Migrations taking close to an hour to run

[Ryan Hall]

We're currently running django 1.7.2 and been having performance issues with migrate/makemigrations for the last few months now, but until recently it hasn't caused us too much trouble. Recently, migrate/makemigrations calls have reached a point where they take up to 45minutes to run. Unfortunately, this is due to the fact that we have around 700 models defined with almost 850 migrations since the initial project. Regardless, I've been looking for any ways to speed this process up in 1.7.2/1.7.7/1.8b2/master and found that we have a significant bottleneck with the way states are cloned.

Out of curiousity, I profiled latest master against a set of ~7 migrations to run and found this:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.001    0.001 2687.730 2687.730 manage.py:2(<module>)
        1    0.000    0.000 2687.669 2687.669 django/core/management/__init__.py:325(execute_from_command_line)
        1    0.000    0.000 2687.669 2687.669 django/core/management/__init__.py:265(execute)
        1    0.000    0.000 2680.113 2680.113 django/core/management/base.py:326(run_from_argv)
        1    0.002    0.002 2680.111 2680.111 django/core/management/base.py:361(execute)
        1    0.198    0.198 2678.912 2678.912 django/core/management/commands/migrate.py:50(handle)
        1   10.405   10.405 2676.084 2676.084 django/db/migrations/executor.py:65(migrate)
      803    0.024    0.000 2581.336    3.215 django/db/migrations/migration.py:72(mutate_state)
  7528892   36.422    0.000 1578.058    0.000 django/db/migrations/state.py:468(construct_fields)
     4137    0.504    0.000 1470.027    0.355 django/db/migrations/state.py:81(reload_model)
      847    0.012    0.000 1192.585    1.408 django/db/migrations/state.py:135(clone)
      847    0.939    0.001 1171.224    1.383 django/db/migrations/state.py:138(<dictcomp>)
   402948    6.238    0.000 1170.285    0.003 django/db/migrations/state.py:488(clone)

In migration.py(mutate_state), the project state gets cloned before applying the operations everytime. I realize that the project state is intended to be immutable since the intermediary states need to be used most of the time, but I'm curious if it is required. During phase 1 of migration execution, the state gets cloned on every iteration inside mutate_state at line 90:

        for migration, _ in full_plan:
            if migration in migrations_to_run:
                states[migration] = state.clone()
            state = migration.mutate_state(state)  # state is cloned inside

and also in migrations/graph.py(make_state) at line 274:

        for node in plan:
            project_state = self.nodes[node].mutate_state(project_state)

If I modify mutate_state and add an option to not preserve (no clone) the passed in state in these specific cases, the migration time is cut in half:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.001    0.001  955.337  955.337 manage.py:2(<module>)
        1    0.000    0.000  955.269  955.269 django/core/management/__init__.py:325(execute_from_command_line)
        1    0.000    0.000  955.269  955.269 django/core/management/__init__.py:265(execute)
        1    0.000    0.000  946.453  946.453 django/core/management/base.py:326(run_from_argv)
        1    0.002    0.002  946.450  946.450 django/core/management/base.py:361(execute)
        1    0.169    0.169  945.012  945.012 django/core/management/commands/migrate.py:50(handle)
        1    0.006    0.006  941.582  941.582 django/db/migrations/executor.py:65(migrate)
     4137    0.387    0.000  869.433    0.210 django/db/migrations/state.py:81(reload_model)
      803    0.020    0.000  863.434    1.075 django/db/migrations/migration.py:72(mutate_state)
   100499   28.233    0.000  405.016    0.004 django/apps/registry.py:323(clear_cache)
     1753    0.027    0.000  399.287    0.228 django/db/migrations/operations/fields.py:43(state_forwards)

My question is: does the state need to be cloned in mutate_state during phase 1? The intermediary states are thrown away besides the explicit calls to clone here:

            if migration in migrations_to_run:
                states[migration] = state.clone()

I can create create a pull request if you'd like to see.