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

Pavel Machek
2020-06-12T23:05:58Z

[-- Attachment #1: Type: text/plain, Size: 2585 bytes --]

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
>

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.

Pavel