more docs, fixes, examples

README.md

@@ -545,6 +545,9 @@ Jumping to a label is always safer than a manual skip.
 ; The label can be a numeric or string label, its sole purpose is tying the two together. They must be unique in the program.
 (barrier-open LABEL)
 (barrier-close LABEL)
+
+; Set coroutine scheduler timeslice (in microseconds). Set to zero to disable preemption.
+(rt-opt RT_TIMESLICE Rd'usec)
 ```
 
 ## Arithmetic Module

crsn/src/builtin/parse.rs

@@ -613,7 +613,7 @@ mod test {
                     global_regs: [0; REG_COUNT],
                     ext_data: Default::default(),
                     cr_deadline: None,
-                    critical_section: false,
+                    critical_section: 0,
                 },
                 const_eval_ti: ti.clone(),
                 parsing_expr: false,

crsn/src/runtime/run_thread.rs

@@ -230,7 +230,9 @@ impl RunThread {
                         // Do switch
                         let old = mem::replace(&mut self.state.cr, cr);
                         self.state.parked.push_back(old);
-                        self.state.cr_deadline = Some(now + *self.info.scheduler_interval.read());
+
+                        self.state.start_task_switching();
+
                     } else if let Some(due) = closest_due {
                         let time = due.saturating_duration_since(now);
                         trace!("No thread to switch to, sleep {:?}", time);
@@ -248,7 +250,7 @@ impl RunThread {
 
                     if n_alive == 0 {
                         trace!("Stop task switching, no parked threads are alive");
-                        self.state.cr_deadline = None;
+                        self.state.stop_task_switching(); // This should improve performance in single-threaded mode
                     }
 
                     match self.state.cr.cr_state {

crsn/src/runtime/run_thread/state.rs

@@ -12,7 +12,7 @@ use nudge::{likely};
 use crate::asm::instr::cond::Flag;
 use std::fmt::{Debug, Formatter};
 use std::fmt;
-use std::time::Instant;
+use std::time::{Instant, Duration};
 
 pub struct RunState {
     pub thread_info: Arc<ThreadInfo>,
@@ -95,10 +95,24 @@ impl RunState {
         });
         if self.cr_deadline.is_none() {
             // start context switching
-            self.cr_deadline = Some(Instant::now() + *self.thread_info.scheduler_interval.read());
+            self.start_task_switching();
         }
         handle
     }
+
+    pub(crate) fn start_task_switching(&mut self) {
+        let ival = *self.thread_info.scheduler_interval.read();
+        if ival > Duration::default() {
+            self.cr_deadline = Some(Instant::now() + ival);
+        } else {
+            // Disabled
+            self.cr_deadline = None;
+        }
+    }
+
+    pub(crate) fn stop_task_switching(&mut self) {
+        self.cr_deadline = None;
+    }
 }
 
 impl Debug for RunState {

examples/coroutines3-crit.csn

@@ -1,16 +1,21 @@
 (
     ; This example shows the use of critical sections.
 
+    ; Set short timeslice (50us) to make the effect more pronounced
+    (rt-opt RT_TIMESLICE 50)
+
     (spawn _ unsafe 'A' 'Z')
-    (spawn _ safe '0' '9')
-    (ssleep 2)
+    (spawn _ unsafe 'a' 'z')
+    (spawn _ safe '0' '9') ; Notice that the sequence 0-9 is always printed in its entirety - because it is in a critical section
+    (msleep 200)
+    (halt)
 
     (proc unsafe start end
         ; This can be interrupted any time
         (:x)
+        (yield)
         (ld r0 start)
         (:l)
-            (msleep 5)
             (ld @cout r0)
             (cmp r0 end)
             (j.eq :x)