6 ms·
Why does it need a kernel module ?
by gens 9y ago
Why does it need a kernel module ?
- m82labs 9y agoI was wondering the same. No way I would load a module just to monitor CPU.
- floatboth 9y agoFreeBSD already comes with cpuctl.ko which is used by Intel's pcm.x tools. No way Linux doesn't have something similar…
- mmrezaie 9y agoLinux has MSR module or one can use perf.
- gens 9y agoMaybe for the real-time cache usage display ? Or performance (in the sense of less noise while measuring) ? I'm tempted to try it out and see for myself, though. For posterity; Cpu counters for a lot of things can be gotten from userspace. The linux kernel exposes a lot of things in /proc (procfs) and /sys (sysfs). Programs like perf, slabtop (and probably others, that i can't remember now), use these informations to display more-or-less the same data as this program.
- dom0 9y agoThat's literally the second thing in the README after the fancy GIF.
- gens 9y agoYes, i read it. Most of the things listed there can be gotten from userspace (through cpu instructions/registers, procfs, sysfs). Do you know which one of those things can not be gotten in userspace ? Turbo boost state ? Topology ?
- CalChris 9y agoPCM is similar and is userspace. However on OSX, you need a driver and then various other hoops must be jumped in order to enable it. On Linux, I think it just runs out of the box. But in any case, PCM itself is userspace. https://github.com/opcm/pcm https://github.com/opcm/pcm
- gens 9y agoThere are generic tools for these stuff on linux like perf, powertop, slabtop, lm-sensors, and probably more that idk about. This PCM looks like it provides information that the tools above can't, like pcm and memory bandwidths (unless i misunderstood what it does). Sucks that it's intel only, i'd like to watch memory bandwidth usage on my amd. Unrelated: this reminds me of mcelog, that collects hardware errors.
- wmf 9y agoThe author is probably concerned about the overhead of using N system calls to read N MSRs. It would be cool if eBPF could read MSRs; then this kind of monitoring could be done though perf tracing.
- mmrezaie 9y agoWell using perf or reading MSR doesn't need you writing your kernel module. I think the overhead of using perf or direct access to MSR are kinda similar.
- brendangregg 9y agoRDMSR is an instruction, not a syscall. It's not clear to me either why this needs a kernel module. It's similar to i7z, which from memory does not use it's own kernel module. eBPF can read the relevant PMCs as of Linux 4.9. eg, the llcstat tool in bcc does this. (It was something I pushed for, and Alexei delivered)
- wmf 9y agoYou already know this, but for completeness you can't RDMSR from userspace, so to read MSRs you can use the msr kernel module which requires 2 system calls per MSR (which is what i7z does) or you can write your own kernel module which requires fewer system calls. Clearly a lot of people in this thread disagree with the author's decision. Can eBPF read non-PMC registers like IA32_PERF_STATUS? I'm not getting much by googling. Thanks for all your work on this topic; it's really educational.
- krylon 9y agoMy thought exactly. It looked really nice, but I do not need that data bad enough to load a third-party kernel module. Or mess with my grub configuration.