Dirty limit

The dirty limit, short for dirty page rate upper limit, is a new capability introduced in the 8.1 QEMU release that uses a new algorithm based on the KVM dirty ring to throttle down the guest during live migration.

The algorithm framework is as follows:

main   --------------> throttle thread ------------> PREPARE(1) <--------
thread  \                                                |              |
         \                                               |              |
          \                                              V              |
           -\                                        CALCULATE(2)       |
             \                                           |              |
              \                                          |              |
               \                                         V              |
                \                                    SET PENALTY(3) -----
                 -\                                      |
                   \                                     |
                    \                                    V
                     -> virtual CPU thread -------> ACCEPT PENALTY(4)

When the qmp command qmp_set_vcpu_dirty_limit is called for the first time, the QEMU main thread starts the throttle thread. The throttle thread, once launched, executes the loop, which consists of three steps:

  • PREPARE (1)

    The entire work of PREPARE (1) is preparation for the second stage, CALCULATE(2), as the name implies. It involves preparing the dirty page rate value and the corresponding upper limit of the VM: The dirty page rate is calculated via the KVM dirty ring mechanism, which tells QEMU how many dirty pages a virtual CPU has had since the last KVM_EXIT_DIRTY_RING_FULL exception; The dirty page rate upper limit is specified by caller, therefore fetch it directly.


    Calculate a suitable sleep period for each virtual CPU, which will be used to determine the penalty for the target virtual CPU. The computation must be done carefully in order to reduce the dirty page rate progressively down to the upper limit without oscillation. To achieve this, two strategies are provided: the first is to add or subtract sleep time based on the ratio of the current dirty page rate to the limit, which is used when the current dirty page rate is far from the limit; the second is to add or subtract a fixed time when the current dirty page rate is close to the limit.


    Set the sleep time for each virtual CPU that should be penalized based on the results of the calculation supplied by step CALCULATE (2).

After completing the three above stages, the throttle thread loops back to step PREPARE (1) until the dirty limit is reached.

On the other hand, each virtual CPU thread reads the sleep duration and sleeps in the path of the KVM_EXIT_DIRTY_RING_FULL exception handler, that is ACCEPT PENALTY (4). Virtual CPUs tied with writing processes will obviously exit to the path and get penalized, whereas virtual CPUs involved with read processes will not.

In summary, thanks to the KVM dirty ring technology, the dirty limit algorithm will restrict virtual CPUs as needed to keep their dirty page rate inside the limit. This leads to more steady reading performance during live migration and can aid in improving large guest responsiveness.