Which threads are being blocked

[NickHocking]

Hi,

We have an application that a large part of it occasionally goes completely unresponsive for around 60 seconds.
We've got a permanent vmstat running and it shows that two processes/threads were blocking for the entire time of the "outage"
--------------------------------------r b w
Tue Jul 3 13:29:41 AEST 2018 2 0 0 97106288 103012448 10 17 0 24 24 0 0 0 0 0 0 3672 6054 3310 0 0 100
Tue Jul 3 13:29:57 AEST 2018 0 0 0 97106288 103012096 13 18 0 56 56 0 0 1 0 0 0 8531 8591 8729 0 0 99
Tue Jul 3 13:30:13 AEST 2018 1 2 0 96952256 102819624 9 16 0 16 16 0 0 1 0 0 0 3349 6904 3564 0 0 99
Tue Jul 3 13:30:29 AEST 2018 0 2 0 96952256 102819408 197 624 0 0 0 0 0 0 0 0 0 3214 3936 2936 0 0 100
Tue Jul 3 13:30:45 AEST 2018 0 2 0 96952248 102819096 7 18 0 0 0 0 0 0 0 0 0 2946 2461 2753 0 0 100
Tue Jul 3 13:31:01 AEST 2018 0 2 0 96952248 102818872 13 18 0 40 40 0 0 0 0 0 0 3233 2616 3351 0 0 100
Tue Jul 3 13:31:17 AEST 2018 1 2 0 97106128 102927504 787 671 8 4596 4517 0 0 136 0 0 1 34686 80817 32663 2 3 95
Tue Jul 3 13:31:33 AEST 2018 1 0 0 97106168 102940712 12 16 0 48 48 0 0 0 0 0 0 15261 15143 15893 0 0 99
Tue Jul 3 13:31:49 AEST 2018 0 0 0 97106168 102941168 759 1223 0 16 16 0 0 0 0 0 1 3633 6074 3897 0 1 99

How do I find out (using DTrace or otherwise) what the blocking processes/threads were and what they were blocking on.

Cheers
Nick