readahead(8)

📦
version 0.36.1 · 2 versions
, in centos-stream 10, centos-stream 9

💅 readahead(8) — like, the manual page, but make it cute ✨

Okay bestie, so readahead is this icon in the System Manager's Manual and she is literally here to show you the performance of your read-ahead cache. No cap, she's serving main character energy for your kernel. 🎀

🌸 NAME 🌸

readahead — literally shows you the performance of the read-ahead cache, periodt.

✨ SYNOPSIS ✨

readahead [-d DURATION]

🦋 DESCRIPTION 🦋

So the way this works: readahead shows you how the read-ahead caching is doing on your system under a given load, so you can investigate any caching issues you're lowkey stressed about. It shows a count of unused pages in the cache and also prints a histogram showing how long they've been chilling there. Obsessed. 💖

This tool traces the __do_page_cache_readahead() kernel function to track entry and exit in the readahead mechanism in the kernel, and then uses __page_cache_alloc() and mark_page_accessed() functions to calculate the age of the page in the cache and see how many are left unaccessed. It's giving ✨ forensic vibes ✨.

🚨 Since this uses BPF, only the root user can use this tool. Sorry bestie, no root no slay. 🔥

💀 NOTE ON KPROBES USAGE 💀

Since the tool uses Kprobes, depending on your linux kernel's compilation, these functions may be inlined and hence not available for Kprobes. 😭 To see whether you have the functions available, check vmlinux source and binary to confirm whether inlining is happening or not. You can also check /proc/kallsyms on the host and verify if the target functions are present there before using this. Do your vibe check first, bestie, don't just run it blind. 🙈

💜 REQUIREMENTS 💜

  • CONFIG_BPF
  • bcc

That's it, that's the list. Iconic. 👑

🌈 OPTIONS 🌈

  • -h — prints the usage message, like a little friendly reminder. 🫶
  • -d DURATION — traces the read-ahead caching system for DURATION seconds. Time it like a reel. ⏱️

🍓 EXAMPLES 🍓

Trace for 30 seconds and show histogram of page age (ms) in read-ahead cache along with unused page count:

# readahead -d 30

Literally that simple. Ate. 💅

🔥 OVERHEAD 🔥

The kernel functions instrumented by this program could be high-frequency depending on the profile of the application (for example sequential IO). We advise the users to measure and monitor the overhead before leaving this turned on in production environments. 💀

Translation: measure first, bestie. Don't just leave it running in prod and hope for the best. That's not the move. 🚫

🧚 SOURCE 🧚

This originated as a bpftrace tool from the book "BPF Performance Tools", published by Addison Wesley (2019):

http://www.brendangregg.com/bpf-performance-tools-book.html

See the book for more documentation on this tool. It's giving required reading. 📚

This version is in the BCC repository:

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool. Go look, it's cute. 🌸

🌟 OS 🌟

Linux. Obviously. 🐧

⭐ STABILITY ⭐

Unstable — in development. She's a work in progress, and honestly? Same. 🫶

👑 AUTHOR 👑

Suchakra Sharma. Respect the icon who built her. 🙌

🎀 SEE ALSO 🎀

Go read those too, iykyk. 💖

And that's the whole page, bestie! 🌈 Shoutout to the BCC crew and the USER COMMANDS source, dated 2020-08-20 — mwah! 💋 No cap, you're now fully educated on readahead. Slay. ✨

📦 where this page comes from