diff --git a/lib/httpx/pool.rb b/lib/httpx/pool.rb
index f4156ff5..fcecfa30 100644
--- a/lib/httpx/pool.rb
+++ b/lib/httpx/pool.rb
@@ -14,7 +14,7 @@ module HTTPX
 
     def initialize
       @resolvers = {}
-      @_resolver_monitors = {}
+      @_resolver_ios = {}
       @timers = Timers::Group.new
       @selector = Selector.new
       @connections = []
@@ -33,9 +33,8 @@ module HTTPX
           throw(:jump_tick)
         end
 
-        @selector.select(timeout) do |monitor|
-          monitor.io.call
-        end
+        @selector.select(timeout, &:call)
+
         @timers.fire
       end
     rescue StandardError => e
@@ -85,7 +84,7 @@ module HTTPX
       resolver << connection
       return if resolver.empty?
 
-      @_resolver_monitors[resolver] ||= @selector.register(resolver)
+      @_resolver_ios[resolver] ||= @selector.register(resolver)
     end
 
     def on_resolver_connection(connection)
@@ -117,8 +116,7 @@ module HTTPX
       @resolvers.delete(resolver_type)
 
       @selector.deregister(resolver)
-      monitor = @_resolver_monitors.delete(resolver)
-      monitor.close if monitor
+      @_resolver_ios.delete(resolver)
       resolver.close unless resolver.closed?
     end
 
diff --git a/lib/httpx/selector.rb b/lib/httpx/selector.rb
index 91535689..c59092b7 100644
--- a/lib/httpx/selector.rb
+++ b/lib/httpx/selector.rb
@@ -30,56 +30,20 @@ class HTTPX::Selector
 
   using IOExtensions unless IO.method_defined?(:wait) && IO.instance_method(:wait).arity == 2
 
-  #
-  # I/O monitor
-  #
-  class Monitor
-    attr_accessor :io
-
-    def initialize(io, reactor)
-      @io = io
-      @reactor = reactor
-      @closed = false
-    end
-
-    # closes +@io+, deregisters from reactor (unless +deregister+ is false)
-    def close(deregister = true)
-      return if @closed
-
-      @closed = true
-      @reactor.deregister(@io) if deregister
-    end
-
-    def closed?
-      @closed
-    end
-
-    # :nocov:
-    def to_s
-      "#<#{self.class}: #{@io}(closed:#{@closed}) #{@io.interests} #{object_id.to_s(16)}>"
-    end
-    # :nocov:
-  end
-
   def initialize
-    @selectables = {}
+    @selectables = []
   end
 
   # deregisters +io+ from selectables.
   def deregister(io)
-    monitor = @selectables.delete(io)
-    monitor.close(false) if monitor
+    @selectables.delete(io)
   end
 
   # register +io+.
   def register(io)
-    monitor = @selectables[io]
-    return if monitor
+    return if @selectables.include?(io)
 
-    monitor = Monitor.new(io, self)
-    @selectables[io] = monitor
-
-    monitor
+    @selectables << io
   end
 
   private
@@ -88,57 +52,78 @@ class HTTPX::Selector
   WRITE_INTERESTS = %i[w rw].freeze
 
   def select_many(interval)
-    begin
-      r = nil
-      w = nil
+    selectables, r, w = nil
 
-      @selectables.each_key do |io|
-        interests = io.interests
+    # first, we group IOs based on interest type. On call to #interests however,
+    # things might already happen, and new IOs might be registered, so we might
+    # have to start all over again. We do this until we group all selectables
+    loop do
+      begin
+        r = nil
+        w = nil
 
-        (r ||= []) << io if READ_INTERESTS.include?(interests)
-        (w ||= []) << io if WRITE_INTERESTS.include?(interests)
+        selectables = @selectables
+        @selectables = []
+
+        selectables.each do |io|
+          interests = io.interests
+
+          (r ||= []) << io if READ_INTERESTS.include?(interests)
+          (w ||= []) << io if WRITE_INTERESTS.include?(interests)
+        end
+
+        if @selectables.empty?
+          @selectables = selectables
+          break
+        else
+          @selectables = [*selectables, @selectables]
+        end
+      rescue StandardError
+        @selectables = selectables if selectables
+        raise
       end
+    end
 
+    # TODO: what to do if there are no selectables?
+
+    begin
       readers, writers = IO.select(r, w, nil, interval)
 
       raise HTTPX::TimeoutError.new(interval, "timed out while waiting on select") if readers.nil? && writers.nil?
     rescue IOError, SystemCallError
-      @selectables.reject! { |io, _| io.closed? }
+      @selectables.reject!(&:closed?)
       retry
     end
 
     readers.each do |io|
-      monitor = @selectables[io]
-      next unless monitor
+      yield io
 
       # so that we don't yield 2 times
       writers.delete(io)
-
-      yield monitor
     end if readers
 
     writers.each do |io|
-      monitor = @selectables[io]
-      next unless monitor
-
-      yield monitor
+      yield io
     end if writers
   end
 
   def select_one(interval)
-    io, monitor = @selectables.first
+    io = @selectables.first
 
-    result = case io.interests
+    interests = io.interests
+
+    result = case interests
              when :r then io.to_io.wait_readable(interval)
              when :w then io.to_io.wait_writable(interval)
              when :rw then io.to_io.wait(interval, :read_write)
+             when nil then return
     end
 
     raise HTTPX::TimeoutError.new(interval, "timed out while waiting on select") unless result
 
-    yield monitor
+    yield io
   rescue IOError, SystemCallError
-    @selectables.reject! { |ios, _| ios.closed? }
+    @selectables.reject!(&:closed?)
   end
 
   def select(interval, &block)