Understanding the overhead of the spin-lock loop in CMT architectures

Spin locks are a synchronization mechanisms used to provide mutual exclusion to shared software resources. Spin locks are used over other synchronization mechanisms in several situations, like when the average waiting time to obtain the lock is short, in which case the probability of getting the lock is high, or when it is no possible to use other synchronization mechanisms. In this paper, we study the effect that the execution of the Linux spin-lock loop in the Sun UltraSPARC T1 and T2 processors introduces on other running tasks, especially in the worst case scenario where the workload shows high contention on a lock. For this purpose, we create a task that continuously executes the spin-lock loop and execute several instances of this task together with another active tasks. Our results show that, when the spin-lock tasks run with other applications in the same core of a T1 or a T2 processor, they introduce a significant overhead on other applications: 31% in T1 and 42% in T2, on average, respectively. For the T1 and T2 processors, we identify the fetch bandwidth as the main source of interaction between active threads and the spin-lock threads. We, propose 4 different variants of the Linux spin-lock loop that require less fetch bandwidth. Our proposal reduces the overhead of the spin-lock tasks over the other applications down to 3.5% and 1.5% on average, in T1 and T2 respectively. This is a reduction of 28 percentage points with respect to the Linux spin-lock loop for T1. For T2 the reduction is about 40 percentage points.