Messages in this thread | | | Date | Fri, 4 Jul 2014 11:01:47 +1000 | From | Dave Chinner <> | Subject | Re: [regression, 3.16-rc] rwsem: optimistic spinning causing performance degradation |
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[re-added lkml]
On Thu, Jul 03, 2014 at 11:50:20AM -0700, Jason Low wrote: > On Wed, Jul 2, 2014 at 7:32 PM, Dave Chinner <david@fromorbit.com> wrote: > > This is what the kernel profile looks like on the strided run: > > > > - 83.06% [kernel] [k] osq_lock > > - osq_lock > > - 100.00% rwsem_down_write_failed > > - call_rwsem_down_write_failed > > - 99.55% sys_mprotect > > tracesys > > __GI___mprotect > > - 12.02% [kernel] [k] rwsem_down_write_failed > > Hi Dave, > > So with no sign of rwsem_spin_on_owner(), yet with such heavy contention in > osq_lock, this makes me wonder if it's spending most of its time spinning > on !owner while a reader has the lock? (We don't set sem->owner for the readers.) > > If that's an issue, maybe the below is worth a test, in which we'll just > avoid spinning if rwsem_can_spin_on_owner() finds that there is no owner. > If we just had to enter the slowpath yet there is no owner, we'll be conservative > and assume readers have the lock.
That makes it quite a bit faster:
XFS_REPAIR Summary Fri Jul 4 10:39:32 2014
Phase Start End Duration Phase 1: 07/04 10:38:04 07/04 10:38:05 1 second Phase 2: 07/04 10:38:05 07/04 10:38:08 3 seconds Phase 3: 07/04 10:38:08 07/04 10:39:12 1 minute, 4 seconds Phase 4: 07/04 10:39:12 07/04 10:39:21 9 seconds Phase 5: 07/04 10:39:21 07/04 10:39:22 1 second Phase 6: 07/04 10:39:22 07/04 10:39:30 8 seconds Phase 7: 07/04 10:39:30 07/04 10:39:30
Total run time: 1 minute, 26 seconds done
real 1m28.504s user 1m23.990s sys 3m20.132s
So system time goes down massively, and speed comes up to within 30% of the single AG run. But it's still 2-3000 IOPS down, but it's acceptible for the moment.
FWIW, the kernel profile ifor the multi-AG run now looks like:
29.64% [kernel] [k] _raw_spin_unlock_irq - _raw_spin_unlock_irq + 35.34% __schedule - 34.15% call_rwsem_down_write_failed + 99.27% sys_mprotect - 30.02% call_rwsem_down_read_failed 95.59% __do_page_fault - 24.65% [kernel] [k] _raw_spin_unlock_irqrestore - _raw_spin_unlock_irqrestore - 69.38% rwsem_wake - call_rwsem_wake - 83.32% sys_mprotect + 15.54% __do_page_fault + 22.55% try_to_wake_up + 9.77% [kernel] [k] default_send_IPI_mask_sequence_phys - 3.21% [kernel] [k] smp_call_function_many - smp_call_function_many - 99.22% flush_tlb_page + 2.51% [kernel] [k] rwsem_down_write_failed
It's much more like the 3.15 profile - it's only wasting half the CPU spinning on the internal spinlock and it's now going fast enough to be blowing another 10-12% of the CPU time sending tlb flushes to other CPUs....
One thing I did notice, even with the single-AG-at-a-time run, is that the system time is *significantly* reduced with this patch, even though it doesn't change performance.
ie unpatched:
unpatched patched runtime 0m58s 1m2s systime 4m55s 1m1s
So not spinning when there are read holders is a major win even when there are few threads contending on read/write.
FWIW, the rwsems in the struct xfs_inode are often heavily read/write contended, so there are lots of IO related workloads that are going to regress on XFS without this optimisation...
Anyway, consider the patch:
Tested-by: Dave Chinner <dchinner@redhat.com>
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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