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[solved] Performance degradation with kernel 5.8
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VoVaN
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PostPosted: Sat Aug 08, 2020 2:53 pm    Post subject: [solved] Performance degradation with kernel 5.8 Reply with quote

Hello,

After updating kernel from 5.7.12 to 5.8.0-r1 I noticed performance degradation on my desktop:
- kernel compilation time ~26 min instead of ~7
- hdparm -Tt for 5.7
Code:
/dev/sda2:
 Timing cached reads:   25448 MB in  1.99 seconds = 12797.63 MB/sec
 Timing buffered disk reads: 1418 MB in  3.00 seconds = 472.19 MB/sec

- hdparm -Tt for 5.8
Code:
/dev/sda2:
 Timing cached reads:   6686 MB in  1.99 seconds = 3362.73 MB/sec
 Timing buffered disk reads: 1422 MB in  3.00 seconds = 473.53 MB/sec

I'd appreciate any thoughts and thank you in advance!

EDIT: Finally understood why ;-) I've default governor settled to "powersave" and CPU frequency scaling worked perfectly fine. With the new kernels >=5.8 there is more choices for governors, while for kernels <5.8 there was only two powersave and performance. So the new kernel changed behavior of this "powersave" governor, so the CPU frequency is always set to the lowest possible value. The new "ondemand" governor has the same behavior as "powersave" for older kernels. End of the story.


Last edited by VoVaN on Thu Aug 20, 2020 5:02 pm; edited 1 time in total
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Tony0945
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PostPosted: Thu Aug 20, 2020 4:59 pm    Post subject: Reply with quote

Please add [solved] to the title. Just edit the post with the edit button,

And thanks for the info. I'm still on 5.4.48
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Banana
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PostPosted: Thu Aug 20, 2020 7:00 pm    Post subject: Reply with quote

gonna check my settings since I just upgraded to it...
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CaptainBlood
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PostPosted: Thu Aug 20, 2020 7:36 pm    Post subject: Reply with quote

@VoVan
No such an issue on 5.8.2/powersave @building kernel.
GKrellM installed here though.

No idea what's going wrong.

Surprising assumption onpowersave [<5.8.x] = ondemand [5.8.x], any pointer for explanation?

Thks 4 ur attention, interest & support.


Last edited by CaptainBlood on Thu Aug 20, 2020 10:37 pm; edited 2 times in total
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Anon-E-moose
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PostPosted: Thu Aug 20, 2020 7:53 pm    Post subject: Reply with quote

I've used the ondemand governor both in the 5.5 series as well as the 5.8 and haven't noticed any difference in compilation/ebuild times. Actually seems like 5.8 is snappier than the 5.5 series, as well as the system running a degree or two cooler (maybe ryzen improvements)
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swimmer
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PostPosted: Thu Aug 20, 2020 10:49 pm    Post subject: Reply with quote

No problem with the governor so far ...

But there's no possibility to choose any Processor family other than 'Generic-x86-64' ... despite the 'experimental' use-flag :-/
Checking bugs.gentoo.org I saw that it's already reported and a solution should also be implemented but not on my system (5.8.1).
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