On Sat, Jul 11, 2015 at 04:36:54PM -0400, Waiman Long wrote: > In an overcommitted guest where some vCPUs have to be halted to make > forward progress in other areas, it is highly likely that a vCPU > later in the spinlock queue will be spinning while the ones earlier in > the queue would have been halted already. The spinning in the later > vCPUs is then just a waste of precious CPU cycles because they are > not going to get the lock soon as the earlier ones have to be woken > up and take their turn to get the lock. > > This patch implements a wait-early mechanism where the vCPU will > call pv_wait() earlier if the previous vCPU is in the halted state > already. In this case, it will spin less before calling pv_wait(). On > the other hand, if the previous vCPU was running and then becomes > halted, the current vCPU will call pv_wait() immmediately in this case. > > This patch also separates the spin threshold for queue head and > queue nodes. It favors the queue head by allowing it to spin longer > before calling pv_wait().
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