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author | Peter Xu <peterx@redhat.com> | 2024-01-09 14:46:25 +0800 |
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committer | Peter Xu <peterx@redhat.com> | 2024-01-16 11:16:10 +0800 |
commit | 4c6f8a79ae539eeb1f86af6522e4000edde3638b (patch) | |
tree | 8627efcc81cd1af07d876abcdaeab5156209aecc /docs/devel | |
parent | bfb4c7cd99f1c39dedf33381954d03b9f8f244ec (diff) | |
download | qemu-4c6f8a79ae539eeb1f86af6522e4000edde3638b.zip qemu-4c6f8a79ae539eeb1f86af6522e4000edde3638b.tar.gz qemu-4c6f8a79ae539eeb1f86af6522e4000edde3638b.tar.bz2 |
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>
Diffstat (limited to 'docs/devel')
-rw-r--r-- | docs/devel/migration/dirty-limit.rst | 71 | ||||
-rw-r--r-- | docs/devel/migration/features.rst | 1 | ||||
-rw-r--r-- | docs/devel/migration/main.rst | 71 |
3 files changed, 72 insertions, 71 deletions
diff --git a/docs/devel/migration/dirty-limit.rst b/docs/devel/migration/dirty-limit.rst new file mode 100644 index 0000000..8f32329 --- /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 0054e0c..e257d0d 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 051ea43..00b9c3d 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. - |