WMFTimeoutException on Commons for WhatLinksHere
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Description

Error message

[Xg9SJgpAME8AAIMFtwgAAAAA] 2020-01-03 14:41:03: Fatal exception of type "WMFTimeoutException"

#0 /srv/mediawiki/php-1.35.0-wmf.11/includes/exception/MWExceptionHandler.php(200): {closure}(integer)
#1 [internal function]: MWExceptionHandler::handleError(integer, string, string, integer, array)
#2 /srv/mediawiki/php-1.35.0-wmf.11/includes/libs/rdbms/database/DatabaseMysqli.php(46): mysqli->query(string)
#3 /srv/mediawiki/php-1.35.0-wmf.11/includes/libs/rdbms/database/Database.php(1308): Wikimedia\Rdbms\DatabaseMysqli->doQuery(string)
#4 /srv/mediawiki/php-1.35.0-wmf.11/includes/libs/rdbms/database/Database.php(1226): Wikimedia\Rdbms\Database->executeQueryAttempt(string, string, boolean, string, integer)
#5 /srv/mediawiki/php-1.35.0-wmf.11/includes/libs/rdbms/database/Database.php(1162): Wikimedia\Rdbms\Database->executeQuery(string, string, integer)
#6 /srv/mediawiki/php-1.35.0-wmf.11/includes/libs/rdbms/database/Database.php(1828): Wikimedia\Rdbms\Database->query(string, string)
#7 /srv/mediawiki/php-1.35.0-wmf.11/includes/libs/rdbms/database/DBConnRef.php(68): Wikimedia\Rdbms\Database->select(array, array, array, string, array, array)
#8 /srv/mediawiki/php-1.35.0-wmf.11/includes/libs/rdbms/database/DBConnRef.php(318): Wikimedia\Rdbms\DBConnRef->__call(string, array)
#9 /srv/mediawiki/php-1.35.0-wmf.11/includes/specials/SpecialWhatLinksHere.php(185): Wikimedia\Rdbms\DBConnRef->select(array, array, array, string, array, array)
#10 /srv/mediawiki/php-1.35.0-wmf.11/includes/specials/SpecialWhatLinksHere.php(194): SpecialWhatLinksHere->{closure}(Wikimedia\Rdbms\MaintainableDBConnRef, string, string)
#11 /srv/mediawiki/php-1.35.0-wmf.11/includes/specials/SpecialWhatLinksHere.php(98): SpecialWhatLinksHere->showIndirectLinks(integer, Title, integer, integer, integer)
#12 /srv/mediawiki/php-1.35.0-wmf.11/includes/specialpage/SpecialPage.php(575): SpecialWhatLinksHere->execute(NULL)
#13 /srv/mediawiki/php-1.35.0-wmf.11/includes/specialpage/SpecialPageFactory.php(611): SpecialPage->run(NULL)
#14 /srv/mediawiki/php-1.35.0-wmf.11/includes/MediaWiki.php(298): MediaWiki\Special\SpecialPageFactory->executePath(Title, RequestContext)
#15 /srv/mediawiki/php-1.35.0-wmf.11/includes/MediaWiki.php(967): MediaWiki->performRequest()
#16 /srv/mediawiki/php-1.35.0-wmf.11/includes/MediaWiki.php(530): MediaWiki->main()
#17 /srv/mediawiki/php-1.35.0-wmf.11/index.php(46): MediaWiki->run()
#18 /srv/mediawiki/w/index.php(3): require(string)
#19 {main}

Impact

Backlink list cannot be get for this template (at least this configuration: all pages except File namespace). I don’t think may people would want to get it, though.

Notes

The backlink table contains ~30M rows for this template, although I don’t think more than some dozens, at most a few hundreds of pages actually use it since its decomission in November. (The count’s slowly but constantly shrinking since I started watching it ten days ago.) I also got some Varnish errors with Error: 503, Backend fetch failed before this error (I reloaded the page a couple of time in the hope it will once succeed).

Details

Request ID: Xg9SJgpAME8AAIMFtwgAAAAA
Request URL: https://commons.wikimedia.org/w/index.php?title=Special%3AWhatLinksHere&target=Template%3AInformation%2Fauthor+processing&namespace=6&invert=1

	Project	Branch	Lines +/-	Subject
	mediawiki/core	master	+21 -7	Do inverse namespace filtering of links tables with a big IN()

Customize query in gerrit

Event Timeline

Tacsipacsi created this task.Jan 3 2020, 3:30 PM

Restricted Application added subscribers: Liuxinyu970226, Aklapper. · View Herald TranscriptJan 3 2020, 3:30 PM

Aklapper renamed this task from WhatLinksHere timing out on Commons to WMFTimeoutException on Commons for WhatLinksHere.Jan 3 2020, 3:46 PM

Aklapper added a project: Performance Issue.

Aklapper edited Stack Trace. (Show Details)

Masumrezarock100 added a project: Commons.Jan 4 2020, 9:47 AM

Masumrezarock100 moved this task from Incoming to Tracking bugs on the Commons board.

Krinkle updated the task description. (Show Details)Jan 7 2020, 8:07 PM

Krinkle edited Stack Trace. (Show Details)

Krinkle moved this task from Untriaged to Jan2020/1.35-wmf.14 on the Wikimedia-production-error board.Jan 8 2020, 5:59 PM

It works when hide the transclusion: https://commons.wikimedia.org/w/index.php?title=Special%3AWhatLinksHere&target=Template%3AInformation%2Fauthor+processing&namespace=6&invert=1&hidetrans=1

That means the query on the templatelinks table timeouts out (There are currently 16M entries there)

I would expect this working. For backlinks the column tl_from_namespace and an index tl_backlinks_namespace was created.
Maybe the database does not pick up the (right) index to exclude the ns 6 here

SELECT page_id, 
       page_namespace, 
       page_title, 
       rd_from, 
       rd_fragment, 
       page_is_redirect 
FROM   (SELECT tl_from, 
               rd_from, 
               rd_fragment 
        FROM   `templatelinks` 
               LEFT JOIN `redirect` 
                      ON ( ( rd_from = tl_from ) 
                           AND rd_title = 'Information/author_processing' 
                           AND ( rd_interwiki = '' 
                                  OR rd_interwiki IS NULL ) 
                           AND rd_namespace = 10 ) 
               JOIN `page` 
                 ON (( tl_from = page_id )) 
        WHERE  tl_namespace = 10 
               AND tl_title = 'Information/author_processing' 
               AND ( tl_from_namespace != 6 ) 
        ORDER  BY tl_from 
        LIMIT  102) `temp_backlink_range` 
       JOIN `page` 
         ON (( tl_from = page_id )) 
ORDER  BY page_id 
LIMIT  51

API also picks them up:
https://commons.wikimedia.org/w/api.php?action=query&list=embeddedin&eilimit=50&eititle=Template%3AInformation%2Fauthor+processing&einamespace=0|1|2|3|4|5|7|8|9|10|11|12|13|14|15|100|101|102|103|104|105|106|107|460|461|486|487|490|491|828|829|1198|1199|2300|2301|2302|2303|2600

But it does not have an invert option (you have to create a list with the namespace missing) and it is missing the LEFT JOIN mit redirect table:

SELECT /*! STRAIGHT_JOIN */ page_id, 
                            page_title, 
                            page_namespace, 
                            page_is_redirect, 
                            page_namespace AS `from_ns` 
FROM   `templatelinks`, 
       `page` 
WHERE  ( tl_from = page_id ) 
       AND tl_title = 'Information/author_processing' 
       AND tl_namespace = 10 
       AND tl_from_namespace IN ( 0, 1, 2, 3, 
                                  4, 5, 7, 8, 
                                  9, 10, 11, 12, 
                                  13, 14, 15, 100, 
                                  101, 102, 103, 104, 
                                  105, 106, 107, 460, 
                                  461, 486, 487, 490, 
                                  491, 828, 829, 1198, 
                                  1199, 2300, 2301, 2302, 
                                  2303, 2600 ) 
ORDER  BY tl_from_namespace, 
          tl_from 
LIMIT  51

In T241837#6298504, @Umherirrender wrote:
It works when hide the transclusion: https://commons.wikimedia.org/w/index.php?title=Special%3AWhatLinksHere&target=Template%3AInformation%2Fauthor+processing&namespace=6&invert=1&hidetrans=1

That means the query on the templatelinks table timeouts out (There are currently 16M entries there)

I would expect this working. For backlinks the column tl_from_namespace and an index tl_backlinks_namespace was created.
Maybe the database does not pick up the (right) index to exclude the ns 6 here
SELECT page_id, 
       page_namespace, 
       page_title, 
       rd_from, 
       rd_fragment, 
       page_is_redirect 
FROM   (SELECT tl_from, 
               rd_from, 
               rd_fragment 
        FROM   `templatelinks` 
               LEFT JOIN `redirect` 
                      ON ( ( rd_from = tl_from ) 
                           AND rd_title = 'Information/author_processing' 
                           AND ( rd_interwiki = '' 
                                  OR rd_interwiki IS NULL ) 
                           AND rd_namespace = 10 ) 
               JOIN `page` 
                 ON (( tl_from = page_id )) 
        WHERE  tl_namespace = 10 
               AND tl_title = 'Information/author_processing' 
               AND ( tl_from_namespace != 6 ) 
        ORDER  BY tl_from 
        LIMIT  102) `temp_backlink_range` 
       JOIN `page` 
         ON (( tl_from = page_id )) 
ORDER  BY page_id 
LIMIT  51

This query is quite crazy indeed:

+------+-------------+---------------+--------+---------------------------------------------+--------------+---------+-----------------------------------+----------+------------------------------------+
| id   | select_type | table         | type   | possible_keys                               | key          | key_len | ref                               | rows     | Extra                              |
+------+-------------+---------------+--------+---------------------------------------------+--------------+---------+-----------------------------------+----------+------------------------------------+
|    1 | PRIMARY     | <derived2>    | ALL    | NULL                                        | NULL         | NULL    | NULL                              |      102 | Using temporary; Using filesort    |
|    1 | PRIMARY     | page          | eq_ref | PRIMARY                                     | PRIMARY      | 4       | temp_backlink_range.tl_from       |        1 |                                    |
|    2 | DERIVED     | templatelinks | ref    | PRIMARY,tl_namespace,tl_backlinks_namespace | tl_namespace | 261     | const,const                       | 40839520 | Using index condition; Using where |
|    2 | DERIVED     | redirect      | eq_ref | PRIMARY,rd_ns_title                         | PRIMARY      | 4       | commonswiki.templatelinks.tl_from |        1 | Using where                        |
|    2 | DERIVED     | page          | eq_ref | PRIMARY                                     | PRIMARY      | 4       | commonswiki.templatelinks.tl_from |        1 | Using index                        |
+------+-------------+---------------+--------+---------------------------------------------+--------------+---------+-----------------------------------+----------+------------------------------------+
5 rows in set (0.013 sec)

However using tl_backlinks_namespace doesn't really make any difference, it keeps scanning millions of rows.

+------+-------------+---------------+--------+------------------------+------------------------+---------+-----------------------------------+-----------+------------------------------------------+
| id   | select_type | table         | type   | possible_keys          | key                    | key_len | ref                               | rows      | Extra                                    |
+------+-------------+---------------+--------+------------------------+------------------------+---------+-----------------------------------+-----------+------------------------------------------+
|    1 | PRIMARY     | <derived2>    | ALL    | NULL                   | NULL                   | NULL    | NULL                              |       102 | Using temporary; Using filesort          |
|    1 | PRIMARY     | page          | eq_ref | PRIMARY                | PRIMARY                | 4       | temp_backlink_range.tl_from       |         1 |                                          |
|    2 | DERIVED     | templatelinks | range  | tl_backlinks_namespace | tl_backlinks_namespace | 4       | NULL                              | 431574438 | Using where; Using index; Using filesort |
|    2 | DERIVED     | redirect      | eq_ref | PRIMARY,rd_ns_title    | PRIMARY                | 4       | commonswiki.templatelinks.tl_from |         1 | Using where                              |
|    2 | DERIVED     | page          | eq_ref | PRIMARY                | PRIMARY                | 4       | commonswiki.templatelinks.tl_from |         1 | Using index                              |
+------+-------------+---------------+--------+------------------------+------------------------+---------+-----------------------------------+-----------+------------------------------------------+
5 rows in set (0.002 sec)

Adding Platform Engineering to see if they can come up with some other strategy here. Maybe splitting the query?
Removing DBA tag as there is no actionable for us at the moment, but I will stay on the task to follow up

• Gilles moved this task from Inbox to Radar on the Performance-Team board.Jul 13 2020, 8:40 PM

• Gilles edited projects, added Performance-Team (Radar); removed Performance-Team.

All right, seems worth a look.

• eprodromou moved this task from Inbox to Later on the Platform Team Workboards (Clinic Duty Team) board.Jul 21 2020, 8:54 PM

Krinkle moved this task from Limbo to Perf recommendation on the Performance-Team (Radar) board.Jul 27 2020, 11:34 PM

I think it will be fast if the != 6 is replaced by IN() as in the API query. Then it can use the tl_backlinks_namespace index efficiently. It appears to treat the IN() like a union.

I ran the query above with "Information" instead of "Information/author_processing", since the latter no longer has many rows. It was slow the first time (15s) and moderately slow on subsequent runs (~0.4s). I reduced it down to:

SELECT tl_from FROM   `templatelinks`  WHERE  tl_namespace = 10  AND tl_title = 'Information'  AND ( tl_from_namespace != 6 )  ORDER  BY tl_from  LIMIT  102;

There's no joins or anything complicated, but it was still slow (0.51s). But the equivalent query with IN():

SELECT tl_from FROM   `templatelinks`  WHERE  tl_namespace = 10  AND tl_title = 'Information'  AND ( tl_from_namespace IN ( 0, 1, 2, 3,  4, 5, 7, 8,  9, 10, 11, 12,  13, 14, 15, 100,  101, 102, 103, 104,  105, 106, 107, 460,  461, 486, 487, 490,  491, 828, 829, 1198,  1199, 2300, 2301, 2302,  2303, 2600 ) ) ORDER  BY tl_from  LIMIT  102;

reported that it ran in 0.00s. Applying this method to the full query, with subselect and joins, it ran in 0.01s.

Change 655806 had a related patch set uploaded (by Tim Starling; owner: Tim Starling):
[mediawiki/core@master] Do inverse namespace filtering of links tables with a big IN()

https://gerrit.wikimedia.org/r/655806

gerritbot added a project: Patch-For-Review.Jan 13 2021, 3:33 AM

Patch looks good, +1'd. Giving a bit more time for @Marostegui or others to common on the solution then will +2 if there have been no objections.

Clarakosi moved this task from Later to Waiting for Review on the Platform Team Workboards (Clinic Duty Team) board.Jan 14 2021, 2:35 PM

In T241837#6742126, @tstarling wrote:
I think it will be fast if the != 6 is replaced by IN() as in the API query. Then it can use the tl_backlinks_namespace index efficiently. It appears to treat the IN() like a union.

I ran the query above with "Information" instead of "Information/author_processing", since the latter no longer has many rows. It was slow the first time (15s) and moderately slow on subsequent runs (~0.4s). I reduced it down to:
SELECT tl_from FROM   `templatelinks`  WHERE  tl_namespace = 10  AND tl_title = 'Information'  AND ( tl_from_namespace != 6 )  ORDER  BY tl_from  LIMIT  102;
There's no joins or anything complicated, but it was still slow (0.51s). But the equivalent query with IN():
SELECT tl_from FROM   `templatelinks`  WHERE  tl_namespace = 10  AND tl_title = 'Information'  AND ( tl_from_namespace IN ( 0, 1, 2, 3,  4, 5, 7, 8,  9, 10, 11, 12,  13, 14, 15, 100,  101, 102, 103, 104,  105, 106, 107, 460,  461, 486, 487, 490,  491, 828, 829, 1198,  1199, 2300, 2301, 2302,  2303, 2600 ) ) ORDER  BY tl_from  LIMIT  102;
reported that it ran in 0.00s. Applying this method to the full query, with subselect and joins, it ran in 0.01s.

The second query is definitely better indeed, using tl_backlinks_namespace looks much better than tl_namespace. (300 rows scanned vs 250k) I have run the query on a cold host and the difference is quite big, 16 seconds vs 0.04
Which is also shown on the handler's read:

| Handler_read_next          | 47733 |

| Handler_read_next          | 358   |

Thanks for looking, sounds like a win. Patch merged.

BPirkle closed this task as Resolved.Jan 14 2021, 6:12 PM

BPirkle claimed this task.

Change 655806 merged by jenkins-bot:
[mediawiki/core@master] Do inverse namespace filtering of links tables with a big IN()