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

Pavel Machek
2020-06-13T06:48:48Z

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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;