MySQL query cache stats - why Qcache_lowmem_prunes is high?

I have the following query cache settings query_cache_type = 1 query_cache_limit = 1M query_cache_size = 32M After running for one full day, I have the following stats mysql> show global status like 'qca%'; +-------------------------+----------+ | Variable_name | Value | +-------------------------+----------+ | Qcache_free_blocks | 66 | | Qcache_free_memory | 30462784 | | Qcache_hits | 1995904 | | Qcache_inserts | 2197056 | | Qcache_lowmem_prunes | 531214 | | Qcache_not_cached | 40683 | | Qcache_queries_in_cache | 1429 | | Qcache_total_blocks | 2946 | +-------------------------+----------+ 8 rows in set (0.00 sec) The Qcache_lowmem_prunes is relatively high, but Qcache_free_memory is also high, what are the reason? Are there any way to have a high hit rate by tuning the above config?