let me know if you have any specific testing scenario.
> >
>
> Can you test the scenario mentioned by Konstantin Knizhnik [1] where
> this patch has shown significant gain?  You might want to use a higher
> core count machine to test it.

create table test (i int, md5 text);

insert.sql:
begin;
insert into test select i, md5(i::text) from generate_series(1,1000) AS i;
end;

pgbench command:
./pgbench postgres  -c 1000 -j 36 -T 180 -P 10 -f insert.sql >> results.txt
TPS on HEAD:
Run 1) : 608.908721
Run 2) : 599.962863
Run 3) : 606.378819
Run 4) : 607.174076
Run 5) : 598.531958

TPS with v14 patch: ( N_RELEXTLOCK_ENTS = 1024)
Run 1) : 649.488472
Run 2) : 657.902261
Run 3) : 654.478580
Run 4) : 648.085126
Run 5) : 647.171482

--
Thanks and Regards
Mahendra Singh Thalor
EnterpriseDB: http://www.enterprisedb.com