&gt; Except instead of a variable normally a &#x27;test&#x27; instruction is used on a page of memory, and instead of setting the variable, the permission on the page are changed triggering a page fault.<p>Which GCs do that? The only one I know is Azul&#x27;s Pauseless JVM GC, but that&#x27;s a kind of a special case, given that it needs support from the kernel.