# Stages of Computation

This page describes all of the parts of computation, some common causes of slowness, and how to effectively profile. This is intended for more advanced users who are encountering slowdowns on larger computations.

## Graph Construction

Operations on Dask collections (array, dataframe, bag, delayed) build task graphs. These are dictionaries of Python functions that include an entry every time some function needs to run on some chunk of data. When these dictionaries become large (millions of tasks) the overhead of constructing them can become considerable. Additionally the code that builds the graphs may itself be inefficient.

Fortunately, this computation is all happening in normal Python right on your own computer, so you can profile it just as you would any other Python code on your computer using tools like the cProfile module, or the `%prun` or `%snakeviz` IPython magics.

Assuming that no obvious cause comes up when profiling, a common solution to this problem is to reduce your graph size by increasing your chunk size if possible, or manually batching many operations into fewer functions.

## Graph Optimization

Just before you submit the graph to be executed, Dask sees if it can clean up the graph a bit. This helps to remove unnecessary work, and sometimes swaps out more efficient operations. As before though, if your graph is very large (millions of tasks) then this can take some time.

Also as before, this is all happening in Python on your local machine. You can profile optimization separately from computation with the `dask.optimize` function.

    # x, y = dask.compute(x, y)
    x, y = dask.optimize(x, y)

It’s rare for people to change optimization. It is rarely the main cause of slowdown.

## Graph serialization

When you are using the distributed scheduler the graph must be sent to the scheduler process, and from there to the workers. To send this data it must first be converted into bytes. This serialization process can sometimes be expensive either if the objects that you’re passing around are very complex, or if they are very large.

The easiest way to profile this is to profile the `persist` call with the distributed scheduler. This will include both the optimization phase above, as well as the serialization and some of the communication phase below (serialization is often the largest component). Fortunately `persist` returns immediately after, not waiting for the computation to actually finish.

Most often the cause of long serialization times is placing large objects like NumPy arrays or Pandas dataframes into your graph repeatedly. Dask will usually raise a warning when it notices this. Often the best solution is to read your data in as a task instead of include it directly, pre-scatter large data, or wrap them in `dask.delayed`. Sometimes serialization is caused by other issues with complex objects. These tend to be very library specific, and so it is hard to provide general guidelines for them.

## Graph Communication

The graph must then be communicated to the scheduler. You can watch the `/system` tab of the dashboard to watch network communication to and from the scheduler. There is no good way to profile this.

## Scheduling

The scheduler now receives the graph, and must populate its internal data structures to be able to efficiently schedule these tasks to the various workers.

It is only after these data structures are populated that the dashboard will show any activity. All time between pressing `compute/persist` and seeing activity is taken up in the stages above.

You can profile scheduling costs with the `/profile-server` page of the dashboard. However, this is rarely useful for users because, unless you’re willing to dive into the scheduling code, it is hard to act here. Still, the interested user may find the profile of the inter workings of the scheduler of interest.

If scheduling is expensive then the best you can do is to reduce your graph size, often by increasing chunk size.

## Execution

Finally your workers get sent some tasks and get to run them. Your code runs on a thread of a worker, and does whatever it was told to do.

Dask’s Dashboard is a good tool to profile and investigate performance here, particularly the `/status` and `/profile` pages.

Accelerating this phase is often up to the author of the tasks that you are submitting. This might be you if you are using custom code, or the NumPy or Pandas developers. We encourage you to consider efficient libraries like Cython, Numba, or any other solution that is commonly used to accelerate Python code.