diff --git a/spec/std/fiber/execution_context/parallel_spec.cr b/spec/std/fiber/execution_context/parallel_spec.cr
index f049073b88a7..f41259450d77 100644
--- a/spec/std/fiber/execution_context/parallel_spec.cr
+++ b/spec/std/fiber/execution_context/parallel_spec.cr
@@ -1,5 +1,6 @@
 {% skip_file unless flag?(:execution_context) %}
 require "spec"
+require "wait_group"
 
 describe Fiber::ExecutionContext::Parallel do
   it ".new" do
@@ -41,4 +42,43 @@ describe Fiber::ExecutionContext::Parallel do
       Fiber::ExecutionContext::Parallel.new("test", size: 5..1)
     end
   end
+
+  it "#resize" do
+    ctx = Fiber::ExecutionContext::Parallel.new("ctx", 1)
+    running = Atomic(Bool).new(true)
+    wg = WaitGroup.new
+
+    10.times do
+      wg.add(1)
+
+      ctx.spawn do
+        while running.get(:relaxed)
+          sleep(10.microseconds)
+        end
+      ensure
+        wg.done
+      end
+    end
+
+    # it grows
+    ctx.resize(4)
+    ctx.capacity.should eq(4)
+
+    # it shrinks
+    ctx.resize(2)
+    ctx.capacity.should eq(2)
+
+    # it doesn't change
+    ctx.resize(2)
+    ctx.capacity.should eq(2)
+
+    10.times do
+      n = rand(1..4)
+      ctx.resize(n)
+      ctx.capacity.should eq(n)
+    end
+
+    running.set(false)
+    wg.wait
+  end
 end
diff --git a/spec/std/fiber/execution_context/runnables_spec.cr b/spec/std/fiber/execution_context/runnables_spec.cr
index 3930bf2f07a6..87d9566e0e8c 100644
--- a/spec/std/fiber/execution_context/runnables_spec.cr
+++ b/spec/std/fiber/execution_context/runnables_spec.cr
@@ -71,6 +71,32 @@ describe Fiber::ExecutionContext::Runnables do
     end
   end
 
+  describe "#drain" do
+    it "drains the local queue into the global queue" do
+      fibers = 6.times.map { |i| new_fake_fiber("f#{i}") }.to_a
+
+      # local enqueue + overflow
+      g = Fiber::ExecutionContext::GlobalQueue.new(Thread::Mutex.new)
+      r = Fiber::ExecutionContext::Runnables(6).new(g)
+
+      # empty
+      r.drain
+      g.size.should eq(0)
+
+      # full
+      fibers.each { |f| r.push(f) }
+      r.drain
+      r.shift?.should be_nil
+      g.size.should eq(6)
+
+      # refill half (1 pop + 2 grab) and drain again
+      g.unsafe_grab?(r, divisor: 1)
+      r.drain
+      r.shift?.should be_nil
+      g.size.should eq(5)
+    end
+  end
+
   describe "#bulk_push" do
     it "fills the local queue" do
       l = Fiber::List.new
diff --git a/src/fiber/execution_context/parallel.cr b/src/fiber/execution_context/parallel.cr
index 2052ff6588ed..1fa32eed5cd1 100644
--- a/src/fiber/execution_context/parallel.cr
+++ b/src/fiber/execution_context/parallel.cr
@@ -69,7 +69,6 @@ module Fiber::ExecutionContext
 
     @parked = Atomic(Int32).new(0)
     @spinning = Atomic(Int32).new(0)
-    @capacity : Int32
 
     # :nodoc:
     protected def self.default(maximum : Int32) : self
@@ -102,12 +101,12 @@ module Fiber::ExecutionContext
       @condition = Thread::ConditionVariable.new
 
       @global_queue = GlobalQueue.new(@mutex)
-      @schedulers = Array(Scheduler).new(@capacity)
-      @threads = Array(Thread).new(@capacity)
+      @schedulers = Array(Scheduler).new(capacity)
+      @threads = Array(Thread).new(capacity)
 
       @rng = Random::PCG32.new
 
-      start_schedulers
+      start_schedulers(capacity)
       @threads << hijack_current_thread(@schedulers.first) if hijack
 
       ExecutionContext.execution_contexts.push(self)
@@ -120,7 +119,7 @@ module Fiber::ExecutionContext
 
     # The maximum number of threads that can be started.
     def capacity : Int32
-      @capacity
+      @schedulers.size
     end
 
     # :nodoc:
@@ -140,7 +139,7 @@ module Fiber::ExecutionContext
     # OPTIMIZE: consider storing schedulers to an array-like object that would
     # use an atomic/fence to make sure that @size can only be incremented
     # *after* the value has been written to @buffer.
-    private def start_schedulers
+    private def start_schedulers(capacity)
       capacity.times do |index|
         @schedulers << Scheduler.new(self, "#{@name}-#{index}")
       end
@@ -176,6 +175,71 @@ module Fiber::ExecutionContext
       end
     end
 
+    # Resizes the context to the new *maximum* parallelism.
+    #
+    # The new *maximum* can grow, in which case more schedulers are created to
+    # eventually increase the parallelism.
+    #
+    # The new *maximum* can also shrink, in which case the overflow schedulers
+    # are removed and told to shutdown immediately. The actual shutdown is
+    # cooperative, so running schedulers won't stop until their current fiber
+    # tries to switch to another fiber.
+    def resize(maximum : Int32) : Nil
+      raise ArgumentError.new("Parallelism can't be less than one.") if maximum < 1
+      removed_schedulers = nil
+
+      @mutex.synchronize do
+        # can run in parallel to #steal that dereferences @schedulers (once)
+        # without locking the mutex, so we dup the schedulers, mutate the copy,
+        # and eventually assign the copy as @schedulers; this way #steal can
+        # safely access the array (never mutated).
+        new_capacity = maximum
+        old_threads = @threads
+        old_schedulers = @schedulers
+        old_capacity = capacity
+
+        if new_capacity > old_capacity
+          @schedulers = Array(Scheduler).new(new_capacity) do |index|
+            old_schedulers[index]? || Scheduler.new(self, "#{@name}-#{index}")
+          end
+          threads = Array(Thread).new(new_capacity)
+          old_threads.each { |thread| threads << thread }
+          @threads = threads
+        elsif new_capacity < old_capacity
+          # tell the overflow schedulers to shutdown
+          removed_schedulers = old_schedulers[new_capacity..]
+          removed_schedulers.each(&.shutdown!)
+
+          # resize
+          @schedulers = old_schedulers[0...new_capacity]
+          @threads = old_threads[0...new_capacity]
+
+          # reset @parked counter (we wake all parked threads) so they can
+          # shutdown (if told to):
+          woken_threads = @parked.get(:relaxed)
+          @parked.set(0, :relaxed)
+
+          # update @spinning prior to unpark threads; we use acquire release
+          # semantics to make sure that all the above stores are visible before
+          # the following wakeup calls (maybe not needed, but let's err on the
+          # safe side)
+          @spinning.add(woken_threads, :acquire_release)
+
+          # wake every waiting thread:
+          @condition.broadcast
+          @event_loop.interrupt
+        end
+      end
+
+      return unless removed_schedulers
+
+      # drain the local queues of removed schedulers since they're no longer
+      # available for stealing
+      removed_schedulers.each do |scheduler|
+        scheduler.@runnables.drain
+      end
+    end
+
     # :nodoc:
     def spawn(*, name : String? = nil, same_thread : Bool, &block : ->) : Fiber
       raise ArgumentError.new("#{self.class.name}#spawn doesn't support same_thread:true") if same_thread
@@ -200,11 +264,12 @@ module Fiber::ExecutionContext
     protected def steal(& : Scheduler ->) : Nil
       return if capacity == 1
 
+      schedulers = @schedulers
       i = @rng.next_int
-      n = @schedulers.size
+      n = schedulers.size
 
       n.times do |j|
-        if scheduler = @schedulers[(i &+ j) % n]?
+        if scheduler = schedulers[(i &+ j) % n]?
           yield scheduler
         end
       end
@@ -271,8 +336,8 @@ module Fiber::ExecutionContext
           # we must also decrement the number of parked threads because another
           # thread could lock the mutex and increment @spinning again before the
           # signaled thread is resumed
-          spinning = @spinning.add(1, :acquire_release)
-          parked = @parked.sub(1, :acquire_release)
+          @spinning.add(1, :acquire_release)
+          @parked.sub(1, :acquire_release)
 
           @condition.signal
         end
@@ -282,11 +347,11 @@ module Fiber::ExecutionContext
       # check if we can start another thread; no need for atomics, the values
       # shall be rather stable over time and we check them again inside the
       # mutex
-      return if @threads.size == capacity
+      return if @threads.size >= capacity
 
       @mutex.synchronize do
         index = @threads.size
-        return if index == capacity # check again
+        return if index >= capacity # check again
 
         @threads << start_thread(@schedulers[index])
       end
diff --git a/src/fiber/execution_context/parallel/scheduler.cr b/src/fiber/execution_context/parallel/scheduler.cr
index cf8df4c74bdd..b91d96cc31ac 100644
--- a/src/fiber/execution_context/parallel/scheduler.cr
+++ b/src/fiber/execution_context/parallel/scheduler.cr
@@ -29,6 +29,7 @@ module Fiber::ExecutionContext
       @spinning = false
       @waiting = false
       @parked = false
+      @shutdown = false
 
       protected def initialize(@execution_context, @name)
         @global_queue = @execution_context.global_queue
@@ -36,6 +37,10 @@ module Fiber::ExecutionContext
         @event_loop = @execution_context.event_loop
       end
 
+      protected def shutdown! : Nil
+        @shutdown = true
+      end
+
       # :nodoc:
       def spawn(*, name : String? = nil, same_thread : Bool, &block : ->) : Fiber
         raise RuntimeError.new("#{self.class.name}#spawn doesn't support same_thread:true") if same_thread
@@ -86,6 +91,8 @@ module Fiber::ExecutionContext
       end
 
       private def quick_dequeue? : Fiber?
+        return if @shutdown
+
         # every once in a while: dequeue from global queue to avoid two fibers
         # constantly respawing each other to completely occupy the local queue
         if (@tick &+= 1) % 61 == 0
@@ -121,8 +128,21 @@ module Fiber::ExecutionContext
         Crystal.trace :sched, "started"
 
         loop do
+          if @shutdown
+            spin_stop
+            @runnables.drain
+
+            # we may have been the last running scheduler, waiting on the event
+            # loop while there are pending events for example; let's resume a
+            # scheduler to take our place
+            @execution_context.wake_scheduler
+
+            Crystal.trace :sched, "shutdown"
+            break
+          end
+
           if fiber = find_next_runnable
-            spin_stop if @spinning
+            spin_stop
             resume fiber
           else
             # the event loop enqueued a fiber (or was interrupted) or the
@@ -145,6 +165,8 @@ module Fiber::ExecutionContext
 
         # nothing to do: start spinning
         spinning do
+          return if @shutdown
+
           yield @global_queue.grab?(@runnables, divisor: @execution_context.size)
 
           if @execution_context.lock_evloop? { @event_loop.run(pointerof(list), blocking: false) }
@@ -189,10 +211,12 @@ module Fiber::ExecutionContext
         # loop: park the thread until another scheduler or another context
         # enqueues a fiber
         @execution_context.park_thread do
+          # don't park the thread when told to shutdown
+          return if @shutdown
+
           # by the time we acquire the lock, another thread may have enqueued
           # fiber(s) and already tried to wakeup a thread (race) so we must
           # check again; we don't check the scheduler's local queue (it's empty)
-
           yield @global_queue.unsafe_grab?(@runnables, divisor: @execution_context.size)
           yield try_steal?
 
diff --git a/src/fiber/execution_context/runnables.cr b/src/fiber/execution_context/runnables.cr
index cc8be5322498..674461a03402 100644
--- a/src/fiber/execution_context/runnables.cr
+++ b/src/fiber/execution_context/runnables.cr
@@ -75,25 +75,54 @@ module Fiber::ExecutionContext
 
       # first, try to grab half of the fibers from local queue
       batch = uninitialized Fiber[N] # actually N // 2 + 1 but that doesn't compile
-      n.times do |i|
-        batch.to_unsafe[i] = @buffer.to_unsafe[(head &+ i) % N]
-      end
-      _, success = @head.compare_and_set(head, head &+ n, :acquire_release, :acquire)
+      _, success = try_grab(batch.to_unsafe, head, n)
       return false unless success
 
-      # append fiber to the batch
+      # append fiber to the batch and push to global queue
       batch.to_unsafe[n] = fiber
+      push_to_global_queue(batch.to_unsafe, n &+ 1)
+      true
+    end
 
-      # link the fibers
+    # Transfers every fiber in the local runnables queue to the global queue.
+    # This will grab the global lock.
+    #
+    # Can be executed by any scheduler.
+    def drain : Nil
+      batch = uninitialized Fiber[N]
+      n = 0
+
+      head = @head.get(:acquire) # sync with other consumers
+      loop do
+        tail = @tail.get(:acquire) # sync with the producer
+
+        n = (tail &- head)
+        return if n == 0 # queue is empty
+
+        # try to grab everything from local queue
+        head, success = try_grab(batch.to_unsafe, head, n)
+        break if success
+      end
+
+      push_to_global_queue(batch.to_unsafe, n)
+    end
+
+    private def try_grab(batch, head, n)
       n.times do |i|
-        batch.to_unsafe[i].list_next = batch.to_unsafe[i &+ 1]
+        batch[i] = @buffer.to_unsafe[(head &+ i) % N]
       end
-      list = Fiber::List.new(batch.to_unsafe[0], batch.to_unsafe[n], size: (n &+ 1).to_i32)
+      @head.compare_and_set(head, head &+ n, :acquire_release, :acquire)
+    end
 
-      # now put the batch on global queue (grabs the global lock)
-      @global_queue.bulk_push(pointerof(list))
+    private def push_to_global_queue(batch, n)
+      # link the fibers
+      (n &- 1).times do |i|
+        batch[i].list_next = batch[i &+ 1]
+      end
+      list = Fiber::List.new(batch[0], batch[n &- 1], size: n.to_i32)
 
-      true
+      # and put the batch on global queue (grabs the global lock)
+      @global_queue.bulk_push(pointerof(list))
     end
 
     # Tries to enqueue all the fibers in *list* into the local queue. If the