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    SubjectRe: 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
    >

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