v2->v3:
- Moved deferred kicking enablement patch forward & move back
the kick-ahead patch to make the effect of kick-ahead more visible.
- Reworked patch 6 to make it more readable.
- Reverted back to use state as a tri-state variable instead of
adding an additional bistate variable.
- Added performance data for different values of PV_KICK_AHEAD_MAX.
- Add a new patch to optimize PV unlock code path performance.
v1->v2:
- Take out the queued unfair lock patches
- Add a patch to simplify the PV unlock code
- Move pending bit and statistics collection patches to the front
- Keep vCPU kicking in pv_kick_node(), but defer it to unlock time
when appropriate.
- Change the wait-early patch to use adaptive spinning to better
balance the difference effect on normal and over-committed guests.
- Add patch-to-patch performance changes in the patch commit logs.
This patchset tries to improve the performance of both normal and
over-commmitted VM guests. The kick-ahead and adaptive spinning
patches are inspired by the "Do Virtual Machines Really Scale?" blog
from Sanidhya Kashyap.
Patch 1 simplifies the unlock code by doing unconditional vCPU kick
when _Q_SLOW_VAL is set as the chance of spurious wakeup showing
up in the statistical data that I collected was very low (1 or 2
occasionally).
Patch 2 adds pending bit support to pvqspinlock improving performance
at light load.
Patch 3 allows the collection of various count data that are useful
to see what is happening in the system. They do add a bit of overhead
when enabled slowing performance a tiny bit.
Patch 4 is an enablement patch for deferring vCPU kickings from the
lock side to the unlock side.
Patch 5 enables multiple vCPU kick-ahead's at unlock time, outside of
the critical section which can improve performance in overcommitted
guests and sometime even in normal guests.
Patch 6 enables adaptive spinning in the queue nodes. This patch can
lead to pretty big performance increase in over-committed guest at
the expense of a slight performance hit in normal guests.
Patch 7 optimizes the PV unlock code path performance.
Performance measurements were done on a 32-CPU Westmere-EX and
Haswell-EX systems. The Westmere-EX system got the most performance
gain from patch 5, whereas the Haswell-EX system got the most gain
from patch 6 for over-committed guests.
The table below shows the Linux kernel build times for various
values of PV_KICK_AHEAD_MAX on an over-committed 48-vCPU guest on
the Westmere-EX system:
PV_KICK_AHEAD_MAX Patches 1-5 Patches 1-6
----------------- ----------- -----------
1 9m46.9s 11m10.1s
2 9m40.2s 10m08.3s
3 9m36.8s 9m49.8s
4 9m35.9s 9m38.7s
5 9m35.1s 9m33.0s
6 9m35.7s 9m28.5s
With patches 1-5, the performance wasn't very sensitive to different
PV_KICK_AHEAD_MAX values. Adding patch 6 into the mix, however, changes
the picture quite dramatically. There is a performance regression if
PV_KICK_AHEAD_MAX is too small. Starting with a value of 4, increasing
PV_KICK_AHEAD_MAX only gets us a minor benefit.
Waiman Long (7):
locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL
locking/pvqspinlock: Add pending bit support
locking/pvqspinlock: Collect slowpath lock statistics
locking/pvqspinlock: Enable deferment of vCPU kicking to unlock call
locking/pvqspinlock: Allow vCPUs kick-ahead
locking/pvqspinlock: Queue node adaptive spinning
locking/pvqspinlock, x86: Optimize PV unlock code path
arch/x86/Kconfig | 7 +
arch/x86/include/asm/qspinlock_paravirt.h | 60 ++++
kernel/locking/qspinlock.c | 38 ++-
kernel/locking/qspinlock_paravirt.h | 537 +++++++++++++++++++++++++++--
4 files changed, 604 insertions(+), 38 deletions(-)
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