Threads in a GIL 25 Nov 2013

MRI Ruby has a global interpreter lock (GIL), meaning that even when writing multi-threaded Ruby only a single thread can be on-CPU at any given point in time. Other distributions of Ruby have done away with the GIL, but even in MRI threads can be useful. The Sidekiq background worker gem takes advantage of this, running multiple workers in separate threads within a single process.

If the workload of a job blocks on I/O, Ruby can context-switch to other threads and do other work until the I/O finishes. This could happen when the workload reaches out to an external API, shells out to another command, or is accessing the file system. Depending on how a job is written, writes to external storage like an NFS server could block on I/O.

If the workload of a process does not block on I/O, it will not benefit from thread switching under a GIL, as it will be, instead, CPU-bound. In this case, multiple processes will be more efficient, and will be able to take better advantage of multi-core systems.

So… why not skip threads and just deal with processes? A number of reasons.

Memory allocation

Threads are more memory efficient. Fewer processes means less virtual memory allocation, allowing for more workers on fewer hosts. This can result in considerably less cost over the course of a year. Garbage collection fixes in Ruby 2 promise better shared memory management between forked processes, though I have yet to see this have a material effect in production.

Context switching

Context switching between processes is more expensive than context switching between threads. This is because process context switching involves switching out the entire memory address space. Thread switching happens within the same address space.

In Solaris it is relatively easy to set CPU affinity for processes, so that as they switch on and off CPU they're more likely to benefit from CPU caches. In practice we've never had to do this.

Connection management and blocking I/O

Even when pooling database connections through a connection manager like PGBouncer, more processes means more idle connections, and if you're not careful (i.e. you don't monitor connection count) it's easy to accidentally max out your database connection limit. We are particularly aggressive about monitoring and managing active connections to PostgreSQL, having burned ourselves a few times. When working with multiple threads that block on I/O, we sometimes go so far as to force ActiveRecord to release connections back into its connection pool before starting long, blocking requests.

For instance:

class PushNotificationWorker < WaneloWorker
  def perform!(user_id, message)
    user = User.find(user_id)
    ActiveRecord::Base.clear_active_connections!, message).deliver!

This way other threads are able to reuse the connection, even before the first thread finishes.

Sleepy actors

Sometimes a workload will purposefully block, for instance in a daemon process that only wakes up every N seconds to do work. The spanx gem works this way, with multiple actors in separate threads.

It's much easier to manage a single process daemon, from an operational point of view, than a set of daemons. In SmartOS this means that the SMF definition for the service does not have to manage multiple processes. Additionally this prevents a situation where one actor may not start, which might happen with multi-process design. It's much less confusing to type "svcadm disable spanx-watcher" when there's a problem, than to track down four separate services in order to stop them all.