diff --git a/lib/Slic3r/GCode/PlaceholderParser.pm b/lib/Slic3r/GCode/PlaceholderParser.pm
index 9f45dfe9b..d391456b2 100644
--- a/lib/Slic3r/GCode/PlaceholderParser.pm
+++ b/lib/Slic3r/GCode/PlaceholderParser.pm
@@ -34,7 +34,9 @@ sub apply_config {
     my ($self, $config) = @_;
     
     # options with single value
-    my @opt_keys = grep !$Slic3r::Config::Options->{$_}{multiline}, @{$config->get_keys};
+    my @opt_keys = grep $Slic3r::Config::Options->{$_}{cli} !~ /\@$/,
+        grep !$Slic3r::Config::Options->{$_}{multiline},
+        @{$config->get_keys};
     $self->_single_set($_, $config->serialize($_)) for @opt_keys;
 
     # options with multiple values
diff --git a/lib/Slic3r/GUI/Plater.pm b/lib/Slic3r/GUI/Plater.pm
index 4946d5a34..e3b8dbd4a 100644
--- a/lib/Slic3r/GUI/Plater.pm
+++ b/lib/Slic3r/GUI/Plater.pm
@@ -33,6 +33,7 @@ our $PROGRESS_BAR_EVENT      : shared = Wx::NewEventType;
 our $MESSAGE_DIALOG_EVENT    : shared = Wx::NewEventType;
 our $EXPORT_COMPLETED_EVENT  : shared = Wx::NewEventType;
 our $EXPORT_FAILED_EVENT     : shared = Wx::NewEventType;
+our $PROCESS_COMPLETED_EVENT : shared = Wx::NewEventType;
 
 use constant CANVAS_SIZE => [335,335];
 use constant FILAMENT_CHOOSERS_SPACING => 3;
@@ -218,6 +219,14 @@ sub new {
         $self->on_export_failed;
     });
     
+    EVT_COMMAND($self, -1, $PROCESS_COMPLETED_EVENT, sub {
+        my ($self, $event) = @_;
+        
+        Slic3r::debugf "Background processing completed.\n";
+        $self->{process_thread}->detach if $self->{process_thread};
+        $self->{process_thread} = undef;
+    });
+    
     $self->{canvas}->update_bed_size;
     $self->update;
     
@@ -439,6 +448,9 @@ sub objects_loaded {
     $self->{list}->Update;
     $self->{list}->Select($obj_idxs->[-1], 1);
     $self->object_list_changed;
+    
+    # TODO: start timer for new export thread
+    $self->start_background_process;
 }
 
 sub remove {
@@ -664,10 +676,60 @@ sub split_object {
     $self->load_model_objects(@model_objects);
 }
 
+sub start_background_process {
+    my ($self) = @_;
+    
+    return if !$Slic3r::have_threads;
+    return if !@{$self->{objects}};
+    
+    if ($self->{process_thread}) {
+        warn "Can't start new process thread because one is already running\n";
+        return;
+    }
+    
+    # It looks like declaring a local $SIG{__WARN__} prevents the ugly
+    # "Attempt to free unreferenced scalar" warning...
+    local $SIG{__WARN__} = Slic3r::GUI::warning_catcher($self);
+    
+    # don't start process thread if config is not valid
+    eval {
+        # this will throw errors if config is not valid
+        $self->skeinpanel->config->validate;
+        $self->{print}->validate;
+    };
+    return if $@;
+    
+    # apply extra variables
+    {
+        my $extra = $self->skeinpanel->extra_variables;
+        $self->{print}->placeholder_parser->set($_, $extra->{$_}) for keys %$extra;
+    }
+    
+    # start thread
+    $self->{process_thread} = threads->create(sub {
+        $self->{print}->process;
+        Wx::PostEvent($self, Wx::PlThreadEvent->new(-1, $PROCESS_COMPLETED_EVENT, undef));
+        Slic3r::thread_cleanup();
+    });
+    Slic3r::debugf "Background processing started.\n";
+}
+
+sub stop_background_process {
+    my ($self) = @_;
+    
+    if ($self->{process_thread}) {
+        Slic3r::debugf "Killing background process.\n";
+        $self->{process_thread}->kill('KILL')->join;
+        $self->{process_thread} = undef;
+    } else {
+        Slic3r::debugf "No background process running.\n";
+    }
+}
+
 sub export_gcode {
     my $self = shift;
     
-    if ($self->{export_thread}) {
+    if ($self->{export_gcode_output_file}) {
         Wx::MessageDialog->new($self, "Another slicing job is currently running.", 'Error', wxOK | wxICON_ERROR)->ShowModal;
         return;
     }
@@ -912,6 +974,7 @@ sub update {
 sub on_config_change {
     my $self = shift;
     my ($opt_key, $value) = @_;
+    
     if ($opt_key eq 'extruders_count' && defined $value) {
         my $choices = $self->{preset_choosers}{filament};
         while (@$choices < $value) {
@@ -937,6 +1000,17 @@ sub on_config_change {
         }
         $self->update if $opt_key eq 'print_center';
     }
+    
+    return if !$self->skeinpanel->is_loaded;
+    # TODO: pause export thread
+    my $invalidated = $self->{print}->apply_config($self->skeinpanel->config);
+    if ($invalidated) {
+        # kill export thread
+        $self->stop_background_process;
+        
+        # TODO: start timer for new export thread
+        $self->start_background_process;
+    }
 }
 
 sub list_item_deselected {
diff --git a/lib/Slic3r/GUI/SkeinPanel.pm b/lib/Slic3r/GUI/SkeinPanel.pm
index 13b532635..e489394a4 100644
--- a/lib/Slic3r/GUI/SkeinPanel.pm
+++ b/lib/Slic3r/GUI/SkeinPanel.pm
@@ -31,6 +31,7 @@ sub new {
     my $self = $class->SUPER::new($parent, -1, wxDefaultPosition, wxDefaultSize, wxTAB_TRAVERSAL);
     $self->{mode} = $params{mode};
     $self->{mode} = 'expert' if $self->{mode} !~ /^(?:simple|expert)$/;
+    $self->{loaded} = 0;
     
     $self->{tabpanel} = Wx::Notebook->new($self, -1, wxDefaultPosition, wxDefaultSize, wxNB_TOP | wxTAB_TRAVERSAL);
     $self->{tabpanel}->AddPage($self->{plater} = Slic3r::GUI::Plater->new($self->{tabpanel}), "Plater")
@@ -81,9 +82,15 @@ sub new {
     $self->SetSizer($sizer);
     $self->Layout;
     
+    $self->{loaded} = 1;
     return $self;
 }
 
+sub is_loaded {
+    my ($self) = @_;
+    return $self->{loaded};
+}
+
 sub quick_slice {
     my $self = shift;
     my %params = @_;
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index c9853a1a4..56664fb54 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -50,10 +50,8 @@ sub apply_config {
     if (@$print_diff) {
         $self->config->apply_dynamic($config);
         
-        my $res;
-        $res = $self->invalidate_all_steps
-            if !$self->invalidate_state_by_config_options($print_diff);
-        $invalidated = 1 if $res;
+        $invalidated = 1
+            if $self->invalidate_state_by_config_options($print_diff);
     }
     
     # handle changes to object config defaults
@@ -74,10 +72,8 @@ sub apply_config {
         if (@$diff) {
             $object->config->apply($new);
             
-            my $res;
-            $res = $object->invalidate_all_steps
-                if !$object->invalidate_state_by_config_options($diff);
-            $invalidated = 1 if $res;
+            $invalidated = 1
+                if $self->invalidate_state_by_config_options($diff);
         }
     }
     
@@ -131,10 +127,8 @@ sub apply_config {
                 if (@$region_config_diff) {
                     $region->config->apply($new);
                     foreach my $o (@{$self->objects}) {
-                        my $res;
-                        $res = $o->invalidate_all_steps
-                            if !$o->invalidate_state_by_config_options($region_config_diff);
-                        $invalidated = 1 if $res;
+                        $invalidated = 1
+                            if $o->invalidate_state_by_config_options($region_config_diff);
                     }
                 }
             }
diff --git a/xs/src/Print.cpp b/xs/src/Print.cpp
index 4afb16278..90d758b6d 100644
--- a/xs/src/Print.cpp
+++ b/xs/src/Print.cpp
@@ -5,48 +5,39 @@ namespace Slic3r {
 
 template <class StepClass>
 bool
-PrintState<StepClass>::started(StepClass step) const
+PrintState<StepClass>::is_started(StepClass step) const
 {
-    return this->_started.find(step) != this->_started.end();
+    return this->started.find(step) != this->started.end();
 }
 
 template <class StepClass>
 bool
-PrintState<StepClass>::done(StepClass step) const
+PrintState<StepClass>::is_done(StepClass step) const
 {
-    return this->_done.find(step) != this->_done.end();
+    return this->done.find(step) != this->done.end();
 }
 
 template <class StepClass>
 void
 PrintState<StepClass>::set_started(StepClass step)
 {
-    this->_started.insert(step);
+    this->started.insert(step);
 }
 
 template <class StepClass>
 void
 PrintState<StepClass>::set_done(StepClass step)
 {
-    this->_done.insert(step);
-}
-
-template <class StepClass>
-void
-PrintState<StepClass>::invalidate(StepClass step)
-{
-    this->_started.erase(step);
-    this->_done.erase(step);
+    this->done.insert(step);
 }
 
 template <class StepClass>
 bool
-PrintState<StepClass>::invalidate_all()
+PrintState<StepClass>::invalidate(StepClass step)
 {
-    bool empty = this->_started.empty();
-    this->_started.clear();
-    this->_done.clear();
-    return !empty;  // return true if we invalidated something
+    bool invalidated = this->started.erase(step) > 0;
+    this->done.erase(step);
+    return invalidated;
 }
 
 template class PrintState<PrintStep>;
@@ -256,21 +247,23 @@ PrintObject::invalidate_state_by_config_options(const std::vector<t_config_optio
             steps.insert(posPerimeters);
             steps.insert(posInfill);
         } else {
-            // for legacy, if we can't handle this option let's signal the caller to invalidate all steps
-            return false;
+            // for legacy, if we can't handle this option let's invalidate all steps
+            return this->invalidate_all_steps();
         }
     }
     
-    for (std::set<PrintObjectStep>::const_iterator step = steps.begin(); step != steps.end(); ++step)
-        this->invalidate_step(*step);
+    bool invalidated = false;
+    for (std::set<PrintObjectStep>::const_iterator step = steps.begin(); step != steps.end(); ++step) {
+        if (this->invalidate_step(*step)) invalidated = true;
+    }
     
-    return true;
+    return invalidated;
 }
 
-void
+bool
 PrintObject::invalidate_step(PrintObjectStep step)
 {
-    this->state.invalidate(step);
+    bool invalidated = this->state.invalidate(step);
     
     // propagate to dependent steps
     if (step == posPerimeters) {
@@ -286,6 +279,21 @@ PrintObject::invalidate_step(PrintObjectStep step)
         this->invalidate_step(posPerimeters);
         this->invalidate_step(posSupportMaterial);
     }
+    
+    return invalidated;
+}
+
+bool
+PrintObject::invalidate_all_steps()
+{
+    // make a copy because when invalidating steps the iterators are not working anymore
+    std::set<PrintObjectStep> steps = this->state.started;
+    
+    bool invalidated = false;
+    for (std::set<PrintObjectStep>::const_iterator step = steps.begin(); step != steps.end(); ++step) {
+        if (this->invalidate_step(*step)) invalidated = true;
+    }
+    return invalidated;
 }
 
 
@@ -403,21 +411,23 @@ Print::invalidate_state_by_config_options(const std::vector<t_config_option_key>
         } else if (*opt_key == "brim_width") {
             steps.insert(psBrim);
         } else {
-            // for legacy, if we can't handle this option let's signal the caller to invalidate all steps
-            return false;
+            // for legacy, if we can't handle this option let's invalidate all steps
+            return this->invalidate_all_steps();
         }
     }
     
-    for (std::set<PrintStep>::const_iterator step = steps.begin(); step != steps.end(); ++step)
-        this->invalidate_step(*step);
+    bool invalidated = false;
+    for (std::set<PrintStep>::const_iterator step = steps.begin(); step != steps.end(); ++step) {
+        if (this->invalidate_step(*step)) invalidated = true;
+    }
     
-    return true;
+    return invalidated;
 }
 
-void
+bool
 Print::invalidate_step(PrintStep step)
 {
-    this->state.invalidate(step);
+    bool invalidated = this->state.invalidate(step);
     
     // propagate to dependent steps
     if (step == psSkirt) {
@@ -428,6 +438,21 @@ Print::invalidate_step(PrintStep step)
             (*object)->invalidate_step(posSupportMaterial);
         }
     }
+    
+    return invalidated;
+}
+
+bool
+Print::invalidate_all_steps()
+{
+    // make a copy because when invalidating steps the iterators are not working anymore
+    std::set<PrintStep> steps = this->state.started;
+    
+    bool invalidated = false;
+    for (std::set<PrintStep>::const_iterator step = steps.begin(); step != steps.end(); ++step) {
+        if (this->invalidate_step(*step)) invalidated = true;
+    }
+    return invalidated;
 }
 
 
diff --git a/xs/src/Print.hpp b/xs/src/Print.hpp
index de9fad731..7b1734be9 100644
--- a/xs/src/Print.hpp
+++ b/xs/src/Print.hpp
@@ -27,16 +27,14 @@ enum PrintObjectStep {
 template <class StepType>
 class PrintState
 {
-    private:
-    std::set<StepType> _started, _done;
-    
     public:
-    bool started(StepType step) const;
-    bool done(StepType step) const;
+    std::set<StepType> started, done;
+    
+    bool is_started(StepType step) const;
+    bool is_done(StepType step) const;
     void set_started(StepType step);
     void set_done(StepType step);
-    void invalidate(StepType step);
-    bool invalidate_all();
+    bool invalidate(StepType step);
 };
 
 // A PrintRegion object represents a group of volumes to print
@@ -108,7 +106,8 @@ class PrintObject
     
     // methods for handling state
     bool invalidate_state_by_config_options(const std::vector<t_config_option_key> &opt_keys);
-    void invalidate_step(PrintObjectStep step);
+    bool invalidate_step(PrintObjectStep step);
+    bool invalidate_all_steps();
     
     private:
     Print* _print;
@@ -155,7 +154,8 @@ class Print
     
     // methods for handling state
     bool invalidate_state_by_config_options(const std::vector<t_config_option_key> &opt_keys);
-    void invalidate_step(PrintStep step);
+    bool invalidate_step(PrintStep step);
+    bool invalidate_all_steps();
 
     private:
     void clear_regions();
diff --git a/xs/xsp/Print.xsp b/xs/xsp/Print.xsp
index 6510cd38e..d246dbfe5 100644
--- a/xs/xsp/Print.xsp
+++ b/xs/xsp/Print.xsp
@@ -86,11 +86,10 @@ _constant()
     void delete_support_layer(int idx);
     
     bool invalidate_state_by_config_options(std::vector<std::string> opt_keys);
-    void invalidate_step(PrintObjectStep step);
-    bool invalidate_all_steps()
-        %code%{ RETVAL = THIS->state.invalidate_all(); %};
+    bool invalidate_step(PrintObjectStep step);
+    bool invalidate_all_steps();
     bool step_done(PrintObjectStep step)
-        %code%{ RETVAL = THIS->state.done(step); %};
+        %code%{ RETVAL = THIS->state.is_done(step); %};
     void set_step_done(PrintObjectStep step)
         %code%{ THIS->state.set_done(step); %};
     void set_step_started(PrintObjectStep step)
@@ -141,11 +140,10 @@ _constant()
         %code%{ RETVAL = THIS->regions.size(); %};
     
     bool invalidate_state_by_config_options(std::vector<std::string> opt_keys);
-    void invalidate_step(PrintStep step);
-    bool invalidate_all_steps()
-        %code%{ RETVAL = THIS->state.invalidate_all(); %};
+    bool invalidate_step(PrintStep step);
+    bool invalidate_all_steps();
     bool step_done(PrintStep step)
-        %code%{ RETVAL = THIS->state.done(step); %};
+        %code%{ RETVAL = THIS->state.is_done(step); %};
     void set_step_done(PrintStep step)
         %code%{ THIS->state.set_done(step); %};
     void set_step_started(PrintStep step)