Query cache was never filled up

[Reynard H]

Hi,

I have setup query_cache with this settings

mysql> show variables like '%query_cache%';
+------------------------------+-----------+
| Variable_name | Value |
+------------------------------+-----------+
| have_query_cache | YES |
| query_cache_limit | 4194304 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 268435456 |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |
+------------------------------+-----------+

mysql> SHOW STATUS LIKE 'Qcache%';
+-------------------------+-----------+
| Variable_name | Value |
+-------------------------+-----------+
| Qcache_free_blocks | 8540 |
| Qcache_free_memory | 217974872 |
| Qcache_hits | 45906852 |
| Qcache_inserts | 31292381 |
| Qcache_lowmem_prunes | 3389670 |
| Qcache_not_cached | 4702447 |
| Qcache_queries_in_cache | 28918 |
| Qcache_total_blocks | 66504 |
+-------------------------+-----------+

So mysql never seem to fill the entire cache size (in this case < 20%), as shown by the high number of Qcache_free_memory. this is the case everytime I check the status. (so it's not because it hasn't warm up). and another problematic thing is that Qcache_lowmem_prunes is really high, which means that it prunes the cache because of low memory but there are so much unused memory.

I'm using mysql 5.0.91 on RHEL5. Any idea what's going on here?
any hints would be greatly appreciated and thanks for reading :)