Re: [PATCH v4 0/9] mm/swap: Regular page swap optimizations

["Huang\, Ying" <ying.huang@intel.com>]

Minchan Kim <minchan@kernel.org> writes:

> Hi,
>
> On Thu, Jan 05, 2017 at 09:33:55AM +0800, Huang, Ying wrote:
>> Hi, Minchan,
>> 
>> Minchan Kim <minchan@kernel.org> writes:
>> [snip]
>> >
>> > The patchset has used several techniqueus to reduce lock contention, for example,
>> > batching alloc/free, fine-grained lock and cluster distribution to avoid cache
>> > false-sharing. Each items has different complexity and benefits so could you
>> > show the number for each step of pathchset? It would be better to include the
>> > nubmer in each description. It helps how the patch is important when we consider
>> > complexitiy of the patch.
>> 
>> Here is the test data.
>
> Thanks!
>
>> 
>> We test the vm-scalability swap-w-seq test case with 32 processes on a
>> Xeon E5 v3 system.  The swap device used is a RAM simulated PMEM
>> (persistent memory) device.  To test the sequential swapping out, the
>> test case created 32 processes, which sequentially allocate and write to
>> the anonymous pages until the RAM and part of the swap device is used
>> up.
>> 
>> The patchset is rebased on v4.9-rc8.  So the baseline performance is as
>> follow,
>> 
>>   "vmstat.swap.so": 1428002,
>
> What does it mean? vmstat.pswpout?

This is the average of swap.so column of /usr/bin/vmstat output.  We run
/usr/bin/vmstat with,

/usr/bin/vmstat -n 1

>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_swap.shrink_page_list": 13.94,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_inactive_list.shrink_node_memcg":
>> 13.75,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.swap_info_get.swapcache_free.__remove_mapping.shrink_page_list": 7.05,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.swap_info_get.page_swapcount.try_to_free_swap.swap_writepage": 7.03,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.__swap_duplicate.swap_duplicate.try_to_unmap_one.rmap_walk_anon": 7.02,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.get_swap_page.add_to_swap.shrink_page_list.shrink_inactive_list": 6.83,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.page_check_address_transhuge.page_referenced_one.rmap_walk_anon.rmap_walk": 0.81,
>
> Numbers mean overhead percentage reported by perf?

Yes.
 
>> >> Patch 1 is a clean up patch.
>> >
>> > Could it be separated patch?
>> >
>> >> Patch 2 creates a lock per cluster, this gives us a more fine graind lock
>> >>         that can be used for accessing swap_map, and not lock the whole
>> >>         swap device
>> 
>> After patch 2, the result is as follow,
>> 
>>   "vmstat.swap.so": 1481704,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_swap.shrink_page_list": 27.53,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_inactive_list.shrink_node_memcg":
>> 27.01,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.free_pcppages_bulk.drain_pages_zone.drain_pages.drain_local_pages": 1.03,
>> 
>> The swap out throughput is at the same level, but the lock contention on
>> swap_info_struct->lock is eliminated.
>> 
>> >> Patch 3 splits the swap cache radix tree into 64MB chunks, reducing
>> >>         the rate that we have to contende for the radix tree.
>> >
>> 
>> After patch 3,
>> 
>>   "vmstat.swap.so": 2050097,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.get_swap_page.add_to_swap.shrink_page_list.shrink_inactive_list": 43.27,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.get_page_from_freelist.__alloc_pages_nodemask.alloc_pages_vma.handle_mm_fault": 4.84,
>> 
>> The swap out throughput is improved about ~43% compared with baseline.
>> The lock contention on swap cache radix tree lock is eliminated.
>> swap_info_struct->lock in get_swap_page() becomes the most heavy
>> contended lock.
>
> The numbers are great! Please include those into each patchset.
> And I ask one more thing I said earlier about patch 2.
>
> ""
> I hope you make three steps to review easier. You can create some functions like
> swap_map_lock and cluster_lock which are wrapper functions just hold swap_lock.
> It doesn't change anything performance pov but it clearly shows what kinds of lock
> we should use in specific context.
>
> Then, you can introduce more fine-graind lock in next patch and apply it into
> those wrapper functions.
>  
> And last patch, you can adjust cluster distribution to avoid false-sharing.
> And the description should include how it's bad in testing so it's worth.
> ""
>
> It makes review more easier, I believe.

Sorry, personally, I don't like this way to organize the patchset.  So
unless more people have this requirement, I still want to keep the
current way.

Best Regards,
Huang, Ying

>> 
>> >
>> >> Patch 4 eliminates unnecessary page allocation for read ahead.
>> >
>> > Could it be separated patch?
>> >
>> >> Patch 5-9 create a per cpu cache of the swap slots, so we don't have
>> >>         to contend on the swap device to get a swap slot or to release
>> >>         a swap slot.  And we allocate and release the swap slots
>> >>         in batches for better efficiency.
>> 
>> After patch 9,
>> 
>>   "vmstat.swap.so": 4170746,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.swapcache_free_entries.free_swap_slot.free_swap_and_cache.unmap_page_range": 13.91,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.get_page_from_freelist.__alloc_pages_nodemask.alloc_pages_vma.handle_mm_fault": 8.56,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.get_page_from_freelist.__alloc_pages_slowpath.__alloc_pages_nodemask.alloc_pages_vma":
>> 2.56,
>>   "perf-profile.calltrace.cycles-pp._raw_spin_lock.get_swap_pages.get_swap_page.add_to_swap.shrink_page_list": 2.47,
>> 
>> The swap out throughput is improved about 192% compared with the
>> baseline.  There are still some lock contention for
>> swap_info_struct->lock, but the pressure begins to shift to buddy system
>> now.
>> 
>> Best Regards,
>> Huang, Ying
>> 
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