WT high memory usage due to high amount of free memory accumulated by TCMalloc

Environment:

• Stand alone mongod
• 3.1.6v
• WiredTiger configured with zlib compression and 3GB cache size

Observation/Issues:

1. When running w/ 100 + threads, WT cache reached to 8GB (2.6 x original cache size).

1. When running w/ 9 threads, WT cache reached to 5GB (1.6 x original cache size).

Breakdown of the 5G memory highlight the followings:

3 GB in the cache size (as expected)
1.7 GB - Current_total_thread_cache_bytes (MB) (higher than expectation)
0.45 GB - Total central_cache_free_bytes (MB) (higher than expectation)

Problem: Our memory cache policy set for 1GB free, however the machine accumulate memory above this threshold.