Matthew Pickering authored Dec 05, 2024 and Marge Bot committed Dec 12, 2024

Consider a module M importing modules A, B and C.

We can waste a lot of work depending on the order that the modules are
checked for family consistency.

Consider that C imports A and B. When compiling C we must have already
checked A and B for consistency, therefore if C is processed first then
A and B will not need to be checked for consistency again.

If A and B are compared first, then the consistency checks will be
performed against (wasted as we already performed them for C).

At the moment the order which modules are checked is non-deterministic.

Clearly we should engineer that C is checked before B and A, but by what
scheme?

A simple one is to observe that if a module M is in the transitive
closure of X then the size of the consistent family set of M is less
than or equal to size of the consistent family set of X.

Therefore by sorting the imports by the size of the consistent family
set and processing the largest first, you make sure to process modules
in topological order.

In practice we have observed that this strategy has reduced the amount
of consistency checks performed.

One solution to #25554