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[筆記] 測試 postgresql 的pg_prewarm 對效能的影響 / test pg_prewarm in postgresql 11

老闆提到想要把新系統的 postgresql 資料庫都撈到記憶體裡面

但是否決了我提出的ramdisk 作法(因為當機的話,資料就沒了)

在找資料的時候,發現了這個postgresql 的 pg_prewarm extension

好像有點意思?就來測試看看吧!

只是目前還不知道該怎麼解讀測試的數據就是了…

幹!林北真的不是 DBA 啦 =.=

安裝系統、postgresql 資料庫什麼的就不提了,那不是這次的重點

修改 postgresql.conf

編輯postgresql.conf,開啟平行處理以及設定可用記憶體容量

這台測試機的環境是一台三代i7 , 24G RAM , 240G SSD,安裝debian 10(buster)

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# load libiriaes
# 其實這次不會用到pg_stat_statements ,不過出於習慣,還是加入開機自動載入吧
shared_preload_libraries = 'pg_stat_statements'

#------------------------------------------------------------------------------
# CUSTOMIZED OPTIONS
#------------------------------------------------------------------------------

max_connections = 20
shared_buffers = 6GB
effective_cache_size = 18GB
maintenance_work_mem = 1536MB
checkpoint_completion_target = 0.7
wal_buffers = 16MB
default_statistics_target = 100
random_page_cost = 1.1
effective_io_concurrency = 200
work_mem = 78643kB
min_wal_size = 1GB
max_wal_size = 2GB
max_worker_processes = 8
max_parallel_workers_per_gather = 4
max_parallel_workers = 8

重新啟動postgresql ,準備開始測試囉!

轉換成 postgres 身份後,進入 psql

建立測試資料庫

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postgres=# create database test;
CREATE DATABASE
postgres=#

連接測試資料庫、建立pg_prewarm extension

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postgres=# \c test ;
You are now connected to database "test" as user "postgres".
test=# CREATE EXTENSION pg_prewarm;
CREATE EXTENSION
test=#

建立測試資料表,塞入500萬筆資料

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test=# \timing
Timing is on.
test=# CREATE TABLE test_tbl AS
SELECT floor(random() * (9923123) + 1)::int FROM generate_series(1, 5000000) AS id;
SELECT 5000000
Time: 2940.602 ms (00:02.941)
test=#

檢查看看剛剛建立的table 用了多少空間

哎呀,看起來用得不多啊

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test=# SELECT pg_size_pretty(pg_relation_size('test_tbl'));
173 MB

玩大一點,塞個一億筆資料好了

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test=# drop table test_tbl;
Time: 0.361 ms
test=# CREATE TABLE test_tbl AS
SELECT floor(random() * (99343) + 1)::int FROM generate_series(1, 100000000) AS id;
SELECT 100000000
Time: 6321.415 ms (00:06.321)

test=# SELECT pg_size_pretty(pg_relation_size('test_tbl'));
pg_size_pretty | 3457 MB

Time: 0.589 ms
test=#

好,現在資料庫長到3457MB了

先來執行一些初步的取得基本數據

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test=# explain (analyze,buffers) select count(*) from test_tbl;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
Finalize Aggregate (cost=755978.52..755978.53 rows=1 width=8) (actual time=3331.917..3331.918 rows=1 loops=1)
Buffers: shared hit=160 read=442318
-> Gather (cost=755978.10..755978.51 rows=4 width=8) (actual time=3331.876..3333.674 rows=5 loops=1)
Workers Planned: 4
Workers Launched: 4
Buffers: shared hit=160 read=442318
-> Partial Aggregate (cost=754978.10..754978.11 rows=1 width=8) (actual time=3329.279..3329.280 rows=1 loops=5)
Buffers: shared hit=160 read=442318
-> Parallel Seq Scan on test_tbl (cost=0.00..692478.08 rows=25000008 width=0) (actual time=0.029..1924.601 rows=20000000 loops=5)
Buffers: shared hit=160 read=442318
Planning Time: 0.040 ms
Execution Time: 3333.729 ms
(12 rows)

(END)

可以看到打中buffer 的部份其實很少,只有 160 ,大部分都是讀進去buffer (442318)

來看看 buffer 的使用狀況

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test=# CREATE EXTENSION pg_buffercache;
CREATE EXTENSION
test=# select c.relname,pg_size_pretty(count(*) * 8192) as buffered,
test-# round(100.0 * count(*) / (
test(# select setting from pg_settings
test(# where name='shared_buffers')::integer,1)
test-# as buffer_percent,
test-# round(100.0*count(*)*8192 / pg_table_size(c.oid),1) as percent_of_relation
test-# from pg_class c inner join pg_buffercache b on b.relfilenode = c.relfilenode inner
test-# join pg_database d on ( b.reldatabase =d.oid and d.datname =current_database())
test-# group by c.oid,c.relname order by 3 desc limit 10;
relname | buffered | buffer_percent | percent_of_relation
--------------+------------+----------------+---------------------
test_tbl | 18 MB | 0.3 | 0.5
pg_am | 8192 bytes | 0.0 | 20.0
pg_index | 24 kB | 0.0 | 37.5
pg_amproc | 32 kB | 0.0 | 50.0
pg_cast | 16 kB | 0.0 | 33.3
pg_depend | 64 kB | 0.0 | 13.3
pg_amop | 48 kB | 0.0 | 54.5
pg_namespace | 8192 bytes | 0.0 | 20.0
pg_opclass | 16 kB | 0.0 | 28.6
pg_aggregate | 8192 bytes | 0.0 | 16.7
(10 rows)

Time: 148.719 ms
test=#

可以看到這個 test_tbl 只有0.5% 被撈到shared_buffers 裡面

接下來就把這個table全部推到shared_buffers 裡面去

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test=# select pg_prewarm('test_tbl','buffer');
pg_prewarm
------------
442478
(1 row)

Time: 1938.043 ms (00:01.938)
test=#

然後再來看一次shared_buffers的使用狀況

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test=# select c.relname,pg_size_pretty(count(*) * 8192) as buffered, 
round(100.0 * count(*) / (
select setting from pg_settings
where name='shared_buffers')::integer,1)
as buffer_percent,
round(100.0*count(*)*8192 / pg_table_size(c.oid),1) as percent_of_relation
from pg_class c inner join pg_buffercache b on b.relfilenode = c.relfilenode inner
join pg_database d on ( b.reldatabase =d.oid and d.datname =current_database())
group by c.oid,c.relname order by 3 desc limit 10;
relname | buffered | buffer_percent | percent_of_relation
--------------+------------+----------------+---------------------
test_tbl | 3457 MB | 56.3 | 100.0
pg_am | 8192 bytes | 0.0 | 20.0
pg_index | 24 kB | 0.0 | 37.5
pg_amproc | 32 kB | 0.0 | 50.0
pg_cast | 16 kB | 0.0 | 33.3
pg_depend | 64 kB | 0.0 | 13.3
pg_amop | 48 kB | 0.0 | 54.5
pg_namespace | 8192 bytes | 0.0 | 20.0
pg_opclass | 16 kB | 0.0 | 28.6
pg_aggregate | 8192 bytes | 0.0 | 16.7
(10 rows)

Time: 2778.354 ms (00:02.778)
test=#

OK ,可以看到 test_tbl 已經通通被載入 shared_buffers 中

buffered 表示表格被載入shared_buffers的大小

buffer_percent 表示這個表格佔用多少shared_buffers 的比例

percent_of_relation 表示這個表格有多少比例被載入 shared_buffers

再來跑一次explain看看狀況

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test=# explain (analyze,buffers) select count(*) from test_tbl;
Time: 3551.785 ms (00:03.552)
Finalize Aggregate (cost=755978.52..755978.53 rows=1 width=8) (actual time=3427.286..3427.287 rows=1 loops=1)
Buffers: shared hit=442478
-> Gather (cost=755978.10..755978.51 rows=4 width=8) (actual time=3427.215..3551.326 rows=5 loops=1)
Workers Planned: 4
Workers Launched: 4
Buffers: shared hit=442478
-> Partial Aggregate (cost=754978.10..754978.11 rows=1 width=8) (actual time=3423.659..3423.659 rows=1 loops=5)
Buffers: shared hit=442478
-> Parallel Seq Scan on test_tbl (cost=0.00..692478.08 rows=25000008 width=0) (actual time=0.017..1976.744 rows=20000000 loops=5)
Buffers: shared hit=442478
Planning Time: 0.039 ms
Execution Time: 3551.365 ms
(12 rows)

這邊就可以看到都是從buffer 讀出來所以 hit=442478

看樣子表格還是太小,所以沒有完全發揮?那再來把表格加大!

先重開一次 postgresql 清除buffer

然後重新建立表格

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test=# drop table test_tbl;
DROP TABLE
Time: 297.493 ms
test=# CREATE TABLE test_tbl AS
test-# SELECT floor(random() * (993343) + 1)::int FROM generate_series(1, 300000000) AS id;
SELECT 300000000
Time: 290660.607 ms (04:50.661)
test=#

一樣,看看用了多少容量

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test=# SELECT pg_size_pretty(pg_relation_size('test_tbl'));
pg_size_pretty
----------------
10 GB
(1 row)

Time: 0.474 ms
test=#

哇哈哈,用了10G ,這次還不撐爆你!

跑explain 看看狀況

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test=# explain (analyze,buffers) select count(*) from test_tbl;
Time: 22909.065 ms (00:22.909)

QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
Finalize Aggregate (cost=2265934.72..2265934.73 rows=1 width=8) (actual time=22906.045..22906.045 rows=1 loops=1)
Buffers: shared hit=2080 read=1325354 dirtied=1295425 written=1295265
-> Gather (cost=2265934.30..2265934.71 rows=4 width=8) (actual time=22905.997..22908.522 rows=5 loops=1)
Workers Planned: 4
Workers Launched: 4
Buffers: shared hit=2080 read=1325354 dirtied=1295425 written=1295265
-> Partial Aggregate (cost=2264934.30..2264934.31 rows=1 width=8) (actual time=22903.473..22903.474 rows=1 loops=5)
Buffers: shared hit=2080 read=1325354 dirtied=1295425 written=1295265
-> Parallel Seq Scan on test_tbl (cost=0.00..2077434.24 rows=75000024 width=0) (actual time=0.040..18374.277 rows=60000000 loops=5)
Buffers: shared hit=2080 read=1325354 dirtied=1295425 written=1295265
Planning Time: 0.094 ms
Execution Time: 22908.571 ms
(12 rows)

看一下現在 shared_buffers 使用狀況

可以看到這個 test_tbl 幾乎沒被放入 shared_buffers 中

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test=# select c.relname,pg_size_pretty(count(*) * 8192) as buffered, 
round(100.0 * count(*) / (
select setting from pg_settings
where name='shared_buffers')::integer,1)
as buffer_percent,
round(100.0*count(*)*8192 / pg_table_size(c.oid),1) as percent_of_relation
from pg_class c inner join pg_buffercache b on b.relfilenode = c.relfilenode inner
join pg_database d on ( b.reldatabase =d.oid and d.datname =current_database())
group by c.oid,c.relname order by 3 desc limit 10;
relname | buffered | buffer_percent | percent_of_relation
--------------+------------+----------------+---------------------
test_tbl | 18 MB | 0.3 | 0.2
pg_am | 8192 bytes | 0.0 | 20.0
pg_index | 24 kB | 0.0 | 37.5
pg_amproc | 32 kB | 0.0 | 50.0
pg_cast | 16 kB | 0.0 | 33.3
pg_depend | 64 kB | 0.0 | 13.3
pg_amop | 48 kB | 0.0 | 54.5
pg_namespace | 8192 bytes | 0.0 | 20.0
pg_opclass | 16 kB | 0.0 | 28.6
pg_aggregate | 8192 bytes | 0.0 | 16.7
(10 rows)

Time: 163.936 ms
test=#

強制把test_tbl 全部塞進 shared_buffers

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test=# select pg_prewarm('test_tbl','buffer');
pg_prewarm
------------
1327434
(1 row)

Time: 7472.805 ms (00:07.473)
test=#

確認一下test_tbl 有沒有被整個塞進去

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test=# select c.relname,pg_size_pretty(count(*) * 8192) as buffered, 
round(100.0 * count(*) / (
select setting from pg_settings
where name='shared_buffers')::integer,1)
as buffer_percent,
round(100.0*count(*)*8192 / pg_table_size(c.oid),1) as percent_of_relation
from pg_class c inner join pg_buffercache b on b.relfilenode = c.relfilenode inner
join pg_database d on ( b.reldatabase =d.oid and d.datname =current_database())
group by c.oid,c.relname order by 3 desc limit 10;
relname | buffered | buffer_percent | percent_of_relation
--------------+------------+----------------+---------------------
test_tbl | 6142 MB | 100.0 | 59.2
pg_am | 8192 bytes | 0.0 | 20.0
pg_index | 24 kB | 0.0 | 37.5
pg_amproc | 32 kB | 0.0 | 50.0
pg_cast | 16 kB | 0.0 | 33.3
pg_depend | 24 kB | 0.0 | 5.0
pg_amop | 40 kB | 0.0 | 45.5
pg_namespace | 8192 bytes | 0.0 | 20.0
pg_opclass | 16 kB | 0.0 | 28.6
pg_aggregate | 8192 bytes | 0.0 | 16.7
(10 rows)

Time: 4985.366 ms (00:04.985)
test=#

GOOD ! let’s do explain again !

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test=# explain (analyze,buffers) select count(*) from test_tbl;
Time: 11451.188 ms (00:11.451)
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
Finalize Aggregate (cost=2265934.72..2265934.73 rows=1 width=8) (actual time=11231.664..11231.664 rows=1 loops=1)
Buffers: shared hit=785963 read=541471
-> Gather (cost=2265934.30..2265934.71 rows=4 width=8) (actual time=11231.606..11450.719 rows=5 loops=1)
Workers Planned: 4
Workers Launched: 4
Buffers: shared hit=785963 read=541471
-> Partial Aggregate (cost=2264934.30..2264934.31 rows=1 width=8) (actual time=11228.829..11228.830 rows=1 loops=5)
Buffers: shared hit=785963 read=541471
-> Parallel Seq Scan on test_tbl (cost=0.00..2077434.24 rows=75000024 width=0) (actual time=0.037..6414.711 rows=60000000 loops=5)
Buffers: shared hit=785963 read=541471
Planning Time: 0.039 ms
Execution Time: 11450.781 ms
(12 rows)

確認一下,果然大部分都打到cache 了,但是因為shared_buffers 不夠大,所以還會從磁碟讀取一部分

而且時間也比之前還沒塞進shared_buffers 的時候要快了不少

22908.571 –> 11450.781 ms


從這次的測試看來,我想如果有足夠大的記憶體,能夠把資料表都塞入shared_buffers 中

應該可以帶來不錯的效能增幅!