Re: 5.7-rc0: kswapd eats cpu during a disk test?!

[Pavel Machek <pavel@ucw.cz>]

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Hi!

> > > Performance of spinning rust is down, too, on x60:
> > >
> > > pavel@amd:~/misc/hw/hdd1t$ sudo ddrescue --force /dev/sda1 /dev/null
> > > GNU ddrescue 1.19
> > > Press Ctrl-C to interrupt
> > > rescued:     2147 MB,  errsize:       0 B,  current rate:    3080 kB/s
> > >    ipos:     2147 MB,   errors:       0,    average rate:    5382 kB/s
> > >       opos:     2147 MB, run time:    6.65 m,  successful read:
> > >       0 s ago
> > >       Finished
> > > pavel@amd:~/misc/hw/hdd1t$ uname -a
> > > Linux amd 5.7.0-next-20200611+ #123 SMP PREEMPT Thu Jun 11
> > >  15:41:22 CEST 2020 i686 GNU/Linux
> > >
> > > And there's something clearly wrong here:
> > >
> > >     966 root      20   0       0      0      0 R  94.4  0.0   8:18.82   kswapd0
> > >   23933 root      20   0    4612   1112   1028 D  80.6  0.0   0:26.40   ddrescue
> > >  
> > 
> > Same x60 under older kernel:
> > 
> > pavel@amd:/data/fast/pavel$ sudo ddrescue --force /dev/sda4 /dev/null
> > GNU ddrescue 1.19
> > Press Ctrl-C to interrupt
> > rescued:     6593 MB,  errsize:       0 B,  current rate:   60424 kB/s
> >    ipos:     6593 MB,   errors:       0,    average rate:   95563 kB/s
> > 
> >   3539 root      20   0    4616   1136   1048 D  21.4  0.0   0:15.63 ddrescue
> >    865 root      20   0       0      0      0 S   6.9  0.0   0:04.91  kswapd0
> > 
> > Linux amd 4.6.0+ #172 SMP Sun Aug 14 11:25:34 CEST 2016 i686 GNU/Linux
> > 
> > These are more reasonable numbers.
> 
> Treat referenced & active pages as reclaim cost.

That helps a bit, but we are still nowehere near v4.6 performance:

pavel@amd:/data/fast/pavel$ sudo ddrescue --force /dev/sda2 /dev/null
GNU ddrescue 1.19
Press Ctrl-C to interrupt
rescued:     8559 MB,  errsize:       0 B,  current rate:   53084 kB/s
   ipos:     8559 MB,   errors:       0,    average rate:   71327 kB/s

 3933 root      20   0    4612   1216   1128 R  68.6  0.0   0:12.14  ddrescue
   967 root      20   0       0      0      0 R  64.4  0.0   1:18.67  kswapd0
 
(I applied the patch on 5.7.0-next-20200611+).

Do we know which original change is causing this? Any other ideas?

Thanks,
									Pavel

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2020,6 +2020,7 @@ static void shrink_active_list(unsigned
>  	struct page *page;
>  	unsigned nr_deactivate, nr_activate;
>  	unsigned nr_rotated = 0;
> +	unsigned nr_refered = 0;
>  	int file = is_file_lru(lru);
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>  
> @@ -2070,7 +2071,8 @@ static void shrink_active_list(unsigned
>  				nr_rotated += hpage_nr_pages(page);
>  				list_add(&page->lru, &l_active);
>  				continue;
> -			}
> +			} else if (!file)
> +				nr_refered++;
>  		}
>  
>  		ClearPageActive(page);	/* we are de-activating */
> @@ -2098,6 +2100,14 @@ static void shrink_active_list(unsigned
>  	free_unref_page_list(&l_active);
>  	trace_mm_vmscan_lru_shrink_active(pgdat->node_id, nr_taken, nr_activate,
>  			nr_deactivate, nr_rotated, sc->priority, file);
> +	if (file)
> +		sc->file_cost += nr_rotated;
> +	else
> +		/*
> +		 * add cost to avoid swapin in the near future which incurs IO
> +		 * on top of reclaim
> +		 */
> +		sc->anon_cost += nr_refered;
>  }
>  
>  unsigned long reclaim_pages(struct list_head *page_list)
> @@ -2311,11 +2321,13 @@ static void get_scan_count(struct lruvec
>  	file_cost = total_cost + sc->file_cost;
>  	total_cost = anon_cost + file_cost;
>  
> -	ap = swappiness * (total_cost + 1);
> -	ap /= anon_cost + 1;
> -
> -	fp = (200 - swappiness) * (total_cost + 1);
> -	fp /= file_cost + 1;
> +	ap = swappiness * total_cost;
> +	if (anon_cost)
> +		ap /= anon_cost;
> +
> +	fp = (200 - swappiness) * total_cost;
> +	if (file_cost)
> +		fp /= file_cost;
>  
>  	fraction[0] = ap;
>  	fraction[1] = fp;

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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