Re: REGRESSION: Performance regressions from switching anon_vma->lock to mutex

Shaohua Li
2011-06-15T01:27:00Z

On Wed, 2011-06-15 at 08:29 +0800, Tim Chen wrote:
> It seems like that the recent changes to make the anon_vma->lock into a
> mutex (commit 2b575eb6) causes a 52% regression in throughput (2.6.39 vs
> 3.0-rc2) on exim mail server workload in the MOSBENCH test suite.
>
> Our test setup is on a 4 socket Westmere EX system, with 10 cores per
> socket. 40 clients are created on the test machine which send email
> to the exim server residing on the sam test machine.
>
> Exim forks off child processes to handle the incoming mail, and the
> process exits after the mail delivery completes. We see quite a bit of
> acquisition of the anon_vma->lock as a result.
>
> On 2.6.39, the contention of anon_vma->lock occupies 3.25% of cpu.
> However, after the switch of the lock to mutex on 3.0-rc2, the mutex
> acquisition jumps to 18.6% of cpu. This seems to be the main cause of
> the 52% throughput regression.
>
> Other workloads which have a lot of forks/exits may be similarly
> affected by this regression. Workloads which are vm lock intensive
> could be affected too.
>
> I've listed the profile of 3.0-rc2 and 2.6.39 below for comparison.
>
> Thanks.
>
> Tim
>
>
> ---------------------------
> 3.0-rc2 profile:
>
> - 18.60% exim [kernel.kallsyms] [k] __mutex_lock_common.clone.5
> - __mutex_lock_common.clone.5
> - 99.99% __mutex_lock_slowpath
> - mutex_lock
> - 99.54% anon_vma_lock.clone.10
> + 38.99% anon_vma_clone
> + 37.56% unlink_anon_vmas
> + 11.92% anon_vma_fork
> + 11.53% anon_vma_free
> + 4.03% exim [kernel.kallsyms] [k] _raw_spin_lock_irqsave
> - 3.00% exim [kernel.kallsyms] [k] do_raw_spin_lock
> - do_raw_spin_lock
> - 94.11% _raw_spin_lock
> + 47.32% __mutex_lock_common.clone.5
> + 14.23% __mutex_unlock_slowpath
> + 4.06% handle_pte_fault
> + 3.81% __do_fault
> + 3.16% unmap_vmas
> + 2.46% lock_flocks
> + 2.43% copy_pte_range
> + 2.28% __task_rq_lock
> + 1.30% __percpu_counter_add
> + 1.30% dput
> + 1.27% add_partial
> + 1.24% free_pcppages_bulk
> + 1.07% d_alloc
> + 1.07% get_page_from_freelist
> + 1.02% complete_walk
> + 0.89% dget
> + 0.71% new_inode
> + 0.61% __mod_timer
> + 0.58% dup_fd
> + 0.50% double_rq_lock
> + 3.66% _raw_spin_lock_irq
> + 0.87% _raw_spin_lock_bh
> + 2.90% exim [kernel.kallsyms] [k] page_fault
> + 2.25% exim [kernel.kallsyms] [k] mutex_unlock
>
>
> -----------------------------------
>
> 2.6.39 profile:
> + 4.84% exim [kernel.kallsyms] [k] page_fault
> + 3.83% exim [kernel.kallsyms] [k] clear_page_c
> - 3.25% exim [kernel.kallsyms] [k] do_raw_spin_lock
> - do_raw_spin_lock
> - 91.86% _raw_spin_lock
> + 14.16% unlink_anon_vmas
> + 12.54% unlink_file_vma
> + 7.30% anon_vma_clone_batch
what are you testing? I didn't see Andi's batch anon->lock for fork
patches are merged in 2.6.39.

Thanks,
Shaohua