Htop Explained

[for_xyz]

The problem with htop on Linux is that once there are 200+ processes running on the system htop takes significant portion of CPU time and utilization. This is because htop has to go through each process entry in procfs (open, read, close) every second and parse the text context instead of just calling appropriate syscall like on the OpenBSD and friends to retrieve such information.

It would help if kernel provided process information in binary form instead of serializing it into text. Or even better to provide specific syscalls for it like on macOS, Windows, OpenBSD, Solaris and others.

[herpderperator]

Significant in what way? I created 400 processes + 328 threads on a 10-year-old CPU and htop is not using more than 1.3% CPU on a machine with 800% available CPU power (quad-core, 8-thread)[0]. That means 0.16% total CPU used. While I agree that it is _less_ efficient than some other ways, in what way is that _significant_?

[0] https://i.imgur.com/onNSHQw.png

[mhd]

I remember way back in the early years of the Third Age, I had to write a process accounting system that supported all kinds of Unices (HP-UX, AIX, Solaris, FreeBSD, Linux ...). And you're right, there's a plethora of other options other than procfs, although IIRC, Linux wasn't the only one I had to support with that variant. Wasn't Solaris one of them?

[microcolonel]

I will say, htop is a lot more efficient than GNU top, despite its functionality. I do not use it in the (default?) mode where it lists all the threads, because that is nuts.

[dfox]

Traditional Unix implementation of ps and similar tools work by directly reading the appropriate data structures from kernel (through /dev/kmem or something to that effect). Modern BSD implementations have libkvm.a, that abstracts that somehow, but still directly reads stuff from kernel space.

I don't know, but I don't see doing random reads from kernel memory as particularly sane API to get list of processes and procfs is several orders of magnitude cleaner solution.

[dana321]

Its not meant to be used 24/7, its just used as a guide/diagnsostic to how the machine and its processes are performing! I use it for maybe about 10 seconds.

[seppel]

Well, you are right (and top has the same problem if not worse), but I'm always surprised what these tools are doing that they take significat CPU time. Parsing 200 virtual text files every second for sure should not.

> Or even better to provide specific syscalls for it like on macOS, Windows, OpenBSD, Solaris and others.

top manages to take 6% CPU on my Macbook.

[zantana]

I have wondered if it made sense to re-write these top style tools using ebpf https://lwn.net/Articles/740157/ but I think ebpf requires root.

[coleifer]

Lennart, we know it's you.

[shanecoin]

One of my favorite things about htop are some of the projects that have been created that are modeled after htop but focus on information other than system resources.

Cointop [0] is one of these projects that comes to mind.

[0] https://github.com/miguelmota/cointop

[the_pwner224]

intel_gpu_top helped me solve a mysterious performance issue on a MacBook after countless hours of fruitless investigation. Overheating and throttling was an issue but even after I fixed it the system would lag hard - instantly when I used the external 4k display, and after a while on the internal 1440p screen. Turns out cool-retro-term was maxing out the integrated Intel GPU which caused the entire system to stutter and lag.

Unfortunately both the MBP and my current XPS 15 are unable to drive cool-retro-term on a 4k display with the CPU integrated graphics, and they both overheat and throttle if I use the nvidia graphics card :/

It's a really cool terminal though: https://github.com/Swordfish90/cool-retro-term

[MasterScrat]

Most importantly, nvtop: https://github.com/Syllo/nvtop

"NVIDIA GPUs htop like monitoring tool"

[Pawamoy]

Shameless plug: aria2p. I built an interactive interface very similar to htop to see your aria2 downloads progress.

https://github.com/pawamoy/aria2p

[NelsonMinar]

htop is an excellent tool. I appreciate his valiant effort to explain what load average is; it's confused Unix users forever. His explanation is more or less right but I think misses a bit of context about the old days of Unix performance.

It used to be in the early 90s that Unix systems were quite frequently I/O bound; disks were slow and typically bottlenecked through one bus and lots of processes wanted disk access all the time. Swapping was also way more common. Load average is more or less a quick count of all processes that are either waiting on CPU or waiting on I/O. Either was likely in an old Unix system, so adding them all up was sensible.

In modern systems your workload is probably very specifically disk bound, or CPU bound, or network bound. It's much better to actually look at all three kinds of load separately. htop is a great tool for doing that! but the summary load average number is not so useful anymore.

[herpderperator]

It should be noted that you shouldn't really do

> $ strace uptime 2>&1 | grep open

Instead, you should do `strace -e open uptime` to select that system call. Like Useless Use of Cat[0], this could be considered a Useless Use of Grep.

[0] https://en.wikipedia.org/wiki/Cat_%28Unix%29#Useless_use_of_...

Edit: Heh, when I went back to continue reading the article this was mentioned on the following line. Oops :)

[jrockway]

These are two different commands. "strace | grep foo" looks for any line containing foo. It will find "foobar" and "food" system calls. It will find the word "foo" in the (abridged) string data that it prints out (read(..., "foo...", ...)).

Meanwhile, strace's -e filter will find exact matches of syscalls that are named on the command line.

Obviously the author wants the second one, but it is hardly useless to grep. And, once you know how to use grep, you know how to do this sort of search for every program ever written. It is nice that strace has its own budget string matching built in... but knowing how strace works only tells you how strace works. Knowing how grep works lets you do this sort of thing to any program that outputs text.

(A lot of work has been done to try and make something as good as "strace -e" but generically; for example, Powershell's Select-Object cmdlet. I have never managed to get that to do anything useful, but they did manage to break grep, erm ... Select-String, in the process. Great work!)

[ddevault]

I disagree in this case, I think it's more Unix-style to use grep when appropriate. You should learn a lot of generally applicable tools, not the intricate details of a few specialized tools. The "useless cat" case can be represented as, instead of grep foo filename, grep foo < filename.

[CUViper]

You might need to cover openat too. I think newer glibc uses the openat(AT_FDCWD,...) syscall for all open calls.

[jimmaswell]

If the cat or grep makes the command more clear or easier to write then it's not useless.

[AdrienLemaire]

I tried both, but the `-e open` option doesn't return anything

  $ strace uptime 2>&1 | grep open
  openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/proc/uptime", O_RDONLY) = 3
  ...

  $ strace -e open uptime         
  10:12:14 up 2 days, 18:52,  1 user,  load average: 1.22, 1.30, 1.71
  +++ exited with 0 +++
  $

strace -- version 5.3 Optional features enabled: stack-trace=libunwind stack-demangle m32-mpers mx32-mpers

After digging in the documentation, I finally found something working:

  $ strace -e trace=openat uptime

EDIT: got it. my system calls are openat, not open. Hence the following works

  $ strace -e openat uptime

[anderspitman]

This article is about htop but explains tons of useful Linux commands and idiosyncrasies along the way.

[ehsankia]

Yeah, honestly htop is just used as an excuse to talk about much much more. And I like how the author works their thought process along the way.

[brown9-2]

This is such a great article - I’ve worked with linux for more than a decade and never really understood what “setuid root” actually meant or that “kill” is a builtin to Bash

[redsparrow]

> I decided to look everything up and document it here.

You can be a hero, too! I find this inspiring. It's nice to see such an accessible and pragmatic way of making a contribution to the community. My very first thought on seeing that was "I could do that!"

Thank you.

[aljarry]

I regret neither Top neither Htop show you estimates of "IO Activity time" like in Windows 10 task manager - I need to use separate iostat to observe that.

I found the "IO Activity time", percentage of time when IO is used, to be a really good indicator of IO load on machine level - neither io-op per second, neither bandwith tell much if you're already using up all available IO. Load does not help here, as number of processes doing IO influences "load" more.

[swiley]

To be fair it’s a far worse problem on Windows (although I guess a lot of my Linux machines are running from ram now so maybe I wouldn’t notice if it weren’t.)

[Avamander]

You also need something like `nethogs` to view network IO, I wish there were a colored, htop-like utility for that as well.

[trasz]

In FreeBSD top(1) does show the IO statistics - just press “m”.

[microcolonel]

On Linux, that information is privileged, IIRC. so since most people use htop as an ordinary user, they wouldn't see that anyway.

[ajryan]

Where exactly can I find this in Windows' Task Manager? I don't see it as a column on the Details pane...

[zymhan]

I'm a fan of iotop for monitoring various IO stats in a top-like interface.

[lma21]

pidstat is also good at showing IO Activity, per process.

[jszymborski]

The one thing I don't get about htop is the progress bars... they never seem to behave the way I'd expect them to based on the percentages, and they've got some colour coding I'm not clear on either... surely there is something I'm missing.

[boomskats]

The bit you’re missing is actually Explained in the link. The colours in the bars refer to threads and their priority.

[catalogia]

Wait, which progress bars? Is it using SIGINFO or something?

[samfriedman]

>There will be a lot of output. We can grep for the open system call. But that will not really work since strace outputs everything to the standard error (stderr) stream. We can redirect the stderr to the standard output (stdout) stream with 2>&1.

Besides being a great explanation of htop, I like the way this article captures the way I - far from a shell guru - tend to think when putting together a few steps in the terminal. And even then it shows that it pays to read the man page too!

[zaphar]

I am convinced that load average on a machine is one of the most misleading statistics you can view. It never means what you think it means and half the time it doesn't even indicate a real problem when it's high.

[vesinisa]

> One process runs for a bit of time, then it is suspended while the other processes waiting to run take turns running for a while. The time slice is usually a few milliseconds so you don't really notice it that much when your system is not under high load. (It'd be really interesting to find out how long time slices usually are in Linux.)

Isn't this the famous kernel HZ? It was originally 100 (interrupts/second), but nowadays often 250 or 1000:

http://man7.org/linux/man-pages/man7/time.7.html

[bri3d]

It’s much more complex than that these days. With the CFS scheduler a process will run for somewhere between the target latency (basically the size of slice that N processes waiting to be scheduled are competing for, I think defaulted to 20ms) as the upper bound and the minimum granularity (the smallest slice that may be granted to a process being scheduled) as the lower bound, I think defaulted to 4ms.

This is made more complex by the ability to configure the scheduler with high granularity, including the ability to schedule different processors and process groups with different schedulers (and the rules that govern how the schedulers can then preempt each other).

[jmercha]

Fascinating! I never knew I needed a htop t-shirt until I read this article!

[jasoneckert]

Excellent htop tutorial - I'll be sharing the link with my students!

[carlchenet]

Awesome article, this HN thread should indicate it was originally published on HN in 2016. As mentioned on the front page "#1 on Hacker News on December 2, 2016"

[helb]

here: https://news.ycombinator.com/item?id=13087904

[zepearl]

Nice explanations.

Btw. additional recommendation (especially when a lot of CPUs and/or disks are involved + you want to keep an eye on multiple things at once): nmon

[pmoriarty]

Is there any way to make it so that htop doesn't clear the screen when it exits?

Seeing the last page of htop's output after it exits is usually useful to me.

[Operyl]

You could suspend the terminal with Ctrl-S and unsuspend with Ctrl-Q?

[rajandatta]

Excellent article and very well done in-depth work.

[kccqzy]

This is a great explanation of everything in the output of htop and related, but I suggest the author clean up the prose a little bit to make it a bit less conversational and easier to read.

[Teknoman117]

another of my favorites is glances.

[ngcc_hk]

Just to say great. Thanks thanks