Before & After Under The Giant Lock

[Robert Watson]

On Sun, 25 Nov 2007, Stephen Montgomery-Smith wrote:

> (Also when I run 4 threads with 2 cpus, each with hyperthreading, it goes 
> 2.5 to 3 times faster - surprising since hyperthreading gets quite bad press 
> for its performance improvements - I should add that Linux didn't do at all 
> well at taking advantage of hyperthreading, running at the same speed as 
> with 2 threads.)

I've seen gradual improvements both in our ability to manage HTT and HTT 
itself.  One of the things that gave HTT a particularly bad reputation was 
that it was first introduced in the P4 Xeon CPU line from Intel, and that line 
had extortionately expensive synchronization instructions compared to either 
prior or later CPU lines.  As a result, even a small amount of synchronization 
(read: kernel locking) quickly ate any benefits of potential parallelism. 
More recent CPUs have managed to reduce "extionate" to "relatively 
expensive", which is much more manageable.

Robert N M Watson
Computer Laboratory
University of Cambridge