problem with gc?

[zhmt via Digitalmars-d-learn]

I am writing a echoclient, as below:

Ptr!Conn conn = connect("127.0.0.1",8881);
ubyte[100] buf;
for(int i=0; i<N; i++)
{
	scope string str = format("%s",i);
	conn.write((cast(ubyte*)str.ptr)[0..str.length]);
	conn.read(buf[0..str.length]);
	n++;
}
conn.close();


When it loops about more than 100000 times,the throughput will 
fall from 60000request/second to 26request/second.

If the code changes a little as below:

  scope string str = format("%s",10000); //changes to a constant

It will runs normally with speed of 60000request/second. I test 
for 13minutes, everything seems well.



What happened when the code changes a little? Who will give an 
explaination,Thanks a lot?