fsleep(9)

✨ fsleep(9) ✨

okay bestie so this is fsleep from the Kernel API Manual, and honestly? she's the friend who always picks the right outfit for the occasion 💅 — a flexible sleep that autoselects the best mechanism. no cap.

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🦋 SYNOPSIS

void fsleep (unsigned long usecs );

like, that's it. one arg. so clean of her 🎀

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🌸 ARGUMENTS

• usecs — the sleep duration you're asking for, measured in microseconds. yes, *micro*seconds. she's precise like that 🫶

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💖 DESCRIPTION

so here's the whole vibe: fsleep scans the situation and picks the mechanism that gives you at most 25% slack on the sleep duration you asked for. maximum 25%, she's not about to let your timing drift off into the void 😭. and because she's got range, she does it with three different moves:

• udelay loop for sleep durations <= 10 microseconds — because spinning up an hrtimer for a sleep that short is giving unnecessary overhead energy 💀 and she is not about that.
• usleep_range for sleep durations where using msleep would land you with slack bigger than 25%. note: this whole decision depends on the granularity of jiffies, so it's not a fixed rule, iykyk ⭐
• msleep for literally all the other sleep durations. she saves the chill one for last 🔥

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🙈 NOTE

real talk: when CONFIG_HIGH_RES_TIMERS is not set, every single sleep gets processed with the granularity of jiffies — and that means the slack might exceed 25%, especially for short sleep durations. so... vibe check your config before you come for her, bestie 😌💀

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🌈 SEE ALSO

Kernel file ./include/linux/delay.h plus her whole friend group: mdelay(9), usleep_range(9), usleep_range_idle(9), ssleep(9) 👑

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anyway that's the whole page, mwah 💜 — this iconic little number lives in ./include/linux and is dated October 2026. see u in the next sleep call, bestie 🧚✨