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

On Thu, Feb 6, 2020 at 1:57 AM Mahendra Singh Thalor <mahi6run(at)gmail(dot)com> wrote:
>
> On Wed, 5 Feb 2020 at 12:07, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > 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.
> >
>
> 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(dot)sql(at)1 -f insert2(dot)sql(at)1 -f insert3(dot)sql(at)1 -f insert4(dot)sql(at)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.

> 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.

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.

> 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.

One thing we can do is to somehow measure the collisions on each bucket.

[1] - https://www.postgresql.org/message-id/ef81da49-d491-db86-3ef6-5138d091fe91%40postgrespro.ru

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com