vacuum_mem

Hi,

I tested vacuum_mem setting under a
4CPU and 4G RAM machine. I am the only person
on that machine.

The table:
tablename | size_kb | reltuples
---------------------------+-------------------------
big_t | 2048392 | 7.51515e+06

Case 1:
1. vacuum full big_t;
2. begin;
update big_t set email = lpad('a', 255, 'b');
rollback;
3. set vacuum_mem=655360; -- 640M
4. vacuum big_t;
It takes 1415,375 ms
Also from top, the max SIZE is 615M while
SHARE is always 566M

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM
TIME COMMAND
5914 postgres 16 0 615M 615M 566M D 7.5 15.8
21:21 postgres: postgres mydb xxx.xxx.xxx.xxx:34361
VACUUM

Case 2:
1. vacuum full big_t;
2. begin;
update big_t set email = lpad('a', 255, 'b');
rollback;
3. set vacuum_mem=65536; -- 64M
4. vacuum big_t;
It takes 1297,798 ms
Also from top, the max SIZE is 615M while
SHARE is always 566M

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM
TIME COMMAND
3613 postgres 15 0 615M 615M 566M D 17.1 15.8
9:04 postgres: postgres mydb xxx.xxx.xxx.xxx:34365
VACUUM

It seems vacuum_mem does not have performance
effect at all.

In reality, we vaccum nightly and I want to find out
which vacuum_mem value is the
best to short vacuum time.

Any thoughts?

Thanks,

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