Tokio Rwlock resources idle

I use tokio-console watch a Rwlock resource, there are seconds idle, i can't understand what cause it.

In my system, i use a Arc<Rwlock<HashMap>> to store tokio channel sneder, and i need get it with low latency. My system load is low, but sometime i find sender is very slow.

It's already resolved, because of sometime i sleep in read lock.