To investigate a perf BF, I ran two patch tests to compare between, the latest master and the latest master with 4.2 wiredtiger.

Latest master:

Latest master with 4.2 wiredtiger:

We also saw that with 4.2 patch several test results dropped, which we do not see with the latest mongodb-4.2:

Observations:

• Moving from the latest wiredtiger in mongo/master to wiredtiger/mongodb-4.2 produced the same perf results for contended_update, but resulted in drop in perf for several other tests
• At this point, we do not have a good explanation of what these results mean
• It is not conclusive where the regression is coming from.

Possibly we can isolate either a change in wiredtiger keeping the latest MongoDB, or a change in MongoDB keeping the latest WiredTiger. Alternatively, there is a complex list of changes involved and we can work to bring back the performance for contended_update back to 4.2 baseline.

I have opened this ticket to address the next steps.