* Peter Zijlstra <pet...@infradead.org> wrote:
> On Tue, Jun 23, 2015 at 07:50:12PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2015 at 04:56:39PM +0200, Daniel Wagner wrote:
> > > flock02
> > > mean variance sigma max
> > > min
> > > tip-1 11.8994 0.5874 0.7664 13.2022
> > > 8.6324
> > > tip-2 11.7394 0.5252 0.7247 13.2540
> > > 9.7513
> > > tip-3 11.8155 0.5288 0.7272 13.2700
> > > 9.9480
> > > tip+percpu-rswem-1 15.3601 0.8981 0.9477 16.8116
> > > 12.6910
> > > tip+percpu-rswem-2 15.2558 0.8442 0.9188 17.0199
> > > 12.9586
> > > tip+percpu-rswem-3 15.5297 0.6386 0.7991 17.4392
> > > 12.7992
> >
> > I did indeed manage to get flock02 down to a usable level and found:
>
> Aside from the flock_lock_file function moving up, we also get an
> increase in _raw_spin_lock.
>
> Before:
>
> 5.17% 5.17% flock02 [kernel.vmlinux] [k]
> _raw_spin_lock
> |
> ---_raw_spin_lock
> |
> |--99.75%-- flock_lock_file_wait
> | sys_flock
> | entry_SYSCALL_64_fastpath
> | flock
> --0.25%-- [...]
>
>
> After:
>
> 7.20% 7.20% flock02 [kernel.vmlinux] [k]
> _raw_spin_lock
> |
> ---_raw_spin_lock
> |
> |--52.23%-- flock_lock_file_wait
> | sys_flock
> | entry_SYSCALL_64_fastpath
> | flock
> |
> |--25.92%-- flock_lock_file
> | flock_lock_file_wait
> | sys_flock
> | entry_SYSCALL_64_fastpath
> | flock
> |
> |--21.42%-- locks_delete_lock_ctx
> | flock_lock_file
> | flock_lock_file_wait
> | sys_flock
> | entry_SYSCALL_64_fastpath
> | flock
> --0.43%-- [...]
>
>
> And its not at all clear to me why this would be. It looks like
> FILE_LOCK_DEFERRED is happening, but I've not yet figured out why that
> would be.
So I'd suggest to first compare preemption behavior: does the workload
context-switch heavily, and is it the exact same context switching rate and are
the points of preemption the same as well between the two kernels?
[ Such high variance is often caused by (dynamically) unstable load balancing
and
the workload never finding a good equilibrium. Any observable locking
overhead
is usually just a second order concern or a symptom. Assuming the workload
context switches heavily. ]
Thanks,
Ingo
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