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SubjectRe: [PATCH v13 10/11] pvqspinlock, x86: Enable PV qspinlock for KVM
On Wed, 29 Oct 2014, Waiman Long wrote:
> AIM7 XFS Disk Test (no overcommit)
> kernel JPM Real Time Sys Time Usr Time
> ----- --- --------- -------- --------
> PV ticketlock 2542373 7.08 98.95 5.44
> PV qspinlock 2549575 7.06 98.63 5.40
> unfairlock 2616279 6.91 97.05 5.42
>
> AIM7 XFS Disk Test (200% overcommit)
> kernel JPM Real Time Sys Time Usr Time
> ----- --- --------- -------- --------
> PV ticketlock 644468 27.93 415.22 6.33
> PV qspinlock 645624 27.88 419.84 0.39

That number is made up by what? ----------------------------^^^^

> unfairlock 695518 25.88 377.40 4.09
>
> AIM7 EXT4 Disk Test (no overcommit)
> kernel JPM Real Time Sys Time Usr Time
> ----- --- --------- -------- --------
> PV ticketlock 1995565 9.02 103.67 5.76
> PV qspinlock 2011173 8.95 102.15 5.40
> unfairlock 2066590 8.71 98.13 5.46
>
> AIM7 EXT4 Disk Test (200% overcommit)
> kernel JPM Real Time Sys Time Usr Time
> ----- --- --------- -------- --------
> PV ticketlock 478341 37.63 495.81 30.78
> PV qspinlock 474058 37.97 475.74 30.95
> unfairlock 560224 32.13 398.43 26.27
>
> For the AIM7 disk workload, both PV ticketlock and qspinlock have
> about the same performance. The unfairlock performs slightly better
> than the PV lock.

Slightly?

Taking the PV locks, which are basically the same for the existing
ticket locks and your new fangled qlocks as a reference then the so
called 'unfair locks' which are just the native locks w/o the PV
nonsense are fundamentally better up to a whopping 18% in the
ext4/200% overcommit case. See below.

> EBIZZY-m Test (no overcommit)
> kernel Rec/s Real Time Sys Time Usr Time
> ----- ----- --------- -------- --------
> PV ticketlock 3255 10.00 60.65 3.62
> PV qspinlock 3318 10.00 54.27 3.60
> unfairlock 2833 10.00 26.66 3.09
>
> EBIZZY-m Test (200% overcommit)
> kernel Rec/s Real Time Sys Time Usr Time
> ----- ----- --------- -------- --------
> PV ticketlock 841 10.00 71.03 2.37
> PV qspinlock 834 10.00 68.27 2.39
> unfairlock 865 10.00 27.08 1.51
>
> futextest (no overcommit)
> kernel kops/s
> ----- ------
> PV ticketlock 11523
> PV qspinlock 12328
> unfairlock 9478
>
> futextest (200% overcommit)
> kernel kops/s
> ----- ------
> PV ticketlock 7276
> PV qspinlock 7095
> unfairlock 5614
>
> The ebizzy and futextest have much higher spinlock contention than
> the AIM7 disk workload. In this case, the unfairlock performs worse
> than both the PV ticketlock and qspinlock. The performance of the 2
> PV locks are comparable.

While I can see that the PV lock stuff performs 13% better for the
ebizzy no overcommit case, what about the very interresting numbers
for the same test with 200% overcommit?

The regular lock has a slightly better performance, but significantly
less sys/usr time. How do you explain that?

'Lies, damned lies and statistics' comes to my mind.

Thanks,

tglx


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