--- title: "[筆記] 測試 postgresql 的pg_prewarm 對效能的影響 / test pg_prewarm in postgresql 11" date: 2019-12-20T14:31:42+08:00 draft: false noSummary: false categories: ['筆記'] image: https://h.cowbay.org/images/post-default-9.jpg tags: ['postgresql'] author: "Eric Chang" keywords: - postgresql - pg_prewarm --- 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) ``` # 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 ``` postgres=# create database test; CREATE DATABASE postgres=# ``` #### 連接測試資料庫、建立pg_prewarm extension ``` 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 ``` 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 ``` 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** ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ``` 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 ! ``` 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！