| Message ID | 59985366f38053caac40c14d86b2a50bead944f6.1697502089.git.yong.huang@smartx.com |
|---|---|
| State | New |
| Headers | show |
| Series | dirtylimit: miscellaneous patches | expand |
Hyman Huang <yong.huang@smartx.com> writes: > The dirty limit feature has been introduced since the 8.1 > QEMU release but has not reflected in the document, add a > section for that. > > Signed-off-by: Hyman Huang <yong.huang@smartx.com> > --- > docs/devel/migration.rst | 71 ++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 71 insertions(+) > > diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst > index c3e1400c0c..1cbec22e2a 100644 > --- a/docs/devel/migration.rst > +++ b/docs/devel/migration.rst > @@ -588,6 +588,77 @@ 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 prepared for the second stage, s/prepare/preparation/ might be more appropriate > + 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_RULL exception; The dirty page rate upper s/RULL/FULL > + 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 There's a non-breaking space artifact between 'the' and 'dirty' > + 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_RULL exception handler, that s/RULL/FULL > +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. > + > Postcopy > ========
diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst index c3e1400c0c..1cbec22e2a 100644 --- a/docs/devel/migration.rst +++ b/docs/devel/migration.rst @@ -588,6 +588,77 @@ 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 prepared 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_RULL 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_RULL 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. + Postcopy ========
The dirty limit feature has been introduced since the 8.1 QEMU release but has not reflected in the document, add a section for that. Signed-off-by: Hyman Huang <yong.huang@smartx.com> --- docs/devel/migration.rst | 71 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 71 insertions(+)