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

[Andrew Morton]

On Wed, 20 May 2020 11:15:02 +0800 Huang Ying <ying.huang@xxxxxxxxx> 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?

> 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?

Does the ram disk actually set SWP_SOLIDSTATE?