This patchset is the KVM-side implementation of a (new) dirty
"quota" based throttling algorithm that selectively throttles vCPUs
based on their individual contribution to overall memory dirtying and also
dynamically adapts the throttle based on the available network bandwidth.
Overview
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To throttle memory dirtying, we propose to set a limit on the number of
pages a vCPU can dirty in given fixed microscopic size time intervals. This
limit depends on the network throughput calculated over the last few
intervals so as to throttle the vCPUs based on available network bandwidth.
We are referring to this limit as the "dirty quota" of a vCPU and
the fixed size intervals as the "dirty quota intervals".
One possible approach to distributing the overall scope of dirtying for a
dirty quota interval is to equally distribute it among all the vCPUs. This
approach to the distribution doesn't make sense if the distribution of
workloads among vCPUs is skewed. So, to counter such skewed cases, we
propose that if any vCPU doesn't need its quota for any given dirty
quota interval, we add this quota to a common pool. This common pool (or
"common quota") can be consumed on a first come first serve basis
by all vCPUs in the upcoming dirty quota intervals.
Design
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Initialization
vCPUDirtyQuotaContext keeps the dirty quota context for each vCPU. It keeps
the number of pages the vCPU has dirtied (dirty_counter) in the ongoing
dirty quota interval and the maximum number of dirties allowed for the vCPU
(dirty_quota) in the ongoing dirty quota interval.
struct vCPUDirtyQuotaContext {
u64 dirty_counter;
u64 dirty_quota;
};
The flag dirty_quota_migration_enabled determines whether dirty quota-based
throttling is enabled for an ongoing migration or not.
Handling page dirtying
When the guest tries to dirty a page, it leads to a vmexit as each page is
write-protected. In the vmexit path, we increment the dirty_counter for the
corresponding vCPU. Then, we check if the vCPU has exceeded its quota. If
yes, we exit to userspace with a new exit reason KVM_EXIT_DIRTY_QUOTA_FULL.
This "quota full" event is further handled on the userspace side.
Please find the KVM Forum presentation on dirty quota-based throttling
here: https://www.youtube.com/watch?v=ZBkkJf78zFA
Shivam Kumar (6):
Define data structures needed for dirty quota migration.
Allocate memory for dirty quota context and initialize dirty quota
migration flag.
Add dirty quota migration capability and handle vCPU page fault for
dirty quota context.
Increment dirty counter for vmexit due to page write fault.
Exit to userspace when dirty quota is full.
Free space allocated for the vCPU's dirty quota context upon destroy.
arch/x86/kvm/Makefile | 3 ++-
arch/x86/kvm/x86.c | 9 ++++++++
include/linux/dirty_quota_migration.h | 21 ++++++++++++++++++
include/linux/kvm_host.h | 3 +++
include/uapi/linux/kvm.h | 2 ++
virt/kvm/dirty_quota_migration.c | 31 ++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 32 ++++++++++++++++++++++++++-
7 files changed, 99 insertions(+), 2 deletions(-)
create mode 100644 include/linux/dirty_quota_migration.h
create mode 100644 virt/kvm/dirty_quota_migration.c
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2.22.3
