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SubjectPerformance regression from switching lock to rw-sem for anon-vma tree
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Ingo,

At the time of switching the anon-vma tree's lock from mutex to
rw-sem (commit 5a505085), we encountered regressions for fork heavy workload.
A lot of optimizations to rw-sem (e.g. lock stealing) helped to
mitigate the problem. I tried an experiment on the 3.10-rc4 kernel
to compare the performance of rw-sem to one that uses mutex. I saw
a 8% regression in throughput for rw-sem vs a mutex implementation in
3.10-rc4.

For the experiments, I used the exim mail server workload in
the MOSBENCH test suite on 4 socket (westmere) and a 4 socket
(ivy bridge) with the number of clients sending mail equal
to number of cores. The mail server will
fork off a process to handle an incoming mail and put it into mail
spool. The lock protecting the anon-vma tree is stressed due to
heavy forking. On both machines, I saw that the mutex implementation
has 8% more throughput. I've pinned the cpu frequency to maximum
in the experiments.

I've tried two separate tweaks to the rw-sem on 3.10-rc4. I've tested
each tweak individually.

1) Add an owner field when a writer holds the lock and introduce
optimistic spinning when an active writer is holding the semaphore.
It reduced the context switching by 30% to a level very close to the
mutex implementation. However, I did not see any throughput improvement
of exim.

2) When the sem->count's active field is non-zero (i.e. someone
is holding the lock), we can skip directly to the down_write_failed
path, without adding the RWSEM_DOWN_WRITE_BIAS and taking
it off again from sem->count, saving us two atomic operations.
Since we will try the lock stealing again later, this should be okay.
Unfortunately it did not improve the exim workload either.

Any suggestions on the difference between rwsem and mutex performance
and possible improvements to recover this regression?

Thanks.

Tim

vmstat for mutex implementation:
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
38 0 0 130957920 47860 199956 0 0 0 56 236342 476975 14 72 14 0 0
41 0 0 130938560 47860 219900 0 0 0 0 236816 479676 14 72 14 0 0

vmstat for rw-sem implementation (3.10-rc4)
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
40 0 0 130933984 43232 202584 0 0 0 0 321817 690741 13 71 16 0 0
39 0 0 130913904 43232 224812 0 0 0 0 322193 692949 13 71 16 0 0


Profile for mutex implementation:
5.02% exim [kernel.kallsyms] [k] page_fault
3.67% exim [kernel.kallsyms] [k] anon_vma_interval_tree_insert
2.66% exim [kernel.kallsyms] [k] unmap_single_vma
2.15% exim [kernel.kallsyms] [k] do_raw_spin_lock
2.14% exim [kernel.kallsyms] [k] page_cache_get_speculative
2.04% exim [kernel.kallsyms] [k] copy_page_rep
1.58% exim [kernel.kallsyms] [k] clear_page_c
1.55% exim [kernel.kallsyms] [k] cpu_relax
1.55% exim [kernel.kallsyms] [k] mutex_unlock
1.42% exim [kernel.kallsyms] [k] __slab_free
1.16% exim [kernel.kallsyms] [k] mutex_lock
1.12% exim libc-2.13.so [.] vfprintf
0.99% exim [kernel.kallsyms] [k] find_vma
0.95% exim [kernel.kallsyms] [k] __list_del_entry

Profile for rw-sem implementation
4.88% exim [kernel.kallsyms] [k] page_fault
3.43% exim [kernel.kallsyms] [k] anon_vma_interval_tree_insert
2.65% exim [kernel.kallsyms] [k] unmap_single_vma
2.46% exim [kernel.kallsyms] [k] do_raw_spin_lock
2.25% exim [kernel.kallsyms] [k] copy_page_rep
2.01% exim [kernel.kallsyms] [k] page_cache_get_speculative
1.81% exim [kernel.kallsyms] [k] clear_page_c
1.51% exim [kernel.kallsyms] [k] __slab_free
1.12% exim libc-2.13.so [.] vfprintf
1.06% exim [kernel.kallsyms] [k] __list_del_entry
1.02% swapper [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
1.00% exim [kernel.kallsyms] [k] find_vma
0.93% exim [kernel.kallsyms] [k] mutex_unlock


turbostat for mutex implementation:
pk cor CPU %c0 GHz TSC %c1 %c3 %c6 CTMP %pc3 %pc6
82.91 2.39 2.39 11.65 2.76 2.68 51 0.00 0.00

turbostat of rw-sem implementation (3.10-rc4):
pk cor CPU %c0 GHz TSC %c1 %c3 %c6 CTMP %pc3 %pc6
80.10 2.39 2.39 14.96 2.80 2.13 52 0.00 0.00






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