diff --git a/lib/Slic3r.pm b/lib/Slic3r.pm
index d4344963f..9019607db 100644
--- a/lib/Slic3r.pm
+++ b/lib/Slic3r.pm
@@ -82,30 +82,37 @@ use constant EXTERNAL_INFILL_MARGIN => 3;
 use constant INSET_OVERLAP_TOLERANCE => 0.2;
 
 # keep track of threads we created
-my @threads = ();
-my $sema = Thread::Semaphore->new;
+my @my_threads = ();
+my @threads : shared = ();
+my $pause_sema = Thread::Semaphore->new;
+my $parallel_sema;
 my $paused = 0;
 
 sub spawn_thread {
     my ($cb) = @_;
     
     my $parent_tid = threads->tid;
+    lock @threads;
     
     @_ = ();
     my $thread = threads->create(sub {
+        @my_threads = ();
+        
         Slic3r::debugf "Starting thread %d (parent: %d)...\n", threads->tid, $parent_tid;
         local $SIG{'KILL'} = sub {
             Slic3r::debugf "Exiting thread %d...\n", threads->tid;
+            $parallel_sema->up if $parallel_sema;
             kill_all_threads();
             Slic3r::thread_cleanup();
             threads->exit();
         };
         local $SIG{'STOP'} = sub {
-            $sema->down;
-            $sema->up;
+            $pause_sema->down;
+            $pause_sema->up;
         };
         $cb->();
     });
+    push @my_threads, $thread->tid;
     push @threads, $thread->tid;
     return $thread;
 }
@@ -118,10 +125,15 @@ sub parallelize {
         my $q = Thread::Queue->new;
         $q->enqueue(@items, (map undef, 1..$params{threads}));
         
+        $parallel_sema = Thread::Semaphore->new(-$params{threads});
+        $parallel_sema->up;
         my $thread_cb = sub {
             # execute thread callback
             $params{thread_cb}->($q);
             
+            # signal the parent thread that we're done
+            $parallel_sema->up;
+            
             # cleanup before terminating thread
             Slic3r::thread_cleanup();
             
@@ -133,17 +145,17 @@ sub parallelize {
             # The downside to using this exit is that we can't return
             # any value to the main thread but we're not doing that
             # anymore anyway.
-            # collect_cb is completely useless now
-            # and should be removed from the codebase.
             threads->exit;
         };
-        $params{collect_cb} ||= sub {};
             
         @_ = ();
         my @my_threads = map spawn_thread($thread_cb), 1..$params{threads};
-        foreach my $th (@my_threads) {
-            $params{collect_cb}->($th->join);
-        }
+        
+        # We use a semaphore instead of $th->join because joined threads are
+        # not listed by threads->list or threads->object anymore, thus can't
+        # be signalled.
+        $parallel_sema->down;
+        $_->detach for @my_threads;
     } else {
         $params{no_threads_cb}->();
     }
@@ -202,40 +214,43 @@ sub thread_cleanup {
 }
 
 sub get_running_threads {
-    return grep defined($_), map threads->object($_), @threads;
+    return grep defined($_), map threads->object($_), @_;
 }
 
 sub kill_all_threads {
-    # detach any running thread created in the current one
-    my @killed = ();
-    foreach my $thread (get_running_threads()) {
-        Slic3r::debugf "Thread %d killing %d...\n", threads->tid, $thread->tid;
-        $thread->kill('KILL');
-        push @killed, $thread;
+    # if we're the main thread, we send SIGKILL to all the running threads
+    if (threads->tid == 0) {
+        lock @threads;
+        foreach my $thread (get_running_threads(@threads)) {
+            Slic3r::debugf "Thread %d killing %d...\n", threads->tid, $thread->tid;
+            $thread->kill('KILL');
+        }
+        
+        # unlock semaphore before we block on wait
+        # otherwise we'd get a deadlock if threads were paused
+        resume_threads();
     }
     
-    # unlock semaphore before we block on wait
-    # otherwise we'd get a deadlock if threads were paused
-    resume_threads();
-    foreach my $thread (@killed) {
-        Slic3r::debugf "  Threads %d waiting for %d...\n", threads->tid, $thread->tid;
+    # in any thread we wait for our children
+    foreach my $thread (get_running_threads(@my_threads)) {
+        Slic3r::debugf "  Thread %d waiting for %d...\n", threads->tid, $thread->tid;
         $thread->join;  # block until threads are killed
         Slic3r::debugf "    Thread %d finished waiting for %d...\n", threads->tid, $thread->tid;
     }
-    @threads = ();
+    @my_threads = ();
 }
 
 sub pause_threads {
     return if $paused;
     $paused = 1;
-    $sema->down;
-    $_->kill('STOP') for get_running_threads();
+    $pause_sema->down;
+    $_->kill('STOP') for get_running_threads(@threads);
 }
 
 sub resume_threads {
     return unless $paused;
     $paused = 0;
-    $sema->up;
+    $pause_sema->up;
 }
 
 sub encode_path {
diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index 4a5ec01fe..166f18792 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -426,7 +426,6 @@ sub make_perimeters {
                 $self->get_layer($i)->make_perimeters;
             }
         },
-        collect_cb => sub {},
         no_threads_cb => sub {
             $_->make_perimeters for @{$self->layers};
         },
@@ -506,7 +505,6 @@ sub infill {
                 $layerm->fills->append($_) for $self->fill_maker->make_fill($layerm);
             }
         },
-        collect_cb => sub {},
         no_threads_cb => sub {
             foreach my $layerm (map @{$_->regions}, @{$self->layers}) {
                 $layerm->fills->clear;