[TYPO3-core] FYI72: #15497: [Caching framework] Remove tagIndex from memcached and apc backend

[Christian Kuhn]

The following patch will be committed in 72 hours if nobody objects:

Type: Performance improvement

Branches: trunk, 4.4, 4.3

BT: http://bugs.typo3.org/view.php?id=15497 [^]
FLOW3 issue: http://forge.typo3.org/issues/8918 [^] (committed)

Problem:
The memcached and apc backend implementations currently store this 
key-value "structure" to map our identifier-data-keys relation:
     * identifier -> data
     * identifier -> keys
     * key -> identifiers
     * tagIndex -> All known tags

The tagIndex doesn't have any benefit at all, it just leads to more 
set() and get() calls.

Solution:
Completely remove tagIndex to save some operations which speeds up the 
backends, unit tests still show green bar.

Numbers:
Performance tests of memcached backend show that the backend speeds up 
between 10 to 100% by removing the tagIndex. As an example, attached 
images show a test which set()'s a raising number of cache entries with 
a single attached tag to the backend. More numbers can be generated with 
ext:enetcacheanalytics (forge) in v4.

Notes:
- Memcache is a simple key-value store and not designed to store 
structures of multiple related entries. Our implementation is a misuse, 
which leads to corrupt caches if memcache evicts entries from cache 
(because it needs space for new elements), or if single servers fail in 
a multi server environment.
- APC backend implementation is pretty much identical to the memcache 
implementation, it's just affected by this change as well, but it's not 
benchmarked on it's own here.

Regards
Christian
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