Evict-server is single-threaded in wiredtiger. Sometimes it becomes the bottleneck under heavy write workloads. As WT cache-used and cache-dirty get too high, app threads are stuck in evict-worker process but won't help.

Single-threaded evict-server may lead 2 problems:

1. Reduce write performance even in multi-core machines.
2. The related cpu core usage spikes to 100% when evict-server is busy.