linux-kernel - [PATCH v5 0/3] KVM: Dynamic Halt-Polling

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Date:	Wed, 2 Sep 2015 14:20:58 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	David Matlack <dmatlack@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>
Subject: [PATCH v5 0/3] KVM: Dynamic Halt-Polling

v4 -> v5:
 * set base case 10us and max poll time 500us
 * handle short/long halt, idea from David, many thanks David ;-)

v3 -> v4:
 * bring back grow vcpu->halt_poll_ns when interrupt arrives and shrinks
   when idle VCPU is detected 

v2 -> v3:
 * grow/shrink vcpu->halt_poll_ns by *halt_poll_ns_grow or /halt_poll_ns_shrink
 * drop the macros and hard coding the numbers in the param definitions
 * update the comments "5-7 us"
 * remove halt_poll_ns_max and use halt_poll_ns as the max halt_poll_ns time,
   vcpu->halt_poll_ns start at zero
 * drop the wrappers 
 * move the grow/shrink logic before "out:" w/ "if (waited)"

v1 -> v2:
 * change kvm_vcpu_block to read halt_poll_ns from the vcpu instead of 
   the module parameter
 * use the shrink/grow matrix which is suggested by David
 * set halt_poll_ns_max to 2ms

There is a downside of always-poll since poll is still happened for idle 
vCPUs which can waste cpu usage. This patchset add the ability to adjust 
halt_poll_ns dynamically, to grow halt_poll_ns when shot halt is detected,  
and to shrink halt_poll_ns when long halt is detected.

There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and halt_poll_ns_shrink. 

						no-poll      always-poll    dynamic-poll
-----------------------------------------------------------------------
Idle (nohz) vCPU %c0     0.15%        0.3%            0.2%  
Idle (250HZ) vCPU %c0    1.1%         4.6%~14%        1.2%
TCP_RR latency           34us         27us            26.7us

"Idle (X) vCPU %c0" is the percent of time the physical cpu spent in
c0 over 60 seconds (each vCPU is pinned to a pCPU). (nohz) means the
guest was tickless. (250HZ) means the guest was ticking at 250HZ.

The big win is with ticking operating systems. Running the linux guest
with nohz=off (and HZ=250), we save 3.4%~12.8% CPUs/second and get close 
to no-polling overhead levels by using the dynamic-poll. The savings
should be even higher for higher frequency ticks.
															
Wanpeng Li (3):
  KVM: make halt_poll_ns per-VCPU
  KVM: dynamic halt_poll_ns adjustment
  KVM: trace kvm_halt_poll_ns grow/shrink

 include/linux/kvm_host.h   |  1 +
 include/trace/events/kvm.h | 30 ++++++++++++++++++++
 virt/kvm/kvm_main.c        | 69 ++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 95 insertions(+), 5 deletions(-)

-- 
1.9.1

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