For safe-looking changes, it's OK to only run a subset of the tests (usually including the tests that directly test the changed library). For changes that are

For safe-looking changes, it's OK to only run a subset of the tests (usually including the tests that directly test the changed library). For changes that are more likely to break distant code, you can run all tests (perhaps bundling together several changes in order not to overload the system). Alternatively you can take the risk of breaking tests post-submit... this is not very good citizenship, but in some cases it might be reasonable (when the risk is small).