readahead(8)

✨ readahead(8) — System Manager's Manual ✨

Okay bestie, let's talk about readahead(8) 💅🎀 This is the manual page for the readahead tool, and it's giving "show me the cache receipts" energy 🧾💖

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🌸 NAME 🌸

readahead — Show performance of read-ahead cache ✨

🦋 SYNOPSIS 🦋

readahead [-d DURATION]

💜 DESCRIPTION 💜

So this tool shows you how well read-ahead caching is doing on your system under a given load 📊 It's the go-to when something about caching feels sus 🕵️‍♀️ It counts how many pages are sitting in the cache unused, and it also prints a histogram of how long they've been hanging out there ⏳

Lowkey the way it works is it traces the __do_page_cache_readahead() kernel function to track entry and exit in the readahead mechanism in the kernel 🔍 Then it uses the __page_cache_alloc() and mark_page_accessed() functions to figure out the age of each page in the cache, and to see how many are left unaccessed 🫶

Since this uses BPF, only the root user can use this tool 🔥 No cap, so don't even try it as a regular user 💀

🙈 NOTE ON KPROBES USAGE 🙈

Okay, real talk: this tool uses Kprobes, so it depends on how your Linux kernel was compiled 🛠️ These functions might be inlined, which means they won't be available for Kprobes 😭 To check, look at the vmlinux source and binary and confirm whether inlining is happening 🔎 You can also check /proc/kallsyms on the host and verify that the target functions are present there before using this 🧚

🎀 REQUIREMENTS 🎀

CONFIG_BPF, bcc 💖

✨ OPTIONS ✨

• -h — Prints the usage message 🌈
• -d DURATION — Traces the read-ahead caching system for DURATION seconds ⏱️

🍓 EXAMPLES 🍓

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

# readahead -d 30

🌈 OVERHEAD 🌈

⚠️ Bestie, watch out! The kernel functions this program instruments could be high-frequency depending on the application's profile (for example, sequential IO) 😱 Please measure and monitor the overhead before leaving this turned on in production environments 🙏

📚 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 🤓

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 💅

🐧 OS 🐧

Linux 🐧

🧪 STABILITY 🧪

Unstable - in development 🚧

👩‍💻 AUTHOR 👩‍💻

Suchakra Sharma 🫶

👑 SEE ALSO 👑

readahead(2), madvise(2) 💫

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Periodt, that's the whole tea ☕💖 Source is the bcc/iovisor project, version and date are USER COMMANDS 2020-08-20 🎀 Stay iconic and keep those caches clean, bestie 🦋✨