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authorNir Weiner <nir.weiner@oracle.com>2019-01-27 12:17:15 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2019-02-20 22:48:50 +0100
commit49113d360bdeb4dd916fb6bffbcc3e157422b6fd (patch)
tree9580d0cf4466aac62107b063a73cf3de6885943f /Documentation
parent7fa08e71b4a0591a518814fa78b32e124f90d587 (diff)
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KVM: Expose the initial start value in grow_halt_poll_ns() as a module parameter
The hard-coded value 10000 in grow_halt_poll_ns() stands for the initial start value when raising up vcpu->halt_poll_ns. It actually sets the first timeout to the first polling session. This value has significant effect on how tolerant we are to outliers. On the standard case, higher value is better - we will spend more time in the polling busyloop, handle events/interrupts faster and result in better performance. But on outliers it puts us in a busy loop that does nothing. Even if the shrink factor is zero, we will still waste time on the first iteration. The optimal value changes between different workloads. It depends on outliers rate and polling sessions length. As this value has significant effect on the dynamic halt-polling algorithm, it should be configurable and exposed. Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com> Reviewed-by: Liran Alon <liran.alon@oracle.com> Signed-off-by: Nir Weiner <nir.weiner@oracle.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/virtual/kvm/halt-polling.txt37
1 files changed, 23 insertions, 14 deletions
diff --git a/Documentation/virtual/kvm/halt-polling.txt b/Documentation/virtual/kvm/halt-polling.txt
index 4a8418318769..4f791b128dd2 100644
--- a/Documentation/virtual/kvm/halt-polling.txt
+++ b/Documentation/virtual/kvm/halt-polling.txt
@@ -53,7 +53,8 @@ the global max polling interval then the polling interval can be increased in
the hope that next time during the longer polling interval the wake up source
will be received while the host is polling and the latency benefits will be
received. The polling interval is grown in the function grow_halt_poll_ns() and
-is multiplied by the module parameter halt_poll_ns_grow.
+is multiplied by the module parameters halt_poll_ns_grow and
+halt_poll_ns_grow_start.
In the event that the total block time was greater than the global max polling
interval then the host will never poll for long enough (limited by the global
@@ -80,22 +81,30 @@ shrunk. These variables are defined in include/linux/kvm_host.h and as module
parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
powerpc kvm-hv case.
-Module Parameter | Description | Default Value
+Module Parameter | Description | Default Value
--------------------------------------------------------------------------------
-halt_poll_ns | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT
- | which defines the ceiling value |
- | of the polling interval for | (per arch value)
- | each vcpu. |
+halt_poll_ns | The global max polling | KVM_HALT_POLL_NS_DEFAULT
+ | interval which defines |
+ | the ceiling value of the |
+ | polling interval for | (per arch value)
+ | each vcpu. |
--------------------------------------------------------------------------------
-halt_poll_ns_grow | The value by which the halt | 2
- | polling interval is multiplied |
- | in the grow_halt_poll_ns() |
- | function. |
+halt_poll_ns_grow | The value by which the | 2
+ | halt polling interval is |
+ | multiplied in the |
+ | grow_halt_poll_ns() |
+ | function. |
--------------------------------------------------------------------------------
-halt_poll_ns_shrink | The value by which the halt | 0
- | polling interval is divided in |
- | the shrink_halt_poll_ns() |
- | function. |
+halt_poll_ns_grow_start | The initial value to grow | 10000
+ | to from zero in the |
+ | grow_halt_poll_ns() |
+ | function. |
+--------------------------------------------------------------------------------
+halt_poll_ns_shrink | The value by which the | 0
+ | halt polling interval is |
+ | divided in the |
+ | shrink_halt_poll_ns() |
+ | function. |
--------------------------------------------------------------------------------
These module parameters can be set from the debugfs files in: