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MDEV-34190: r_engine_stats.pages_read_count is unrealistically low

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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.
This commit is contained in:
Sergei Petrunia
2024-05-27 17:00:12 +03:00
parent 8ed3c37592
commit 513c827041
10 changed files with 163 additions and 4 deletions
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6
mysql-test/main/log_slow_innodb.result
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@ -23,7 +23,7 @@ UPDATE t1 set b=b+1 where a=1 or a=999;
[log_grep.inc] lines: 0
[log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Tmp_tables: \d+ Tmp_disk_tables: \d+$
[log_grep.inc] lines: 0
[log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Pages_accessed: \d+ Pages_read: \d+ Pages_updated: \d+ Old_rows_read: \d+$ expected_matches: 2
[log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Pages_accessed: \d+ Pages_read: \d+ Pages_prefetched: \d+ Pages_updated: \d+ Old_rows_read: \d+$ expected_matches: 2
[log_grep.inc] found expected match count: 2
[log_grep.inc] file: log_slow_innodb-verbosity_1 pattern: ^# Pages_read_time: \d+\.\d+ Engine_time: \d+\.\d+$ expected_matches: 2
[log_grep.inc] found expected match count: 2
@ -47,7 +47,7 @@ SELECT 1;
[log_grep.inc] lines: 0
[log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Tmp_tables: \d+ Tmp_disk_tables: \d+$
[log_grep.inc] lines: 0
[log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Pages_accessed: \d+ Pages_read: \d+ Pages_updated: \d+ Old_rows_read: \d+$
[log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Pages_accessed: \d+ Pages_read: \d+ Pages_prefetched: \d+ Pages_updated: \d+ Old_rows_read: \d+$
[log_grep.inc] lines: 0
[log_grep.inc] file: log_slow_innodb-verbosity_2 pattern: ^# Pages_read_time: \d+\.\d+ Engine_time: \d+\.\d+$
[log_grep.inc] lines: 0
@ -85,7 +85,7 @@ INSERT INTO t1 VALUE(1000) pattern: ^# Tmp_tables: \d+ Tmp_disk_tables: \d+$
[log_grep.inc] lines: 0
[log_grep.inc] file: log_slow_innodb-verbosity_3
--source include/log_slow_start.inc
INSERT INTO t1 VALUE(1000) pattern: ^# Pages_accessed: \d+ Pages_read: \d+ Pages_updated: \d+ Old_rows_read: \d+$
INSERT INTO t1 VALUE(1000) pattern: ^# Pages_accessed: \d+ Pages_read: \d+ Pages_prefetched: \d+ Pages_updated: \d+ Old_rows_read: \d+$
[log_grep.inc] lines: 0
[log_grep.inc] file: log_slow_innodb-verbosity_3
--source include/log_slow_start.inc