Re: [PATCH v2 3/5] mm: use a dedicated workqueue for the free workers

[Aaron Lu]

On Thu, Mar 23, 2017 at 08:38:43AM -0700, Dave Hansen wrote:
> On 03/22/2017 01:41 AM, Aaron Lu wrote:
> > On Wed, Mar 22, 2017 at 03:33:35PM +0900, Minchan Kim wrote:
> >> On Wed, Mar 15, 2017 at 05:00:02PM +0800, Aaron Lu wrote:
> >>> Introduce a workqueue for all the free workers so that user can fine
> >>> tune how many workers can be active through sysfs interface: max_active.
> >>> More workers will normally lead to better performance, but too many can
> >>> cause severe lock contention.
> >>
> >> Let me ask a question.
> >>
> >> How well can workqueue distribute the jobs in multiple CPU?
> > 
> > I would say it's good enough for my needs.
> > After all, it doesn't need many kworkers to achieve the 50% time
> > decrease: 2-4 kworkers for EP and 4-8 kworkers for EX are enough from
> > previous attched data.
> 
> It's also worth noting that we'd like to *also* like to look into
> increasing how scalable freeing pages to a given zone is.

Still on EX, I restricted the allocation to be only on node 1, with
120G memory allocated there:

max_active            time            compared to base  lock from perf
base(no parallel)     3.81s Â3.3%     N/A               <1%
1                     3.10s Â7.7%     â18.6%            14.76%
2                     2.44s Â13.6%    â35.9%            36.95%
4                     2.07s Â13.6%    â45.6%            59.67%
8                     1.98s Â0.4%     â48.0%            62.59%
16                    2.01s Â2.4%     â47.2%            79.62%

If we can improve the scalibility of freeing a given zone, then parallel
free will be able to achieve more.

BTW, the lock is basically pgdat->lru_lock in release_pages and
zone->lock in free_pcppages_bulk:
    62.59%    62.59%  [kernel.kallsyms]  [k] native_queued_spin_lock_slowpath
37.17% native_queued_spin_lock_slowpath;_raw_spin_lock_irqsave;free_pcppages_bulk;free_hot_cold_page;free_hot_cold_page_list;release_pages;free_pages_and_swap_cache;tlb_flush_mmu_free_batches;batch_free_work;process_one_work;worker_thread;kthread;ret_from_fork
25.27% native_queued_spin_lock_slowpath;_raw_spin_lock_irqsave;release_pages;free_pages_and_swap_cache;tlb_flush_mmu_free_batches;batch_free_work;process_one_work;worker_thread;kthread;ret_from_fork