I have run a quick `7z b` benchmark on the lowlatency kernel with and
without `nohz_full` parameter, and the results are fairly close:
No `nohz_full` parameter:
Compressing | Decompressing
Dict Speed Usage R/U Rating | Speed Usage R/U Rating
KiB/s % MIPS MIPS | KiB/s % MIPS MIPS
22: 82694 1333 6033 80445 | 741767 1584 3994 63265
23: 81022 1400 5896 82552 | 736593 1589 4011 63731
24: 79427 1429 5978 85401 | 722675 1581 4011 63433
25: 77665 1459 6077 88676 | 711778 1587 3990 63346
---------------------------------- | ------------------------------
Avr: 1405 5996 84269 | 1585 4002 63444
Tot: 1495 4999 73856
With `nohz_full=1-15` parameter:
Compressing | Decompressing
Dict Speed Usage R/U Rating | Speed Usage R/U Rating
KiB/s % MIPS MIPS | KiB/s % MIPS MIPS
22: 84475 1357 6055 82177 | 738578 1552 4060 62993
23: 80361 1376 5951 81878 | 726439 1482 4240 62852
24: 80275 1437 6006 86312 | 715778 1496 4199 62827
25: 77007 1448 6073 87924 | 708632 1563 4034 63066
---------------------------------- | ------------------------------
Avr: 1404 6021 84573 | 1523 4133 62935
Tot: 1464 5077 73754
In the latter case, decompressing is slightly slower, but definitely not
"800MHz" slower, so it looks like the problem is indeed with frequency
reporting rather than scaling.
** Summary changed:
- Specifying nohz_full disables CPU frequency scaling
+ Specifying nohz_full breaks CPU frequency reporting
** Description changed:
With the lowlatency kernel, if I specify "nohz_full=1-15" boot parameter
- then CPU frequency scaling doesn't work for the logical cores 1-15. That
- is, only logical core 0 shows varying CPU frequency in its
+ then CPU frequency reporting doesn't work for the logical cores 1-15.
+ That is, only logical core 0 shows varying CPU frequency in its
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq file, all other
cores constantly show 800000 in their scaling_cur_freq files (which is
the lowest supported frequency) regardless of the CPU load.
Steps to reproduce:
1. Add "nohz_full=1-15" (specify the core numbers to include all logical
cores except 0) to kernel boot options in /etc/default/grub.
2. Run `sudo update-grub` and reboot.
3. Upon booting, run a multithreaded workload. For example, run `openssl
speed -multi $(nproc --all)`.
4. In another console, monitor CPU frequencies by running `watch cat
/sys/devices/system/cpu/cpu[0-9]*/cpufreq/scaling_cur_freq`.
Actual results:
- All cores specified in "nohz_full" parameter are always at their lowest
+ All cores specified in "nohz_full" parameter always report their lowest
frequency.
+
+ Despite that, the actual performance seems to be as if frequency scaling
+ actually works (i.e. according to benchmarks, the performance seems to
+ be similar with and without the "nohz_full" parameter).
Expected results:
- All cores must scale the frequency up with active load.
+ All cores must report their actual frequency depending on the load.
ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: linux-image-6.5.0-15-lowlatency 6.5.0-15.15.1.1~22.04.1
ProcVersionSignature: Ubuntu 6.5.0-15.15.1.1~22.04.1-lowlatency 6.5.3
Uname: Linux 6.5.0-15-lowlatency x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Tue Jan 30 23:39:51 2024
InstallationDate: Installed on 2015-05-01 (3196 days ago)
InstallationMedia: Kubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
SourcePackage: linux-signed-lowlatency-hwe-6.5
UpgradeStatus: Upgraded to jammy on 2022-05-14 (626 days ago)
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You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux-signed-lowlatency-hwe-6.5 in
Ubuntu.
https://bugs.launchpad.net/bugs/2051733
Title:
Specifying nohz_full breaks CPU frequency reporting
Status in linux-signed-lowlatency-hwe-6.5 package in Ubuntu:
Confirmed
Bug description:
With the lowlatency kernel, if I specify "nohz_full=1-15" boot
parameter then CPU frequency reporting doesn't work for the logical
cores 1-15. That is, only logical core 0 shows varying CPU frequency
in its /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq file, all
other cores constantly show 800000 in their scaling_cur_freq files
(which is the lowest supported frequency) regardless of the CPU load.
Steps to reproduce:
1. Add "nohz_full=1-15" (specify the core numbers to include all logical
cores except 0) to kernel boot options in /etc/default/grub.
2. Run `sudo update-grub` and reboot.
3. Upon booting, run a multithreaded workload. For example, run `openssl
speed -multi $(nproc --all)`.
4. In another console, monitor CPU frequencies by running `watch cat
/sys/devices/system/cpu/cpu[0-9]*/cpufreq/scaling_cur_freq`.
Actual results:
All cores specified in "nohz_full" parameter always report their
lowest frequency.
Despite that, the actual performance seems to be as if frequency
scaling actually works (i.e. according to benchmarks, the performance
seems to be similar with and without the "nohz_full" parameter).
Expected results:
All cores must report their actual frequency depending on the load.
ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: linux-image-6.5.0-15-lowlatency 6.5.0-15.15.1.1~22.04.1
ProcVersionSignature: Ubuntu 6.5.0-15.15.1.1~22.04.1-lowlatency 6.5.3
Uname: Linux 6.5.0-15-lowlatency x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Tue Jan 30 23:39:51 2024
InstallationDate: Installed on 2015-05-01 (3196 days ago)
InstallationMedia: Kubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
SourcePackage: linux-signed-lowlatency-hwe-6.5
UpgradeStatus: Upgraded to jammy on 2022-05-14 (626 days ago)
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux-signed-lowlatency-hwe-6.5/+bug/2051733/+subscriptions
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