<p>OCaml runtime is known to do context switch at the point allocation occurs (confirmed on current version 3.11.1).
This snippet shows that context switch never occurs if the thread does not do any allocation, i/o nor synchronization.
compile with:</p>

<p><code>ocamlopt -thread -I +threads -I unix unix.cmxa threads.cmxa filename.ml</code></p>

<p>the function <code>allocation</code> allocates some memory on OCaml side, while <code>allocation2</code> does on C side.
In latter case allocation never occur on OCaml side, hence context doesn't switch, and one of the thread runs forever, while the other starves.</p>

<p><strong>See also:</strong><a href="http://snipplr.com/view/19485/">http://snipplr.com/view/19485/</a></p>

let allocation _ = let rec gen x = if x=0 then [] else x::gen (x-1) in ignore (gen 100000)
let allocation2 _ = ignore (String.create 1000000) (* process runs forever if we use this allocation *)
 
let r = ref false;;
 
let rec forever x = allocation (); r:=true; forever x;;
 
let rec wait _ = if !r then print_string "end.\n" else (allocation (); wait ()) ;;
 
Thread.create (fun _ -> forever ()) ();;
 
wait ();;