Postgres performance not increasing with increase in number of core

Question

I was trying out postgres google-cloud-sql and loaded a simple school schema

 CREATE TABLE school ( 
    id SERIAL NOT NULL PRIMARY KEY,
    name TEXT NOT NULL
);

CREATE TABLE class ( 
    id SERIAL NOT NULL PRIMARY KEY,
    name TEXT,
    school_id INTEGER NOT NULL REFERENCES school
);

CREATE TABLE student ( 
    id SERIAL NOT NULL PRIMARY KEY,
    name TEXT,
    class_id INTEGER NOT NULL REFERENCES class
);

-- ALL id and foreign keys have indexs

Loaded ~15 millions row in total with 1500 school, 500 class per school, 200 student per class.

After that create a simple pgbench script

\setrandom sId1 1 20000000
\setrandom sId2 1 20000000
\setrandom sId3 1 20000000

select count(*) from school s 
join class c on s.id=c.school_id 
join student stu on c.id=stu.class_id where s.id=:sId1;

select count(*) from school s 
join class c on s.id=c.school_id 
join student stu on c.id=stu.class_id where s.id=:sId2;

select count(*) from school s 
join class c on s.id=c.school_id 
join student stu on c.id=stu.class_id where s.id=:sId3;

Now running the the script with

pgbench -c 90 -f ./sql.sql  -n -t 1000

2 cores, 7.5 GB, 90 client --

OUTPUT:
number of transactions actually processed: 90000/90000
tps = 1519.690555 (including connections establishing)
tps = 2320.408683 (excluding connections establishing

26 cores, 30 GB, 90 client-

number of transactions actually processed: 90000/90000
tps = 1553.721286 (including connections establishing)
tps = 2405.664795 (excluding connections establishing)

Question: Why do we have only 80 tps increase from 2 core to 26 cores ?

Answer 1

I asked same question on the postgres irc.

Community was sure that i was maxing out the client pgbench , they suggested to use -j4 in pgbench and tps increased to 23k per sec.