[wp-trac] [WordPress Trac] #52582: wp_cache_* duplicate/redundant storage and insufficient clearing of cache

[WordPress Trac]

#52582: wp_cache_* duplicate/redundant storage and insufficient clearing of cache
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 Reporter:  sormano       |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  General       |    Version:  trunk
 Severity:  normal        |   Keywords:  2nd-opinion needs-testing
  Focuses:                |
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 This is something I'm currently encountering, but have not been able to
 fully investigate myself (yet).

 **Summarized backstory; **
 Using Redis, sometimes maxes memory > needs flush. Started investigating
 whats taking up soo much memory (relatively simple sites).
 ~42,000/67,000 records stored are for `get_comment_child_ids` and
 `get_comments`.
 (By far not that many comments on my sites)
 Looking for a single specific comment ID it is repeated between 30-50
 times, while in theory it should be just once (unless using it in
 different contexts? (->query_vars))

 **Possible cause; **
 For the methods `get_comments` and `fill_descendants` the
 `wp_cache_get_last_changed( 'comment' );` is used to create a cache key
 for the comments / childs.
 https://core.trac.wordpress.org/browser/tags/5.6.1/src/wp-includes/class-
 wp-comment-query.php#L432
 https://core.trac.wordpress.org/browser/tags/5.6.1/src/wp-includes/class-
 wp-comment-query.php#L998

 When a new comment is inserted for example, there is a attempt to delete
 the comment cache by calling `clean_comment_cache()`, but it seems this is
 based on the comment ID, which is not the same as the key / doesn't target
 the childs.
 The 'last_changed' is however changed at the same time in that function;
 https://core.trac.wordpress.org/browser/tags/5.6.1/src/wp-
 includes/comment.php#L3195
 Which I think makes it impossible for the prior cached data to be found
 because it uses that in the key.

 This causes it to store the same data over and over (redundant), even when
 it hasn't changed for those comments, with new cache keys without clearing
 the old ones (duplicate).

 Could be I'm completely off, but wanted to get another pair of eyes on it.
 If whats described is actually happening it looks pretty major for caching
 efficiency.

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Ticket URL: <https://core.trac.wordpress.org/ticket/52582>
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