linux-kernel - [PATCH 0/2] Adaptive halt-polling toggle

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Date:	Tue,  1 Sep 2015 14:41:14 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	kvm@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, pbonzini@...hat.com,
	wanpeng.li@...mail.com, peter@...ser.ca,
	David Matlack <dmatlack@...gle.com>
Subject: [PATCH 0/2] Adaptive halt-polling toggle

This patchset adds a dynamic on/off switch for polling. This patchset
gets good performance on its own for both idle and Message Passing
workloads.

                           no-poll     always-poll    adaptive-toggle
---------------------------------------------------------------------
Idle (nohz) VCPU %c0       0.12        0.32           0.15
Idle (250HZ) VCPU %c0      1.22        6.35           1.27
TCP_RR latency             39 us       25 us          25 us

(3.16 Linux guest, halt_poll_ns=200000)

"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 5% CPUs/second and get close to
no-polling overhead levels by using the adaptive toggle. The savings
should be even higher for higher frequency ticks.

Since we get low idle overhead with polling now, halt_poll_ns defaults
to 200000, instead of 0. We can increase halt_poll_ns a bit more once
we have dynamic halt-polling length adjustments (Wanpeng's patch). We
should however keep halt_poll_ns below 1 ms since that is the tick
frequency used by windows.

David Matlack (1):
  kvm: adaptive halt-polling toggle

Wanpeng Li (1):
  KVM: make halt_poll_ns per-VCPU

 include/linux/kvm_host.h   |   1 +
 include/trace/events/kvm.h |  23 ++++++----
 virt/kvm/kvm_main.c        | 111 ++++++++++++++++++++++++++++++++++-----------
 3 files changed, 99 insertions(+), 36 deletions(-)

-- 
2.5.0.457.gab17608

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