Investigate performance issues surrounding count_usable_descriptors()
According to a gprof output generated by Andrea (#11322 (moved)), her busy Tor node called count_usable_descriptors 65368 times, mostly from router_have_minimum_dir_info(). This is expensive because it iterates over all the nodes and does a lot of siphash / digestmap / tor_memeq stuff.
Why are we calling router_have_minimum_dir_info() so much? Almost entirely because of second_elapsed_callback().
But why is router_have_minimum_dir_info() invoking update_router_have_minimum_dir_info so often? If I'm reading these numbers right, it's doing so once every 5 calls. That's not right; it's supposed to cache the result of update_router_have_minimum_dir_info() for a long time, until somebody calls router_dir_info_changed(). Who is doing that?
According to that profile, the top two callers are:
0.00 0.00 4851/23430 router_add_to_routerlist [266]
0.00 0.00 17823/23430 channel_do_open_actions <cycle 2> [144]router_add_to_routerlist() calls should be clustered; they shouldn't cause most of the re-invocations of update_router_have_minimum_dir_info(). So let's look at channel_do_open_actions().
It's calling router_set_status(), which is calling router_dir_info_changed unconditionally!
Two issues there:
- I'm not sure that router_set_status should be calling router_dir_info_changed at all. Does changing our opinion about a node's is_running status count as a change in whether we know most of the nodes in the network? Should it?
- It surely shouldn't be calling it unconditionally.