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

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Date:   Sat, 13 Jun 2020 00:45:33 +0200
From:   Pavel Machek <pavel@....cz>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...l.org>, mhocko@...e.cz,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: 5.7-rc0: kswapd eats cpu during a disk test?!

Hi!

> +CC linux-mm
> 
> On 5/31/20 12:34 PM, Pavel Machek wrote:
> > Hi!
> > 
> > This is simple cat /dev/sda > /dev/zero... on thinkpad x60 (x86-32),
> > with spinning rust.
> > 
> >   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+  COMMAND
> >    1000 root      20   0       0      0      0 R  53.3  0.0  57:34.93  kswapd0
> >   27897 root      20   0    6976    580    536 R  44.5  0.0   1:44.53  cat
> > 
> > It keeps both CPUs busy... and I don't think that's right.
> 
> Does an older kernel behave differently here?

Let me try on x220 (x86-64, first):

  737 root      20   0    5404    744    680 R  31.2   0.0   0:09.98 cat        
 1024 root      20   0       0      0      0 S  21.4   0.0 165:22.68 kswapd0    

That was with ssd, result with spinning rust is similar:

  859 root      20   0    5404    740    672 D  21.1   0.0   0:03.33 cat        
 1024 root      20   0       0      0      0 R  11.8   0.0 165:33.07 kswapd0    

5.7-rc1+ kernel.

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
  

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

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