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513c827041
The symptoms were: take a server with no activity and a table that's not in the buffer pool. Run a query that reads the whole table and observe that r_engine_stats.pages_read_count shows about 2% of the table was read. Who reads the rest? The cause was that page prefetching done inside InnoDB was not counted. This counts page prefetch requests made in buf_read_ahead_random() and buf_read_ahead_linear() and makes them visible in: - ANALYZE: r_engine_stats.pages_prefetch_read_count - Slow Query Log: Pages_prefetched: This patch intentionally doesn't attempt to count the time to read the prefetched pages: * there's no obvious place where one can do it * prefetch reads may be done in parallel (right?), it is not clear how to count the time in this case.
77 lines
2.5 KiB
Text
77 lines
2.5 KiB
Text
--source include/have_innodb.inc
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--source include/have_sequence.inc
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# Each row is 1K.
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create table t1 (
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a varchar(255),
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b varchar(255),
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c varchar(255),
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d varchar(255),
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primary key(a,b,c,d)
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) engine=innodb;
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# The data size is 16K * 1K = 16M
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# 16M / (page_size=16K) = 1K pages.
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SET unique_checks=0, foreign_key_checks= 0;
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begin;
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insert into t1 select
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repeat(uuid(), 7),
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repeat(uuid(), 7),
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repeat(uuid(), 7),
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repeat(uuid(), 7)
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from seq_1_to_16384;
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insert into t1 values ('z','z','z','z');
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commit;
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--echo # Restart the server to make sure we have an empty InnoDB Buffer Pool
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--echo # (in the test's .opt file we've disabled buffer pool saving/loading
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--echo # and also tried to disable any background activity)
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SET GLOBAL innodb_fast_shutdown=0;
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--source include/restart_mysqld.inc
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set @innodb_pages_read0=
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(select variable_value
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from information_schema.session_status
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where variable_name like 'innodb_pages_read');
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let $analyze_output= `analyze format=json
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select * from t1 force index (PRIMARY) order by a,b,c,d`;
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evalp set @js='$analyze_output';
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set @js=json_extract(@js, '$.query_block.table.r_engine_stats');
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#select @js;
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set @pages_accessed= cast(json_value(@js,'$.pages_accessed') as INT);
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set @pages_read_count= cast(json_value(@js,'$.pages_read_count') as INT);
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set @pages_prefetch_read_count= cast(json_value(@js,'$.pages_prefetch_read_count') as INT);
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select @pages_accessed > 1000 and @pages_accessed < 1500;
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set @total_read = (@pages_read_count + @pages_prefetch_read_count);
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select @pages_accessed*0.75 < @total_read, @total_read < @pages_accessed*1.25;
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set @innodb_pages_read1=
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(select variable_value
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from information_schema.session_status
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where variable_name like 'innodb_pages_read');
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set @innodb_pages_read_incr= (@innodb_pages_read1 - @innodb_pages_read0);
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select @innodb_pages_read_incr > 1000, @innodb_pages_read_incr < 1500;
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let $analyze_output= `analyze format=json
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select * from t1 force index (PRIMARY) order by a,b,c,d`;
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evalp set @js='$analyze_output';
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set @js=json_extract(@js, '$.query_block.table.r_engine_stats');
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--echo # This must just print pages_accessed. No page reads or prefetch reads,
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--echo # because the previous query has read all the needed pages into the
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--echo # buffer pool, which is set to be large enough to accomodate the whole
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--echo # table.
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--replace_regex /[0-9]+/NUMBER/
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select @js;
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set @pages_accessed2= cast(json_value(@js,'$.pages_accessed') as INT);
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select @pages_accessed2 = @pages_accessed;
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drop table t1;
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