Threading best practices in Cats Effect

Posted on January 12, 2021

I regularly get asked what the best way to manage threadpools in Cats Effect is and what ContextShift does so this is my attempt to write a consistent explanation that I can point to. My intention is to cover both Cats Effect 2 and Cats Effect 3, although at the time of writing the latter is at milestone 5 so some details are subject to change. I’ll endeavour to update this should that happen.

High-level goals

The high-level goals of threading are covered in detail by Daniel’s gist so I’ll just give the executive summary. We are aiming for:

  • A single thread pool of roughly the number of available processors for compute-based operations (depending on your application you may get better performance by leaving one or two cores free for GC, etc)
  • An unbounded, cached threadpool for blocking operations
  • 1 or 2 high-priority threads for handling asynchronous I/O events, the handling of which should immediately be shifted to the compute pool

The goal of this is to minimize the number of expensive thread context shifts and to maximize the amount of time that our compute pool is doing useful work.

It is also worth noting that is a poor choice for your compute pool as its fork-join design assumes that there will be blocking operations performed on it and hence it allocates more threads. In addition, there is no way to stop libraries on your classpath from scheduling arbitrary code on it so it is a very unreliable basis for your compute pool.

The IO runloop

A simplified IO might look something like this:

Of course this has no error handling, isn’t stacksafe, doesn’t support asynchronous effects, etc but it’s close enough for illustrative purposes. The key thing to note is that unsafeRun() is a tightly CPU-bound loop evaluating different layers of IO. The situation is just the same when we evaluate the real IO via one of the unsafeRunX methods or as part of an IOApp.


Of course we tend to have many logical threads of execution in our applications. Cats effect trivially supports this via lightweight Fibers, each of which is an instance of the IO runloop. These are run m:n on the OS-level threads (so there is no direct mapping between fibers and threads) and can be created via IO#start, as well as various combinators like IO#race. It is important to note that this is cooperative multi-tasking (as opposed to pre-emptive) so it is the responsibility of a fiber to yield control of the CPU by suspending its runloop periodically. In practice this is rarely an issue as fibers automatically yield at asynchronous boundaries (eg I/O) but it does means that it is actually possible for a fiber to take control of a CPU core and never give it back if it executes a tight CPU-bound loop like

If you have such a loop then you can insert a fairness boundary via IO.shift (CE2 but has other potential side-effects) or IO.cede (CE3), which will give another fiber an opportunity to run on the thread.

Note that the runloop-per-fiber model means that we obtain maximum performance when all of our CPU threads are free to evaluate this runloop for one of our IO fibers.

Thread blocking

A direct consequence of the above is that running blocking code on our compute pool is very bad. If we’re running on a node with 2 CPUs and we evaluate a blocking call like IO(Source.fromFile(path).getLines()) then for the duration of that operation our capacity to evaluate IO fibers is halved. Run two such operations at the same time and your application effectively stops until one of those blocking calls completes.

The solution to this is to shift the execution of the blocking operation to our unbounded, cached threadpool and then shift computation back to the compute pool once the blocking call has completed. We’ll see code samples for this later as it is quite different between CE2 and CE3.

Semantic blocking

Of course, we do also need the ability to tell fibers to wait for conditions to be fulfilled. If we can’t call thread blocking operations (eg Java/Scala builtin locks, semaphores, etc) then what can we do? It seems we need a notion of semantic blocking, where the execution of a fiber is suspended and control of the thread it was running on is yielded.

Cats effect provides various APIs which have these semantics, such as IO.sleep(duration). Indeed this is why you must never call IO(Thread.sleep(duration)) instead, as this is a thread blocking operation whereas IO.sleep is only semantically blocking.

The building block for arbitrary semantic blocking is Deferred, which is a purely functional promise that can only be completed once

Deferred#get is semantically blocking until Deferred#complete is called and cats effect provides many more semantically blocking abstractions like semaphores that are built on top of this.

Summary thus far

So we’ve seen that best performance is achieved when we dedicate use of the compute pool to evaluating IO fiber runloops and ensure that we shift all blocking operations to a separate blocking threadpool. We’ve also seen that many things do not need to block a thread at all - cats effect provides semantic blocking abstractions for waiting for arbtirary conditions to be satisifed. Now it’s time to see the details of how we achieve this in cats effect 2 and 3.

Cats Effect 2

CE2 IOApp provides a fixed execution context sized to the number of available cores for us to use for compute-bound work. This maintains a global queue of runnables awaiting scheduling. Several abstractions are provided to facilitate shifting work to other pools.

Context shift

ContextShift is a pure representation of a threadpool and looks a bit like this:

An instance of this will be backed by some thread pool. IOApp provides an instance which is backed by the default compute pool it provides.

evalOn allows us to shift an operation onto another pool and have the continuation be automatically shifted back eg

shift is a uni-directional shift of thread pool so that the continuation runs on the pool that the ContextShift represents


Blocker was introduced to provide an abstraction for our unbounded pool for blocking operations. It relies upon ContextShift for its actual behaviour and is simply a marker for a threadpool that is suitable for blocking operations.

blockOn behaves exactly like ContextShift#evalOn - the provided fa will be run on the blocker’s pool and then the continuation will run on the pool that cs represents.

A common pattern in libraries for CE2 is to have an API which asks for a Blocker and an implicit ContextShift

In this case you must provide the ContextShift given to you by IOApp (unless you’re absolutely sure you know what you’re doing) as the expectation of the library authors is that they can use that ContextShift to shift execution back to the compute pool after performing any blocking operations on the provided Blocker.

Local reasoning

Unfortunately there are some problems with these abstractions - we lose the ability to reason locally about what thread pool effects are running on.

The problem is that inner could be something like randomCS.shift in which case the continuation (the second print) will be run on whatever thread pool randomCS represents.

In fact, shift is never safe for this reason and evalOn is only safe if the ContextShift in implicit scope represents the threadpool that we were running on before so that we shift back to where we were executing before. Nested evalOn is also prone to non-intuitive behaviour - see this gist for one such example.

What we need is the ability to locally change the threadpool with the guarantee that the continuation will be shifted to the previous pool afterwards. If you are familiar with MonadReader

then you might see that this has exactly the semantics we need, where local is like evalOn in allowing us to locally change the execution context, but it will be restored to the previous value afterwards.


Auto-yielding is the automatic insertion of fiber yields into the runloop to ensure that a single fiber does not hog a CPU core and is not supported in CE2 as yielding requires re-enqueuing the fiber on a global queue and waiting for it to be re-scheduled. This is too expensive to be inserted automatically as the global queue requires coordination between the CPU cores to access this shared resource and will also result in CPU core-local caches being invalidated. If you have a tight CPU-bound loop then you should insert IO.shift where appropriate whilst ensuring that the implicit ContextShift is the one that represents the current execution context (so you don’t accidentally shift the execution to another pool).

Obtaining a handle to the compute pool

Another unfortunate wart is that it is very difficult to obtain a handle to IOApp's compute pool. This can be worked around with IOApp.WithContext but it is somewhat clunky, especially if you want to instantiate the same threadpool as IOApp would otherwise instantiate.

Cats Effect 3

The good news is that CE3 fixes these things and makes other things nicer as well! :) Notably, ContextShift and Blocker are no more.


CE3 introduces a re-designed typeclass Async

which has exactly the MonadReader semantics we discussed above. Note that the execution shifts back to the threadpool defined by Async#executionContext.

Also note that Async[IO].executionContext in IOApp will give us a handle to the compute pool without the WithContext machinery.


CE3 has a builtin blocking which will shift execution to an internal blocking threadpool and shift it back afterwards using Async.

This means that we can simply write

There is also a similar operation interruptible which shifts to the blocking pool but will also attempt to cancel the operation using Thread#interrupt() in the event that the fiber is canceled.

Work-stealing pool

CE3 also has a very exciting custom work-stealing threadpool implementation. This has numerous benefits over the FixedThreadpool used in CE2:

  • It maintains a work queue per core rather than a single global one so contention is dramatically reduced, especially with lots of cores
  • This means that we can implement thread affinity, where a fiber that yields is most likely to be re-scheduled on the same thread. This makes yielding much cheaper as if the fiber is immediately re-scheduled we don’t even have to flush CPU caches
  • Consequently we can support auto-yielding where a fiber will insert an IO.cede every fixed number of iterations of the runloop, stopping a rogue cpu-bound fiber from inadvertently pinning a CPU core

And that’s it!

CE3 drastically simplifies threadpool usage and removes a number of significant gotchas, whilst significantly improving performance. Bring on the release!