topsql: use new cache policy for top-n SQL #25744

crazycs520 · 2021-06-24T09:53:05Z

What problem does this PR solve?

#24875 implement the TOP-SQL feature. But the cache policy was not very good. Here is an example:

suppose the config is:

set @@global.tidb_top_sql_precision_seconds=1;
set @@global.tidb_top_sql_max_statement_count=2;
set @@global.tidb_top_sql_report_interval_seconds=3;

At timestamp_1: the SQL and CPU time is:

SQL1: 100ms
SQL2: 200ms

At timestamp_2: the SQL and CPU time is:

SQL3: 300ms
SQL1: 100ms

At timestamp_3: the SQL and CPU time is:

SQL4: 100ms
SQL1: 100ms

As you can see, between timestamp_1 and timestamp_3, the total SQL and CPU time is:

SQL1: 300ms
SQL2: 200ms
SQL3: 300ms
SQL4: 100ms

So, between timestamp_1 and timestamp_3, the final TOP-2 SQL should be:

SQL1: 300ms
SQL3: 300ms

But before this PR, the final Top-2 SQL is:

SQL2: 200ms
SQL3: 300ms

In this PR, the final TOP-2 SQL is:

SQL1: 300ms
SQL3: 300ms

What is changed and how it works?

Reference: https://github.com/breeswish/topn-stores

This PR uses a new cache policy, which saves each Top-N SQL for each timestamp and calculate the final Top-N SQL between the report interval.

This PR also change the default value of tidb_top_sql_max_statement_count variable from 2000 to 200. Since 200 is big enough for one second.

Related changes

PR to update pingcap/docs/pingcap/docs-cn:
Need to cherry-pick to the release branch

Check List

Tests

Unit test

Side effects

Performance regression
- Consumes more MEM

In the default config:

tidb_top_sql_report_interval_seconds = 60;
tidb_top_sql_precision_seconds = 1;
tidb_top_sql_max_statement_count=200;

MaxSQLTextSize=4 * 1024 
MaxPlanTextSize = 32 * 1024

In the best case, doesn't have too much kind of SQL, the memory usage enlarge 1X. The memory usage is 200*36K * 2 =
14MB.

In the worse case, each second has many different SQL digest, the memory will become 200 * 36K * 60 = 422MB.

Release note

No release note.

Signed-off-by: crazycs <crazycs520@gmail.com>

crazycs520 · 2021-06-25T03:03:33Z

@dragonly @breeswish PTAL

dragonly

Suggest adding a holistic description about the 2-round top N eviction algorithm in the code comments.
Rest LGTM.

util/topsql/reporter/reporter.go

breezewish · 2021-06-28T05:46:59Z

We need to adjust back to maximum 200 statements if we switch the evict policy.

breezewish · 2021-06-28T05:48:06Z

Also do we need to merge with pingcap/tipb#233?

Signed-off-by: crazycs <crazycs520@gmail.com>

crazycs520 · 2021-06-28T07:25:52Z

We need to adjust back to maximum 200 statements if we switch the evict policy.

Nice catch, already change
to 200.

Also do we need to merge with pingcap/tipb#233?

Ok, already update the latest tipb.

Signed-off-by: crazycs <crazycs520@gmail.com>

This reverts commit 0c3a765.

This reverts commit 18b57b7.

Signed-off-by: crazycs <crazycs520@gmail.com>

util/topsql/reporter/reporter.go

breezewish · 2021-06-28T15:25:48Z

util/topsql/reporter/reporter.go

+		records = append(records, v)
+	}
+	var evicted []*dataPoints
+	records, evicted = tsr.getTopNDataPoints(records)


I think it is fine to not to evict it here, since immediately after that the map will be dropped entirely. Maybe we can simply ignore them at the report time?

This evict is use to avoid report useless SQL/Plan. And since the data structure of evicted is a slice, it's a little bit of trouble for ignore them at the report time, since we should traverse the slice for each SQL/Plan record.

How about traversing the records and generate a "retain map" first, then iterate all records in the hash map and send records that only in the retain map?

Signed-off-by: crazycs <crazycs520@gmail.com>

…into new-top-sql-cache

dragonly

LGTM

ti-chi-bot · 2021-06-29T03:52:30Z

@dragonly: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

In response to this:

LGTM

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breezewish · 2021-06-29T04:23:24Z

util/topsql/reporter/reporter.go

-			// When `reportDataChan` receives something, there could be ongoing `RegisterSQL` and `RegisterPlan` running,
-			// who writes to the data structure that `data` contains. So we wait for a little while to ensure that
-			// these writes are finished.
-			time.Sleep(time.Millisecond * 100)


Why don't we need this any more?

Signed-off-by: crazycs <crazycs520@gmail.com>

ti-chi-bot · 2021-06-29T10:27:03Z

@tangenta: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

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ti-chi-bot · 2021-06-29T10:31:33Z

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crazycs520 · 2021-06-29T10:35:46Z

/merge

ti-chi-bot · 2021-06-29T10:35:50Z

This pull request has been accepted and is ready to merge.

Commit hash: 3809ae7

Signed-off-by: crazycs <crazycs520@gmail.com>

crazycs520 · 2021-06-29T12:16:41Z

/merge

ti-chi-bot · 2021-06-29T12:16:44Z

This pull request has been accepted and is ready to merge.

Commit hash: 0d12420

ti-chi-bot · 2021-06-29T12:26:14Z

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/run-all-tests

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crazycs520 added 2 commits June 24, 2021 13:24

init

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add test

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ti-chi-bot added the size/L Denotes a PR that changes 100-499 lines, ignoring generated files. label Jun 24, 2021

Merge branch 'master' into new-top-sql-cache

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breezewish added the sig/diagnosis SIG: Diagnosis label Jun 25, 2021

Merge branch 'master' into new-top-sql-cache

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update tipb and address comment

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crazycs520 added 6 commits June 28, 2021 15:29

fix test

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Revert "update tipb and address comment"

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This reverts commit 0c3a765.

Revert "fix test"

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This reverts commit 18b57b7.

address comment

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remove useless sleep

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Merge branch 'master' into new-top-sql-cache

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crazycs520 added 5 commits June 29, 2021 11:09

address comment

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Merge branch 'master' into new-top-sql-cache

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add comment

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Merge branch 'new-top-sql-cache' of https://github.com/crazycs520/tidb …

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…into new-top-sql-cache

Merge branch 'master' into new-top-sql-cache

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add sleep back

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crazycs520 added 2 commits June 29, 2021 15:08

Merge branch 'master' into new-top-sql-cache

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Merge branch 'master' into new-top-sql-cache

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fix test

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topsql: use new cache policy for top-n SQL #25744

topsql: use new cache policy for top-n SQL #25744

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topsql: use new cache policy for top-n SQL #25744

topsql: use new cache policy for top-n SQL #25744

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What problem does this PR solve?

What is changed and how it works?

Related changes

Check List

Release note

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