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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>
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Dirty limit | ||
=========== | ||
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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. | ||
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The algorithm framework is as follows: | ||
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:: | ||
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------------------------------------------------------------------------------ | ||
main --------------> throttle thread ------------> PREPARE(1) <-------- | ||
thread \ | | | ||
\ | | | ||
\ V | | ||
-\ CALCULATE(2) | | ||
\ | | | ||
\ | | | ||
\ V | | ||
\ SET PENALTY(3) ----- | ||
-\ | | ||
\ | | ||
\ V | ||
-> virtual CPU thread -------> ACCEPT PENALTY(4) | ||
------------------------------------------------------------------------------ | ||
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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: | ||
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- PREPARE (1) | ||
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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. | ||
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- CALCULATE (2) | ||
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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. | ||
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- SET PENALTY (3) | ||
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Set the sleep time for each virtual CPU that should be penalized based | ||
on the results of the calculation supplied by step CALCULATE (2). | ||
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After completing the three above stages, the throttle thread loops back | ||
to step PREPARE (1) until the dirty limit is reached. | ||
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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. | ||
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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. |
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@@ -7,3 +7,4 @@ Migration has plenty of features to support different use cases. | |
:maxdepth: 2 | ||
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postcopy | ||
dirty-limit |
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