Abstract: Methods, systems, and apparatus, including computer programs encoded on computer storage media, for determining image search results. One of the methods includes scheduling a first computation for execution on each of a first plurality of worker processes. The first computation changes a respective state of each of one or more of the first worker processes from a first state to a second state. A respective second computation is scheduled for execution on each of a second plurality of worker, where each respective second computation will use a different value for a particular variable for two or more of the second plurality of worker processes. The respective state of each of the second plurality of worker processes is updated from the second state to a third state, where the third state corresponds to execution of the second computation using a first value of the particular variable.