On 1/9/24 07:46, pet...@redhat.com wrote:
From: Peter Xu <pet...@redhat.com>
Split that into a separate file, put under "features".
Cc: Yong Huang <yong.hu...@smartx.com>
Signed-off-by: Peter Xu <pet...@redhat.com>
Reviewed-by: Cédric Le Goater <c...@redhat.com>
Thanks,
C.
---
docs/devel/migration/dirty-limit.rst | 71 ++++++++++++++++++++++++++++
docs/devel/migration/features.rst | 1 +
docs/devel/migration/main.rst | 71 ----------------------------
3 files changed, 72 insertions(+), 71 deletions(-)
create mode 100644 docs/devel/migration/dirty-limit.rst
diff --git a/docs/devel/migration/dirty-limit.rst
b/docs/devel/migration/dirty-limit.rst
new file mode 100644
index 0000000000..8f32329d5f
--- /dev/null
+++ b/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.
diff --git a/docs/devel/migration/features.rst
b/docs/devel/migration/features.rst
index 0054e0c900..e257d0d100 100644
--- a/docs/devel/migration/features.rst
+++ b/docs/devel/migration/features.rst
@@ -7,3 +7,4 @@ Migration has plenty of features to support different use cases.
:maxdepth: 2
postcopy
+ dirty-limit
diff --git a/docs/devel/migration/main.rst b/docs/devel/migration/main.rst
index 051ea43f0e..00b9c3d32f 100644
--- a/docs/devel/migration/main.rst
+++ b/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.
-
