docs/migration: Split "dirty limit"

Split that into a separate file, put under "features".

Cc: Yong Huang <yong.huang@smartx.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-8-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

docs/devel/migration/dirty-limit.rst

@@ -0,0 +1,71 @@
+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 (2)
+
+     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 PENALTY (3)
+
+     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.

docs/devel/migration/features.rst

@@ -7,3 +7,4 @@ Migration has plenty of features to support different use cases.
    :maxdepth: 2
 
    postcopy
+   dirty-limit

docs/devel/migration/main.rst

@@ -573,74 +573,3 @@ path.
      Return path  - opened by main thread, written by main thread AND postcopy
      thread (protected by rp_mutex)
 
-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 (2)
-
-     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 PENALTY (3)
-
-     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.
-