Default execution context is now parallel (MT:1) (#16136)

The concurrent context doesn't exhibit any performance improvement over the parallel context (see #16135). So we may as well use a parallel context as default context, and allow resizing it for more parallelism (see #15956).

The default configuration is without parallelism (MT:1), thus the execution context remains concurrent only, which is backward compatible (nothing changes).

Resizing the context to enable parallelism will be at the discretion of the application: it can keep using the `CRYSTAL_WORKERS` environment variable, and/or fallback to the number of CPUs or use a hardcoded value, and/or provide a command line argument, or anything else.

Co-authored-by: Johannes Müller <[email protected]>

Author: ysbaddaden

src/fiber/execution_context.cr

@@ -56,34 +56,31 @@ require "./execution_context/*"
 #
 # ## The default execution context
 #
-# The Crystal runtime starts with a single threaded execution context, available
-# as `Fiber::ExecutionContext.default`:
+# The Crystal runtime starts a default execution context exposed as
+# `Fiber::ExecutionContext.default`. This is where the main fiber is running.
+#
+# Its parallelism is set to 1 for backwards compatibility reasons; Crystal used
+# to be single-threaded and concurrent only. You can increase the parallelism at
+# any time using `Parallel#resize`, for example:
 #
 # ```
-# Fiber::ExecutionContext.default.class # => Fiber::ExecutionContext::Concurrent
+# count = Fiber::ExecutionContext.default_workers_count
+# Fiber::ExecutionContext.default.resize(count)
 # ```
-#
-# NOTE: The default context is a `Concurrent` context for backwards
-# compatibility reasons. It might change to a `Parallel` context in the
-# future.
 @[Experimental]
 module Fiber::ExecutionContext
   @@default : ExecutionContext?
 
   # Returns the default `ExecutionContext` for the process, automatically
   # started when the program started.
-  #
-  # NOTE: The default context is a `Concurrent` context for backwards
-  # compatibility reasons. It might change to a `Parallel` context in the
-  # future.
   @[AlwaysInline]
   def self.default : ExecutionContext
     @@default.not_nil!("expected default execution context to have been setup")
   end
 
   # :nodoc:
   def self.init_default_context : Nil
-    @@default = Concurrent.default
+    @@default = Parallel.default(1)
     @@monitor = Monitor.new
   end
 

src/fiber/execution_context/parallel.cr

@@ -4,8 +4,9 @@ require "./parallel/scheduler"
 module Fiber::ExecutionContext
   # Parallel execution context.
   #
-  # Fibers running in the same context run both concurrently and in parallel to each
-  # others, in addition to the other fibers running in other execution contexts.
+  # Fibers running in the same context run both concurrently and in parallel to
+  # each others, in addition to the other fibers running in other execution
+  # contexts.
   #
   # The context internally keeps a number of fiber schedulers, each scheduler
   # being able to start running on a system thread, so multiple schedulers can
@@ -18,6 +19,9 @@ module Fiber::ExecutionContext
   # example not enough fibers, the schedulers will pause themselves and
   # parallelism will decrease.
   #
+  # The parallelism can be as low as 1, in which case the context becomes a
+  # concurrent context (no parallelism) until resized.
+  #
   # For example: we can start a parallel context to run consumer fibers, while
   # the default context produces values. Because the consumer fibers can run in
   # parallel, we must protect accesses to the shared *value* variable. Running