The hard throttling CMS does (blocking the incoming thread that wants
to launch a new merge) can be devastating when it strikes during NRT
It can easily happen if a huge merge is off and running, but then a
tiny merge is needed to clean up recently created segments due to
I think a small change to CMS, whereby it assigns a higher thread
priority to tiny merges than big merges, should allow us to increase
the max merge thread count again, and greatly reduce the chance that
NRT's reopen would hit this.