Threading Model

Similar to other non-blocking event-driven frameworks, Finagle leverages a fixed size thread pool (I/O worker pool) shared between all clients and servers running within the same JVM process. While unlocking tremendous scalability potential (less resources are spent to process more events), this approach comes at the expense of services’ responsiveness, being notably sensitive to blocking operations. Put this way, when blocked, I/O threads can’t process the incoming events (new requests, new connections) thereby degrading the system’s overall velocity.

Somewhat conservative defaults for Finagle’s worker pool size only exaggerate the problem: there are few (two per each logical CPU core with the floor of 8 workers) threads in the pool and blocking even one of them may affect multiple clients and servers. While it’s recommended sticking to these defaults (numerous experiments showed they are good), it’s possible to override the worker pool size with a command line flag.

The following sets the worker pool size to 24.


I/O Thread Affinity

As evident by the flag name, Finagle’s I/O threads are allocated and managed within its underlying networking library, Netty. Built on top of native non-blocking I/O APIs (epoll, kqueue), Netty dictates an affinity between I/O threads and the list of opened file descriptors (network sockets) they are watching over. Once a network connection is established, it’s assigned a serving I/O thread that can never be reassigned. This is why blocking even a single I/O thread halts progress in multiple independent connections (clients, servers).

I/O Threads and User Code

Although not immediately obvious, all users’ code that’s triggered by receiving a message is run on Finagle I/O threads, unless explicitly executed somewhere else (e.g., application’s own thread pool, a FuturePool). This includes server’s Service.apply and all Future callbacks but excludes application startup code (executed in the main thread) and the work triggered by a timeout expiring (executed in the timer thread).

This design is on par with the asynchronous nature of Twitter Futures, which are purely a coordination mechanism that does not describe any execution environment (callbacks are run in whatever thread satisfies a Promise). Given it’s typically I/O threads that satisfy promises (pending future RPC responses), they are also on the hook for running promises’ callbacks. Sticking with the caller thread reduces context switches at the expense of making I/O threads vulnerable to users blocking (or just slow) code.

Blocking Examples

Not only blocking I/O (e.g., calling a JDBC driver, using the JDK’s File API) can block Finagle threads. CPU intensive computations are equally dangerous: keeping I/O threads busy processing application-level work means they underserve critical RPC events.

Consider a CPU-bound server application running an algorithm with profoundly bad asymptotic complexity. Scala’s permutations operation can serve a good example with its O(n!) worst case running time. As implemented below, such a server would have a hard time staying responsive as its I/O threads would be constantly busy (blocked) calculating permutations as opposed to serving networking events.

import com.twitter.finagle.Service
import com.twitter.finagle.http.{Request, Response}
import com.twitter.util.Future

class MyHttpService extends Service[Request, Response] {
  def apply(req: Request): Future[Response] = {
    val rep = Response()
    rep.contentString = req.contentString.permutations.mkString("\n")


A comparably bad thing can happen to clients too. Running permutations as part of any callback (flatMap, onSuccess, onFailure, etc) on a Future returned from a client has the exact same effect - it saturates an I/O thread.

import com.twitter.finagle.Service
import com.twitter.finagle.http.{Request, Response}

def process(client: Service[Request, Response]): Future[String] =
  client(Request()).map(rep => rep.contentString.permutations.mkString("\n"))

Identifying Blocking

General-purpose JVM profilers and even jstack can be quite effective in identifying bottlenecks on the request path (within I/O threads). There are, however, two metrics that can provide a valuable insight with no need for external tools.

  • blocking_ms - a counter of total time spent in Await.result and Await.ready blocking an I/O thread. Refer to this blog post on what to do when this counter is not zero.
  • pending_io_events - a gauge of the number of pending I/O events enqueued in all event loops serving this client or server. When this metric climbs up, it indicates I/O queues are clogged and I/O threads overloaded.

Whereas getting rid of Await on the request path is generally advised, there is no guidance that could be provided with regards to what is a healthy number of pending I/O events. Clearly, striving for “zero” or “near zero” might be a reasonable strategy if taken not as the gold standard but a friendly recommendation. Depending on the workload, even double-digit values could be acceptable for some applications.


Shifting users’ work off of I/O threads can go a long way in improving an application’s responsiveness, minding the increase in context switches as well as associated cost of managing additional JVM threads. However, run your own tests to determine if offloading is good for your service given its traffic profile and resource allocation.

FuturePools provide a convenient API to wrap any expression with a Future that’s scheduled in the underlying ExecutorService. They come in handy for offloading the I/O threads in Finagle while preserving the first-class support to Twitter Futures (interrupts, locals).

Offloading can be done on per-method (endpoint) basis:

import com.twitter.util.{Future, FuturePool}

def offloadedPermutations(s: String, pool: FuturePool): Future[String] =

As well as per entire client or server:

import com.twitter.util.FuturePool
import com.twitter.finagle.Http

val server: Http.Server = Http.server

val client: Http.Client = Http.client

Or per entire application (JVM process), using the command-line flag:

Or to enable with a manually tuned threading configuration:

-com.twitter.finagle.offload.numWorkers=14 -com.twitter.finagle.netty4.numWorkers=10

Offload Admission Control


This is an experimental feature and may undergo rapid developmental changes.


This AC mechanism can only work if global offloading is enabled.

Using system-wide offloading opens the door to an experimental form of admission control. This admission control mechanism, referred to as Offload AC, uses the behavior of the work queue to determine when to reject work. The default behavior is to reject work when there is a 20ms wait in the queue.

This can be enabled using the flag:


Or to manually tune the allowable delay: