Serial Solver Execution¶

You may want to run the solver several times iteratively when evaluating performance. Also, some solvers are more likely to find a good solution by running several iterations with a short timeout than by running a single iteration with a long timeout.Amplify SDK allows running the same combinatorial optimization problem multiple times in a row with the same solver for such needs.

Example of multiple runs¶

First, create a model and solver client as in a usual solve() function execution.

from amplify import VariableGenerator, one_hot, FixstarsClient, solve
from datetime import timedelta

gen = VariableGenerator()
q = gen.array("Binary", 3)

objective = q[0] * q[1] - q[2]
constraint = one_hot(q)

model = objective + constraint

client = FixstarsClient()
# client.token = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
client.parameters.timeout = timedelta(milliseconds=1000)

By passing an integer as the num_solves keyword argument to solve(), the solver is executed iteratively for num_solves times.

result = solve(model, client, num_solves=3)

As in a usual solve() call, the solver returns an instance of the Result class. The return value contains the results of the num_solves iterations.

>>> len(result)
3

Using the best attribute, you can obtain the best solution among those returned by the num_solves runs.

>>> print(f"objective = {result.best.objective}, q = {q.evaluate(result.best.values)}")
objective = -1.0, q = [0. 0. 1.]

Fetching results¶

When solve() is called with the num_solves keyword, the return value is an instance of the Result class, as usual.

The Amplify SDK determines the number of times of the solver executions by the num_solves attribute of the Result class. This attribute usually yields the same value as the value specified to solve() as the num_solves keyword argument. Still, it may be less than the value specified as the num_solves keyword argument, if some of the solver runs, fail for some reason.

>>> result.num_solves
3

Index or iteration access allows you to aggregate all the solutions the solver returns for the num_solves runs. By default, the Amplify SDK sorts them in order of preferred solution, and there is no distinction regarding the number of executions the solution was found.

>>> print(f"objective = {result[0].objective}, q = {q.evaluate(result[0].values)}")
objective = -1.0, q = [0. 0. 1.]

Use the split property to get only the solutions returned by a particular run.

>>> first_result = result.split[0] # extract only the part of the `result` that was obtained at the first run
>>> type(first_result)
<class 'amplify.Result'>
>>> len(first_result)
1
>>> print(f"objective = {first_result.best.objective}, q = {q.evaluate(first_result.best.values)}")
objective = -1.0, q = [0. 0. 1.]

The following are included for each property of the Result object returned by solve() when num_solves is specified, and the Result object represents the result of the i-th run obtained by using the split property on it.

Property name	`Result` returned by `solve()`	`Result` after applying `split`
`best`	The best solution for the `num_solves` runs	The best solution for the `i`-th run
`solutions`	All solutions obtained for the `num_solves` runs	All solutions obtained for the `i`-th run
`intermediate`	Same as usual `solve()` execution	Same as usual `solve()` execution
`embedding`	Same as usual `solve()` execution	Same as usual `solve()` execution
`client_result`	Property obtained for the first run	Property obtained for the `i`-th run
`execution_time`	The sum of the `num_solves` runs	Property obtained for the `i`-th run
`response_time`	The sum of the `num_solves` runs	Property obtained for the `i`-th run
`total_time`	Time from the start to the end of `solve()`	Time used for the `i`-th run