Analyzing Manycore Scaling with Simics

In my previous blog post on multicore scaling investigations with Simics, I tested a simple parallel program on a variety of machines. The scaling obtained was not particularly impressive, especially not on a 60-core target machine. In this post, we will use the Simics timeline view to look a bit closer at what is going on inside the target machines. In particular, with respect to operating system scheduling of the target threads.

Before we look at the runs that indicated a lack of scaling, we should look at a well-behaved case to make sure we have something to compare to. For this, I brought up the 5-core heavy load experiment again, and reran it. Some things in the target setup had changed compared to the last blog post, so the scaling of the length 100 line is a bit different (once again showing the sensitivity of this program to noise and initial conditions just like it would be on physical hardware). If we look at the behavior for a nicely scaling line and compare it to the timeline view plotting the active threads, we can see that nice scaling does indeed correspond to good parallelism.

Continue reading >>