• heavy mixed workload
• Ubuntu 14.04.1 LTS, 3.13.0-32-generic
• VMware, 6 cores

5-second pause in db ops was seen following this message in syslog:

Jan 16 14:39:15 ubuntu kernel: [20023.738805] [sched_delayed] sched: RT throttling activated

• For a second or so from A to B no samples were reported by the external monitoring processes (a mongo shell process calling serverStatus and a python process monitoring system stats.)
• An extremely high context switch rate was reported at A.
• At B the monitoring processes resumed.
• At around B (to within syslog resolution of 1 second) the above message appeared in syslog.
• Starting at B for about 5 seconds db op rate dropped to 0
• However not all activity was blocked: evictions appeared to be continuing, serverStatus was being processed.
• "slots selected for switching that were unavailable" was high from B to C.

It appears that some behavior involving extreme CPU utilization at high priority from A to B, possibly involving context switches, caused the kernel to suspend those threads for 5 seconds. The threads involved with eviction apparently weren't suspended.