Thread

postgresql13-llvm jit-13.5-1PGDG.rhel8.x86_64

Mladen Gogala <gogala.mladen@gmail.com> — 2021-11-15T04:21:54Z

I was trying to upgrade my test 13.4 instance on Oracle Linux 8.4 
(x86_64) to 13.5. I can't upgrade postgresql13-llvm jit because Oracle's 
and Red Hat repositories still don't have the required version of llvm 
(12.1.0.2):

root@postgres mgogala]# rpm -qa|grep llvm
llvm-libs-11.0.1-2.0.1.module+el8.4.0+20397+f876858a.x86_64
llvm7.0-libs-7.0.1-7.el8.x86_64
llvm-test-11.0.1-2.0.1.module+el8.4.0+20397+f876858a.x86_64
llvm-11.0.1-2.0.1.module+el8.4.0+20397+f876858a.x86_64
llvm-static-11.0.1-2.0.1.module+el8.4.0+20397+f876858a.x86_64
llvm-devel-11.0.1-2.0.1.module+el8.4.0+20397+f876858a.x86_64
[root@postgres mgogala]#

I am getting the following error:

postgresql13-llvm jit-13.5-1PGDG.rhel8.x86_64 requires 
libLLVM-12.so()(64bit), but none of the providers can be installed.

There is a CentOS8-stream version which solves the problem but I cannot 
use that in the office. I will probably have to wait for another month 
before OL8 has everything that I need in its repositories. Now, the 
question is what kind of an impact will running without llvm-jit have? 
According to the links below, llvm-jit effects are quite spectacular:

https://llvm.org/devmtg/2016-09/slides/Melnik-PostgreSQLLLVM.pdf

https://www.pgcon.org/2017/schedule/events/1092.en.html

Now, the question is whether anyone on this list can quantify the 
difference? What would be a better option? To wait for the repos to 
receive the necessary packages or to run without llvm-jit? In the office 
I have some rather large databases and CREATE INDEX CONCURRENTLY and 
REINDEX CONCURRENTLY  fixes in 13.5 are highly desired but not at the 
cost of the overall application performance.

Regards


-- 
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
https://dbwhisperer.wordpress.com

Re: postgresql13-llvm jit-13.5-1PGDG.rhel8.x86_64

Pavel Stehule <pavel.stehule@gmail.com> — 2021-11-15T05:04:47Z

Hi


> There is a CentOS8-stream version which solves the problem but I cannot
> use that in the office. I will probably have to wait for another month
> before OL8 has everything that I need in its repositories. Now, the
> question is what kind of an impact will running without llvm-jit have?
> According to the links below, llvm-jit effects are quite spectacular:
>
> https://llvm.org/devmtg/2016-09/slides/Melnik-PostgreSQLLLVM.pdf
>
> https://www.pgcon.org/2017/schedule/events/1092.en.html


When JIT was used on very large query with a lot of CASE expr, then JIT has
a positive effect about 50%. On usual large queries, the effect of JIT was
about 20%. Unfortunately, JIT is sensitive to estimation, and the JIT
sometimes increases seconds to queries, although without JIT this query is
executed in ms. When you use a query that can be well calculated in
parallel, then positive effect of JIT is less.

Regards

Pavel



>
>
>
> --
> Mladen Gogala
> Database Consultant
> Tel: (347) 321-1217
> https://dbwhisperer.wordpress.com
>
>
>
>

Re: postgresql13-llvm jit-13.5-1PGDG.rhel8.x86_64

Mladen Gogala <gogala.mladen@gmail.com> — 2021-11-15T13:56:54Z

On 11/15/21 00:04, Pavel Stehule wrote:
>
> Hi
>
>
>     There is a CentOS8-stream version which solves the problem but I
>     cannot
>     use that in the office. I will probably have to wait for another
>     month
>     before OL8 has everything that I need in its repositories. Now, the
>     question is what kind of an impact will running without llvm-jit
>     have?
>     According to the links below, llvm-jit effects are quite spectacular:
>
>     https://llvm.org/devmtg/2016-09/slides/Melnik-PostgreSQLLLVM.pdf
>
>     https://www.pgcon.org/2017/schedule/events/1092.en.html
>
>
> When JIT was used on very large query with a lot of CASE expr, then 
> JIT has a positive effect about 50%. On usual large queries, the 
> effect of JIT was about 20%. Unfortunately, JIT is sensitive to 
> estimation, and the JIT sometimes increases seconds to queries, 
> although without JIT this query is executed in ms. When you use a 
> query that can be well calculated in parallel, then positive effect of 
> JIT is less.
>
> Regards
>
> Pavel

Thanks Pavel, you answered my question. I'll wait with the upgrade.

Regards

-- 

Mladen Gogala
Database Consultant
Tel: (347) 321-1217
https://dbwhisperer.wordpress.com

Re: postgresql13-llvm jit-13.5-1PGDG.rhel8.x86_64

Rick Otten <rottenwindfish@gmail.com> — 2021-11-15T14:17:14Z

On Mon, Nov 15, 2021 at 8:57 AM Mladen Gogala <gogala.mladen@gmail.com>
wrote:

>
> On 11/15/21 00:04, Pavel Stehule wrote:
>
>
> Hi
>
>
>> There is a CentOS8-stream version which solves the problem but I cannot
>> use that in the office. I will probably have to wait for another month
>> before OL8 has everything that I need in its repositories. Now, the
>> question is what kind of an impact will running without llvm-jit have?
>> According to the links below, llvm-jit effects are quite spectacular:
>>
>> https://llvm.org/devmtg/2016-09/slides/Melnik-PostgreSQLLLVM.pdf
>>
>> https://www.pgcon.org/2017/schedule/events/1092.en.html
>
>
> When JIT was used on very large query with a lot of CASE expr, then JIT
> has a positive effect about 50%. On usual large queries, the effect of JIT
> was about 20%. Unfortunately, JIT is sensitive to estimation, and the JIT
> sometimes increases seconds to queries, although without JIT this query is
> executed in ms. When you use a query that can be well calculated in
> parallel, then positive effect of JIT is less.
>
> Regards
>
> Pavel
>
>
> Thanks Pavel, you answered my question. I'll wait with the upgrade.
>
>
>
FWIW, there was a lively discussion on the postgresql subreddit over the
weekend on JIT:
https://www.reddit.com/r/PostgreSQL/comments/qtsif5/cascade_of_doom_jit_and_how_a_postgres_update_led/

(lively for that subreddit anyway)