Yay, a comment!

>I find it hard to believe that there is minimal overhead from the instrumentation, as the README claims. Surely the additional traps alone introduce an overhead, and tracking the elapsed time of each operation even more so. It would be interesting to know what the actual overhead is.

note that timep does a 2 pass approach - it runs the code with instrumentation just logging the data it needs, and then after the code finishes it goes back and uses that data to generate the profile and flamegraphs. For "overhead" im just talking abut in the initial profiling run...total time to getting results is a bit longer (but still pretty damn fast).

if you run timep with the `-t` flag it will run profiling run of the code (with the trap-based instrumentation enabled and recording) inside of a `time { ... }` block. You can then compare that timed to the running the code without using timep, giving you overhead. I used that method while running a highly parallelized test that, between 28 persistent workers, ran around 67000 individual bash commands (and computed 17.6 million checksums of a bunch of small files in a ramdisk) in 34.5 or so seconds (without timep). using `timep -t` to run the code this increased to 38 seconds. So +10%. And thats really a worst case scenario...it indicates the per command overhead is around 1 ms. what percent if the total run time that translates to depends on the average time-per-command-run for the code you are profiling.

side note: in this case, the total time to generate a profile was ~2.5 minutes and the total time to generate a profile and flamegraphs was ~5 minutes

timep manages to keep it so low because the debug trap instrumentation is 100% bash builtins and doesnt spawn and subshells or forks - so you just have a string of builtin commands with no context switching nor any "copying the environment to fork something" to slow you down.

> The amount of patience required to instrument Bash scripts must've been monumental.

It took literally months to get everything working correctly and all the edge cases worked out properly. Bash does some borderline bizarre things behind the scenes.

> EDIT: I took a look at `timep.bash`, and I may have nightmares tonight... Sweet, fancy Moses.

Its a little bit...involved.

> Also, I really don't like that it downloads and loads some random .so files

So by default it doesnt do this. It has the ability to, but you have to to explicitly tell it to....it wont do it automatically.

By default, it will get the .so file using the base64 blob (in ascii string representation and compressed) that is built into timep.bash. This has sha256 and md5 checksums incorporated into it that get checked when the .so file is re-created using that base64 blob (to ensure no corruption).

the .so file is needed to add a bash loadable builtin that outputs microsecond granularity CPU usage time. Without this it will try and use /proc/stat, which works but the measurement is 10000x more coarse (typically it displays cpu time in number of 10 ms intervals). to get microsecond accuracy you need the .so file.

> Or better yet: make their source visible as well, and have users compile them on their own.

the source is available in the repo at https://github.com/jkool702/timep/blob/main/LIB/LOADABLES/SR...

compile instructions are at the top in commented out lines.

the code is pretty straightforward - it uses getrusage and/or (if available) clock_gettime to get cpu usage for itself and its children.

the `_timep_SETUP` function has the logic included (but not used by default - you have to manually call it with a flag) to let allow you to use a .so file that is in your current directory instead of the one generated from the builtin base64 blob. So, you are able, should you wish, to compile and have timep use your own .so for the loadable.

However, realistically, most people who might be interested in using timep wont do that. So it defaults to fully automating this and having it all self contained in a single script file, while allowing for advanced users to manually override it. I thought that was the best overall way to do it.

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imiric 4 days ago | parent [-]

Hey, apologies for the late response.

> using `timep -t` to run the code this increased to 38 seconds. So +10%.

Thanks. I suppose this will depend on each script, as there is another commenter here claiming that the overhead is much higher.

Re: the binary, that's fine. Your approach is surely easier to use than asking users to compile it themselves, but I would still prefer to have that option. After all, how do I know that that binary came from that source code? So a simple Make target to build it would put my concerns to rest. It's something that only has to be done once, anyway, so it's not a huge inconvenience.

In any case, it's pretty cool that you wrote the CPU time tracking in C. I wasn't even aware that Bash was so easily extensible.

You've clearly put a lot of thought and effort into this project, which is commendable. Good luck!

	▲	jkool702 4 days ago \| parent [-]
		> Thanks. I suppose this will depend on each script, as there is another commenter here claiming that the overhead is much higher. The better way to think about overhead with timep is "average overhead per command run" (or more specifically per debug trap fire). this value wont change all that much between profiling different bash scripts The code that commenter was profiling was a rubix cube solver that was impressively well optimized: 100% builtins, no forking, all the expensive operations were pre-computed and saved in huge lookup tables (some of which had over 400,000 elements), vars passed by reference to avoid making copies, etc. The overhead from timep was about 230 microseconds per command, but that code was averaging a microsecond or two per command. To put it in perspective, bash's overhead any time it calls an external binary is 1-2 ms. so in a script that did nothing but call `/bin/true` repeatedly timep's overhead would probably be a little under 20%. > Re: the binary, that's fine. Your approach is surely easier to use than asking users to compile it themselves, but I would still prefer to have that option. I mean technically you can, but i'll give you that its not really documented unless you read through all the comments in the code. A makefile is probably doable and would make the process more straightforward. that said, its on my to do list to figure out how i can setup a github actions workflow to have github automatically build the .so files for all the different architectures whenever timep.c changes. Perhaps that would alleviate your concern. > After all, how do I know that that binary came from that source code? I mean, you can say that about virtually any compiled binary. sure , some of them (like from your distro's official repos) have been "signed off" on by someone you trust, but that is a leap of faith you have to make with anything you install from a 3rd party. And, in general, i feel like "compiling it yourself" doesnt really make it safer unless you personally (or someone you trust personally) look through the source to check that it doesnt do anything malicious. > I wasn't even aware that Bash was so easily extensible. bash supporting loadable builtins isnt a well known feature. Its really quite handy when you want to do something (e.g., access to a syscall) that bash doesnt support and you cant / dont want to used a external tool for it. IMO, the biggest hurdle behind using them is that they make scripts much less portable - you either need to setup a distribution system for it (and require the target system has internet access, at least briefly) and/or require the target system has a full build environment and can compile it. which are both sorta crappy options. unless, of course, you were to base64 encode the binary and directly include it inside of the script. ;)