While doing some benchmarks, I found out strange performance degradation for MongoDB for read only workload on YCSB (WorkloadC). You can see it on the right side of the graph where the number of clients is getting bigger:

I tried to investigate using perf tool and figured out that the only two metrics that are growing significantly are number of cpu migrations and syscall `sched_yield`. On the next graph you can see the dynamics of cpu migration event, number of `sched_yield` also grew 3 times bigger.

Judging from source code I can assume that it's somehow related to spin locks, since the only place where I found `sched_yield` is `spin_lock.cpp`.