Extreme memory usage when `synchronized( this )` is used

[Daniel Kozák via Digitalmars-d-learn]

On Mon, 11 May 2015 09:09:07 +0000
tcak via Digitalmars-d-learn <digitalmars-d-learn at puremagic.com> wrote:

> [code]
> import std.stdio;
> 
> class Connection{
> 	private void other() shared{}
> 
> 	public void close() shared{
> 		synchronized( this ){
> 			other();
> 		}
> 	}
> 
> 	public void hasData() shared{ writeln("Has Data"); }
> }
> 
> void main() {
> 	for(long i=0; i < 250_000_000; ++i){
> 		auto conn = new shared Connection();
> 
> 		conn.hasData();
> 
> 		conn.close();
> 	}
> }
> [/code]
> 
> With this code, memory usage of program is increasing very fast. 
> In about 10 seconds, it reached 100MB for me.
> 
> If I comment out `synchronized( this )` line with its 
> parentheses, OR remove `(this)` from it, it suddenly turns 
> normal. Very little memory usage.
> 
> What's happening? Is object instance being stored somewhere at 
> each iteration?
> 
> --
> 
> I tried the same thing by creating synchronisation object instead 
> of object itself as blow, still usage lots of memory.
> 
> [code]
> import std.stdio;
> 
> class Connection{
> 	private Object syncObject;
> 
> 	public this() shared{
> 		syncObject = new shared Object();
> 	}
> 
> 	private void other() shared{}
> 
> 	public void close() shared{
> 		synchronized( syncObject ){
> 			other();
> 		}
> 	}
> 
> 	public void hasData() shared{ writeln("Has Data"); }
> }
> 
> void main() {
> 	for(long i=0; i < 250_000_000; ++i){
> 		auto conn = new shared Connection();
> 
> 		conn.hasData();
> 
> 		conn.close();
> 	}
> }
> [/code]

I think synchronize(this) prevents GC from collect memory