TYPO3.Flow

Problem:
The memcached backend implementation currenty stores 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:
Completly remove tagIndex to save some operations which speeds up this backend, unit tests still show green bar.

Numbers:
Performance tests 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.