Re: [PATCH -V2] swap: Reduce lock contention on swap cache from swap slots allocation

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

"Huang, Ying" <ying.huang@intel.com> writes:

> Andrew Morton <akpm@linux-foundation.org> writes:
>
>> On Wed, 20 May 2020 11:15:02 +0800 Huang Ying <ying.huang@intel.com> wrote:
>>
>>> In some swap scalability test, it is found that there are heavy lock
>>> contention on swap cache even if we have split one swap cache radix
>>> tree per swap device to one swap cache radix tree every 64 MB trunk in
>>> commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB trunks").
>>> 
>>> The reason is as follow.  After the swap device becomes fragmented so
>>> that there's no free swap cluster, the swap device will be scanned
>>> linearly to find the free swap slots.  swap_info_struct->cluster_next
>>> is the next scanning base that is shared by all CPUs.  So nearby free
>>> swap slots will be allocated for different CPUs.  The probability for
>>> multiple CPUs to operate on the same 64 MB trunk is high.  This causes
>>> the lock contention on the swap cache.
>>> 
>>> To solve the issue, in this patch, for SSD swap device, a percpu
>>> version next scanning base (cluster_next_cpu) is added.  Every CPU
>>> will use its own per-cpu next scanning base.  And after finishing
>>> scanning a 64MB trunk, the per-cpu scanning base will be changed to
>>> the beginning of another randomly selected 64MB trunk.  In this way,
>>> the probability for multiple CPUs to operate on the same 64 MB trunk
>>> is reduced greatly.  Thus the lock contention is reduced too.  For
>>> HDD, because sequential access is more important for IO performance,
>>> the original shared next scanning base is used.
>>> 
>>> To test the patch, we have run 16-process pmbench memory benchmark on
>>> a 2-socket server machine with 48 cores.  One ram disk is configured
>>
>> What does "ram disk" mean here?  Which drivers(s) are in use and backed
>> by what sort of memory?
>
> We use the following kernel command line
>
> memmap=48G!6G memmap=48G!68G
>
> to create 2 DRAM based /dev/pmem disks (48GB each).  Then we use these
> ram disks as swap devices.
>
>>> as the swap device per socket.  The pmbench working-set size is much
>>> larger than the available memory so that swapping is triggered.  The
>>> memory read/write ratio is 80/20 and the accessing pattern is random.
>>> In the original implementation, the lock contention on the swap cache
>>> is heavy.  The perf profiling data of the lock contention code path is
>>> as following,
>>> 
>>> _raw_spin_lock_irq.add_to_swap_cache.add_to_swap.shrink_page_list:      7.91
>>> _raw_spin_lock_irqsave.__remove_mapping.shrink_page_list:               7.11
>>> _raw_spin_lock.swapcache_free_entries.free_swap_slot.__swap_entry_free: 2.51
>>> _raw_spin_lock_irqsave.swap_cgroup_record.mem_cgroup_uncharge_swap:     1.66
>>> _raw_spin_lock_irq.shrink_inactive_list.shrink_lruvec.shrink_node:      1.29
>>> _raw_spin_lock.free_pcppages_bulk.drain_pages_zone.drain_pages:         1.03
>>> _raw_spin_lock_irq.shrink_active_list.shrink_lruvec.shrink_node:        0.93
>>> 
>>> After applying this patch, it becomes,
>>> 
>>> _raw_spin_lock.swapcache_free_entries.free_swap_slot.__swap_entry_free: 3.58
>>> _raw_spin_lock_irq.shrink_inactive_list.shrink_lruvec.shrink_node:      2.3
>>> _raw_spin_lock_irqsave.swap_cgroup_record.mem_cgroup_uncharge_swap:     2.26
>>> _raw_spin_lock_irq.shrink_active_list.shrink_lruvec.shrink_node:        1.8
>>> _raw_spin_lock.free_pcppages_bulk.drain_pages_zone.drain_pages:         1.19
>>> 
>>> The lock contention on the swap cache is almost eliminated.
>>> 
>>> And the pmbench score increases 18.5%.  The swapin throughput
>>> increases 18.7% from 2.96 GB/s to 3.51 GB/s.  While the swapout
>>> throughput increases 18.5% from 2.99 GB/s to 3.54 GB/s.
>>
>> If this was backed by plain old RAM, can we assume that the performance
>> improvement on SSD swap is still good?
>
> We need really fast disk to show the benefit.  I have tried this on 2
> Intel P3600 NVMe disks.  The performance improvement is only about 1%.
> The improvement should be better on the faster disks, such as Intel
> Optane disk.  I will try to find some to test.

I finally find 2 Intel Optane disks to test.  The pmbench throughput
(page accesses per second) increases ~1.7% with the patch.  The swapin
throughput increases 2% (~1.36 GB/s to ~1.39 GB/s), the swapout
throughput increases 1.7% (~1.61 GB/s to 1.63 GB/s).  Perf profile shows
the CPU cycles on the swap cache radix tree spinlock is reduced from
~1.76% to nearly 0.  So the performance difference is much smaller, but
still measurable.

Best Regards,
Huang, Ying