runnerd_core/sizing/

auto.rs

//! Auto sizing: which profiles' `memory`, `slots` and `cpus` the daemon changes now, from the advice, the runs since
//! each profile's last change and the limits `[sizing]` and each profile set. Pure: the daemon reads the history and
//! when each profile last changed, writes each change through the config check as `PUT /v1/config` does, and records
//! it. Never lowers a profile a run met the cap of, never changes `memory` from fewer than `[sizing] min_jobs` jobs,
//! never lowers within `[sizing] interval` of a profile's last change or raises within `raise_interval`, never
//! leaves a profile outside its bounds, and sizes each profile against the budget the changes before it in the same
//! look leave. Moves `cpus` only on the CPU averages of runs at the profile's current `cpus` since its last change
//! and since the daemon got them, never on CPU peaks: a parallel build peaks at whatever cap it gets.

use std::collections::BTreeMap;
use std::fmt::Write as _;

use crate::api::{
    Advice, ProfileSizing, Resize, Sizing, SizingChange, SizingRefused, cores, job_title,
};
use crate::config::{Admission, ByteSize, Config, Profile, cpu_budget};

use super::{
    BUSY, CpusSet, GRANULE, History, WAITED, Window, advise, busy, first_runner, spread, waits,
};

/// The fewest runs a raise of `slots` or `cpus` reads: the jobs queued since the profile's last change, or one job's
/// runs at its current `cpus`. It does not wait for `min_jobs`: a slot too few costs every queued job a wait now.
const RAISE_RUNS: u64 = 3;

/// A job whose runs averaged this share of their CPU cap or more, in percent, at the median, is held back by it over
/// the whole run, not in a burst: its checkout, cache restore and upload steps barely use a CPU, so the rest of it
/// ran at the cap. On the owner's history no job's median came past 37%, whatever its cap: their builds are bursts.
const BOUND: u64 = 70;

/// `cpus` come down toward this many times the heaviest job's p95 CPU average: a burst at twice its run's average
/// still runs uncapped, and a job back at the cap would average half of it, far under `BOUND`, so a lowering never
/// asks for the raise back.
const HEADROOM: u64 = 2;

/// What auto sizing reads besides the config and the history.
#[derive(Debug, Clone, Copy)]
pub struct Context<'run> {
    /// The slice's `MemoryMax`; `None` when it is unlimited.
    pub memory_max: Option<u64>,
    /// The slice's CPUs, in millicores; `None` when they could not be read, and then `cpus` are never raised.
    pub cpus: Option<u64>,
    /// The history's window; `now` is when the changes are made.
    pub window: Window,
    /// When auto sizing last changed each profile, in seconds, by name, however long ago.
    pub last: &'run BTreeMap<String, u64>,
    /// Each profile's `cpus` as the daemon ran them, oldest first: what a `cpus` change reads runs since, as a
    /// hand edit, a reload or an undo leaves no change of auto sizing's.
    pub cpus_set: &'run [CpusSet],
    /// How many runners each profile runs now, by name.
    pub running: &'run BTreeMap<String, u32>,
}

/// The changes to make now, in order, one per profile at most. Empty unless `[sizing] auto` is on.
///
/// A change that lowers `memory` or `cpus` waits `[sizing] interval` after the profile's last change; one that only
/// raises waits `raise_interval`. Each moves `memory` toward the advice, and `cpus` toward what its runs reach, by at
/// most `[sizing] max_step` percent, `memory` within the profile's `min_memory` and `max_memory`, `cpus` within the
/// slice's CPUs. A profile whose jobs waited for a slot, or whose slots were busy, since its last change gets one
/// more slot when every profile's slots still fit `MemoryMax`: at their caps under `caps` admission, at their jobs'
/// p95 peaks under `measured`; at most `max_slots` when set, and never under what runs. Under `caps` a lowered
/// `memory` adds the slots the advice gives. Each change is sized as if the ones before it were written: they take
/// budget the later ones cannot.
///
/// `cpus` go up `max_step` percent, at least 1, when one job's runs at them since the profile's last change and since
/// the daemon got them (`Context::cpus_set`), 3 at least, averaged `BOUND` percent of them at the median; up to the slice's CPUs, and under `caps` with a CPU budget
/// only as far as every profile's `slots × cpus` fit it. They come down, `interval` after the last change and by
/// `max_step` percent at most, toward `HEADROOM` times the heaviest job's p95 average, rounded up to whole CPUs, at
/// least 1, once `min_jobs` such runs say so and every job among them has a measured average. Only successful runs
/// with a measured average count; see [`next_look`] for the raises it cannot make.
pub fn changes(config: &Config, history: &History, context: Context<'_>) -> Vec<SizingChange> {
    next_look(config, history, context).changes
}

/// What a look does: the changes [`changes`] picks, and the `cpus` raises it cannot make.
#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct Look {
    /// The changes to make now, in order, one per profile at most.
    pub changes: Vec<SizingChange>,
    /// The profiles whose jobs the CPU cap holds back but whose `cpus` cannot go up: the slice's CPUs are unknown or
    /// all theirs, or the CPU budget has no room.
    pub refused: Vec<SizingRefused>,
}

/// The changes [`changes`] picks, and the `cpus` raises the same look cannot make.
pub fn next_look(config: &Config, history: &History, context: Context<'_>) -> Look {
    let rules = &config.sizing;
    if !rules.auto {
        return Look::default();
    }
    let window = context.window;
    let sizing = advise(config, context.memory_max, history, window);
    let now = window.now / 1000;
    let mut budget = Budget::new(config, &sizing.profiles);
    let mut changes = Vec::new();
    let mut refused = Vec::new();
    for advised in &sizing.profiles {
        let Some(profile) = config.profiles.get(&advised.name) else {
            continue;
        };
        let last = context.last.get(&advised.name).copied();
        let since = last.map_or(u64::MAX, |at| now.saturating_sub(at));
        if since < rules.raise_after().as_secs() {
            continue;
        }
        let lowering = if since >= rules.interval.as_secs() {
            Lowering::Allowed
        } else {
            Lowering::Held
        };
        let fresh = Fresh::read(history, advised, last, window);
        let mut reasons = Vec::new();
        let memory = memory(
            config,
            profile,
            advised.advice,
            met_cap(history, &advised.name),
            lowering,
        );
        let running = context.running.get(&advised.name).copied().unwrap_or(0);
        let slots = match config.host.admission {
            Admission::Caps => {
                caps_slots(profile, advised, &fresh, memory, &context, &budget, running)
            }
            Admission::Measured => {
                measured_slots(config, &sizing.profiles, advised, &fresh, &context, &budget)
            }
        };
        let (slots, cpus) = cpus_and_slots(
            config,
            profile,
            advised,
            history,
            fresh.start,
            &context,
            &budget,
            slots,
            lowering,
        );
        if memory != profile.memory.0 {
            reasons.push(memory_reason(
                &sizing,
                advised,
                memory,
                slots,
                config.sizing.max_step,
            ));
        }
        // Under caps a lowered memory's slots are its line's.
        let advised_slots = config.host.admission == Admission::Caps && memory < profile.memory.0;
        if slots > profile.slots && !advised_slots {
            reasons.push(fresh.waited(last.is_some()));
        }
        let cpus = cpus.taken(&advised.name, &mut reasons, &mut refused);
        let from = Resize {
            memory: profile.memory.0,
            slots: profile.slots,
            cpus: cpus.map(|_| profile.cpus),
        };
        let to = Resize {
            memory,
            slots,
            cpus,
        };
        if to == from || slots == 0 || !within(profile, from, to) {
            continue;
        }
        budget.take(&advised.name, profile, from, to);
        changes.push(SizingChange {
            at: now,
            profile: advised.name.clone(),
            from,
            to,
            reason: reasons.join("; "),
            uncertain: false,
        });
    }
    Look { changes, refused }
}

/// Whether a change may lower `memory` or `cpus`: only `interval` after the profile's last change.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Lowering {
    Allowed,
    Held,
}

/// The budget as the changes picked so far leave it.
struct Budget {
    /// `slots × memory` over all profiles, with the mirror.
    caps: u64,
    /// `slots × cpus` over all profiles, in millicores.
    cpus: u64,
    /// Each profile's slots.
    slots: BTreeMap<String, u32>,
}

impl Budget {
    /// Takes what a change of `profile` from `from` to `to` adds, and gives back what it frees.
    fn take(&mut self, name: &str, profile: &Profile, from: Resize, to: Resize) {
        self.caps = self
            .caps
            .saturating_sub(from.memory.saturating_mul(u64::from(from.slots)))
            .saturating_add(to.memory.saturating_mul(u64::from(to.slots)));
        let millicores = |resize: Resize| {
            u64::from(resize.cpus.unwrap_or(profile.cpus)) * 1000 * u64::from(resize.slots)
        };
        self.cpus = self
            .cpus
            .saturating_sub(millicores(from))
            .saturating_add(millicores(to));
        self.slots.insert(name.to_owned(), to.slots);
    }

    /// The budget the config takes as it is.
    fn new(config: &Config, profiles: &[ProfileSizing]) -> Budget {
        Budget {
            caps: config.memory_caps().map_or(u64::MAX, |caps| caps.0),
            cpus: config
                .profiles
                .values()
                .map(|profile| u64::from(profile.cpus) * 1000 * u64::from(profile.slots))
                .sum(),
            slots: profiles
                .iter()
                .map(|advised| (advised.name.clone(), advised.slots))
                .collect(),
        }
    }
}

/// What the jobs queued since a profile's last change say about its slots: a raise reads only them, so one slot
/// that fixed a month's waits is not followed by another every `raise_interval`.
struct Fresh {
    /// When the evidence starts, in milliseconds: the last change, or the window's start.
    start: u64,
    jobs: u64,
    /// Seconds from queued to started, p95.
    wait: u64,
    busy: u32,
}

impl Fresh {
    /// The profile's jobs queued since `last` (seconds), or in the whole window without a last change.
    fn read(
        history: &History,
        advised: &ProfileSizing,
        last: Option<u64>,
        window: Window,
    ) -> Fresh {
        let start = last.map_or(window.since, |at| at.saturating_mul(1000).max(window.since));
        let jobs: Vec<_> = history
            .jobs
            .iter()
            .filter(|job| job.profile == advised.name && job.queued_at >= start)
            .collect();
        Fresh {
            start,
            jobs: jobs.len() as u64,
            wait: waits(&jobs).p95,
            busy: busy(
                history,
                &advised.name,
                advised.slots,
                first_runner(history, start),
                window.now,
            ),
        }
    }

    /// Whether another slot would help: enough jobs, which waited a minute at p95 or kept the slots busy half the
    /// time.
    fn wants_slots(&self) -> bool {
        self.jobs >= RAISE_RUNS && (self.wait >= WAITED || self.busy >= BUSY)
    }

    /// Why a slot is added.
    fn waited(&self, changed: bool) -> String {
        format!(
            "{} jobs {}waited {}s for a slot (p95) and slots were busy {}%; one more fits the slice beside the \
             other profiles'",
            self.jobs,
            if changed {
                "since its last change "
            } else {
                ""
            },
            self.wait,
            self.busy
        )
    }
}

/// Why `memory` changes, as the advice line says it of the `memory` and `slots` this change writes, not the
/// advice's own: a step of `max_step` stops short of the advice's memory, and the slots that fit follow the step.
/// A step short of the advice says what it goes toward.
fn memory_reason(
    sizing: &Sizing,
    advised: &ProfileSizing,
    memory: u64,
    slots: u32,
    max_step: u32,
) -> String {
    let made = ProfileSizing {
        advice: match advised.advice {
            Advice::Lower { .. } => Advice::Lower { memory, slots },
            Advice::Raise { .. } => Advice::Raise { memory, slots },
            other => other,
        },
        ..advised.clone()
    };
    let line = sizing.memory_line(&made);
    match advised.advice.change() {
        Some(advice) if advice.memory != memory => format!(
            "{line}; a step toward {}, at most {max_step}% a change",
            ByteSize(advice.memory)
        ),
        _ => line,
    }
}

/// Whether `to` keeps the profile within its bounds: `memory` within `min_memory` and `max_memory`, and `slots` at
/// most `max_slots` when they grow.
fn within(profile: &Profile, from: Resize, to: Resize) -> bool {
    profile.min_memory.is_none_or(|min| to.memory >= min.0)
        && profile.max_memory.is_none_or(|max| to.memory <= max.0)
        && (to.slots <= from.slots || profile.max_slots.is_none_or(|max| to.slots <= max))
}

/// Whether a run of the profile in the history, cancelled ones too, was killed for memory or met its cap.
fn met_cap(history: &History, profile: &str) -> bool {
    history
        .jobs
        .iter()
        .chain(&history.cancelled)
        .filter(|job| job.profile == profile)
        .any(|job| job.oom_killed || job.memory_cap.is_some_and(|cap| job.met_cap(cap)))
}

/// The profile's next `memory`: the advice's, at most `max_step` percent away and within its bounds, in steps of
/// `GRANULE` toward the advice. A lowering waits while a run met the cap, and while `lowering` holds it.
fn memory(
    config: &Config,
    profile: &Profile,
    advice: Advice,
    met_cap: bool,
    lowering: Lowering,
) -> u64 {
    let current = profile.memory.0;
    let step = current.saturating_mul(u64::from(config.sizing.max_step)) / 100;
    match advice {
        Advice::Lower { memory, .. } if !met_cap && lowering == Lowering::Allowed => {
            let floor = current
                .saturating_sub(step)
                .div_ceil(GRANULE)
                .saturating_mul(GRANULE);
            let bound = profile.min_memory.map_or(0, |min| min.0);
            memory.max(floor).max(bound).min(current)
        }
        Advice::Raise { memory, .. } => {
            let ceiling = current.saturating_add(step) / GRANULE * GRANULE;
            let bound = profile.max_memory.map_or(u64::MAX, |max| max.0);
            memory.min(ceiling).min(bound).max(current)
        }
        _ => current,
    }
}

/// Under `caps`: as many slots as the advice gives for the new memory. A lowering whose jobs waited adds as many as
/// the memory the slots take now holds, up to `max_slots`; a raise keeps as many as the slice holds beside the other
/// profiles, and 0, no change, while more than that run. With the memory kept, one more slot when its jobs waited
/// since the last change and its cap fits `MemoryMax` beside every other cap, up to `max_slots`. Otherwise the same.
fn caps_slots(
    profile: &Profile,
    advised: &ProfileSizing,
    fresh: &Fresh,
    memory: u64,
    context: &Context<'_>,
    budget: &Budget,
    running: u32,
) -> u32 {
    let current = profile.slots;
    let max_slots = profile.max_slots.unwrap_or(u32::MAX);
    let fit = |room: u64| u32::try_from(room / memory.max(1)).unwrap_or(u32::MAX);
    if memory < profile.memory.0 && advised.wants_slots {
        let share = profile.memory.0.saturating_mul(u64::from(current));
        return fit(share).min(max_slots).max(current);
    }
    let Some(memory_max) = context.memory_max else {
        return current;
    };
    if memory > profile.memory.0 {
        let share = profile.memory.0.saturating_mul(u64::from(current));
        let room = memory_max.saturating_sub(budget.caps.saturating_sub(share));
        // Fewer slots than run would leave the running ones over the budget: wait for some to finish.
        let slots = fit(room).min(current);
        return if slots < running { 0 } else { slots };
    }
    if fresh.wants_slots()
        && current < max_slots
        && budget.caps.saturating_add(memory) <= memory_max
    {
        return current + 1;
    }
    current
}

/// Under `measured`: one more slot when its jobs waited since the last change and every profile's slots, this one's
/// with the new one, fit the slice beside the mirror at their jobs' p95 peaks (at its cap, a profile with fewer than
/// `min_jobs` jobs), up to `max_slots`, with the slots the changes before it gave. Without `max_slots` or a
/// `MemoryMax`, nothing bounds slots, so none is added. Otherwise the same.
fn measured_slots(
    config: &Config,
    profiles: &[ProfileSizing],
    advised: &ProfileSizing,
    fresh: &Fresh,
    context: &Context<'_>,
    budget: &Budget,
) -> u32 {
    let current = advised.slots;
    let max_slots = config
        .profiles
        .get(&advised.name)
        .and_then(|profile| profile.max_slots);
    if !fresh.wants_slots() || max_slots.is_some_and(|max| max <= current) {
        return current;
    }
    let Some(memory_max) = context.memory_max else {
        return if max_slots.is_some() {
            current + 1
        } else {
            current
        };
    };
    let peaks: u64 = profiles
        .iter()
        .map(|other| {
            let peak = if matches!(other.advice, Advice::TooFew { .. }) {
                other.memory
            } else {
                other.peaks.memory.p95
            };
            let slots = if other.name == advised.name {
                current + 1
            } else {
                budget
                    .slots
                    .get(&other.name)
                    .copied()
                    .unwrap_or(other.slots)
            };
            peak.saturating_mul(u64::from(slots))
        })
        .fold(config.mirror_memory(), u64::saturating_add);
    if peaks <= memory_max {
        current + 1
    } else {
        current
    }
}

/// The profile's next `cpus` and `slots`, from the `slots` memory and waits chose: under `caps` with a CPU budget,
/// slots past what the budget holds at the new `cpus` are dropped, and the `cpus` sized again at the slots that stay,
/// so an extra slot the budget refuses does not also refuse a CPU raise that fits without it.
#[expect(
    clippy::too_many_arguments,
    reason = "each is one thing a CPU change reads; a struct for them would be read here only"
)]
fn cpus_and_slots(
    config: &Config,
    profile: &Profile,
    advised: &ProfileSizing,
    history: &History,
    since: u64,
    context: &Context<'_>,
    budget: &Budget,
    slots: u32,
    lowering: Lowering,
) -> (u32, CpuStep) {
    let size = |slots: u32| {
        cpus(
            config, profile, advised, history, since, context, budget, slots, lowering,
        )
    };
    // Slots the CPU budget cannot hold would wait for CPU anyway, and their waits would ask for more.
    let fit = |slots: u32, cpus: &CpuStep| {
        let cpus = match cpus {
            CpuStep::To(cpus) => cpus.to,
            CpuStep::Keep | CpuStep::Refused(_) => profile.cpus,
        };
        if slots > profile.slots {
            cpu_slots(config, profile, context, budget, cpus)
                .map_or(slots, |fit| slots.min(fit.max(profile.slots)))
        } else {
            slots
        }
    };
    let mut cpus = size(slots);
    let fitted = fit(slots, &cpus);
    if fitted < slots {
        cpus = size(fitted);
    }
    (fit(fitted, &cpus), cpus)
}

/// Under `caps` admission with a CPU budget, the most slots of the profile at `cpus` each that fit the budget beside
/// every other profile's `slots × cpus`, as the changes before it left them; none more than it has when the slice's
/// CPUs are unknown. `None` when no CPU budget bounds slots.
fn cpu_slots(
    config: &Config,
    profile: &Profile,
    context: &Context<'_>,
    budget: &Budget,
    cpus: u32,
) -> Option<u32> {
    let (Admission::Caps, Some(overcommit)) = (config.host.admission, config.host.cpu_overcommit)
    else {
        return None;
    };
    let Some(slice) = context.cpus else {
        return Some(profile.slots);
    };
    let others = budget
        .cpus
        .saturating_sub(u64::from(profile.cpus) * 1000 * u64::from(profile.slots));
    let room = cpu_budget(slice, overcommit).saturating_sub(others);
    Some(u32::try_from(room / (u64::from(cpus.max(1)) * 1000)).unwrap_or(u32::MAX))
}

/// What a look does with a profile's `cpus`.
enum CpuStep {
    Keep,
    To(Cpus),
    /// Its jobs are held back by the cap, and this says why it cannot go up.
    Refused(String),
}

impl CpuStep {
    /// The new `cpus`, if any, its reason added to `reasons`, or a refused raise to `refused`.
    fn taken(
        self,
        profile: &str,
        reasons: &mut Vec<String>,
        refused: &mut Vec<SizingRefused>,
    ) -> Option<u32> {
        match self {
            CpuStep::To(cpus) => {
                reasons.push(cpus.reason);
                Some(cpus.to)
            }
            CpuStep::Refused(reason) => {
                refused.push(SizingRefused {
                    profile: profile.to_owned(),
                    reason,
                });
                None
            }
            CpuStep::Keep => None,
        }
    }
}

/// A new `cpus` and why.
struct Cpus {
    to: u32,
    reason: String,
}

/// A job: its repository, workflow and name.
type JobKey<'run> = (&'run str, Option<&'run str>, &'run str);

/// When the daemon started running the profile with `cpus`, in ms, by `set`: its first event since the last one
/// with other `cpus`. `now` when its latest has other `cpus`, as before a reload announces them; 0 with no events.
fn cpus_since(set: &[CpusSet], profile: &str, cpus: u32, now: u64) -> u64 {
    let mut since = None;
    let mut seen = false;
    for event in set.iter().filter(|event| event.profile == profile) {
        seen = true;
        if event.cpus != cpus {
            since = None;
        } else if since.is_none() {
            since = Some(event.at);
        }
    }
    since.unwrap_or(if seen { now } else { 0 })
}

/// The CPU averages, millicores, of the profile's runs a `cpus` change reads, by job: successful runs whose container
/// had `cap`, queued at or after `since` (ms), the later of the profile's last change and when it got these `cpus`
/// (`cpus_since`). A run without a measured average adds none; nor does an average of 0, which only a run sampled
/// once records. Its job is still listed, so a lowering can tell a job it knows nothing of from one it lacks.
fn averages<'run>(
    history: &'run History,
    profile: &str,
    cap: u64,
    since: u64,
) -> BTreeMap<JobKey<'run>, Vec<u64>> {
    let mut jobs: BTreeMap<JobKey<'run>, Vec<u64>> = BTreeMap::new();
    for job in &history.jobs {
        if job.profile == profile
            && job.cpu_cap == Some(cap)
            && job.succeeded
            && job.queued_at >= since
        {
            let averages = jobs
                .entry((&job.repository, job.workflow.as_deref(), &job.name))
                .or_default();
            averages.extend(job.cpu_average.filter(|average| *average > 0));
        }
    }
    jobs
}

/// `owner/name workflow / job`, as a CPU change's reason names a job.
fn job_named(job: JobKey<'_>) -> String {
    let (repository, workflow, name) = job;
    format!("{repository} {}", job_title(workflow, name, None))
}

/// The profile's next `cpus`, from the CPU averages of its runs at its current `cpus` since `since` (ms), the last
/// change, and since it got them (`averages`). A raise, `max_step` percent, at least 1, when one job's `RAISE_RUNS` runs or more averaged
/// `BOUND` percent of the cap at the median: up to the slice's CPUs, and under `caps` admission with a CPU budget
/// only as many as every profile's `slots × cpus` fit in it, at the `slots` this change gives; refused, with why,
/// when that is none. A lowering, while `lowering` allows it, with `min_jobs` runs and none of a job without a
/// measured average: toward `HEADROOM` times the
/// heaviest job's p95 average, rounded up to whole CPUs, at least 1, by at most `max_step` percent.
#[expect(
    clippy::too_many_arguments,
    reason = "each is one thing a CPU change reads; a struct for them would be read here only"
)]
fn cpus(
    config: &Config,
    profile: &Profile,
    advised: &ProfileSizing,
    history: &History,
    since: u64,
    context: &Context<'_>,
    budget: &Budget,
    slots: u32,
    lowering: Lowering,
) -> CpuStep {
    let current = profile.cpus;
    let cap = u64::from(current) * 1000;
    let since = since.max(cpus_since(
        context.cpus_set,
        &advised.name,
        current,
        context.window.now,
    ));
    let jobs = averages(history, &advised.name, cap, since);
    let step = (current.saturating_mul(config.sizing.max_step) / 100).max(1);
    let bound = jobs
        .iter()
        .filter(|(_, averages)| averages.len() as u64 >= RAISE_RUNS)
        .map(|(job, averages)| (*job, averages.len(), spread(averages.clone()).p50))
        .filter(|(_, _, median)| median.saturating_mul(100) >= cap.saturating_mul(BOUND))
        .max_by_key(|(_, _, median)| *median);
    if let Some((job, runs, median)) = bound {
        let held = format!(
            "{} runs of {} averaged {} of its {current} CPUs (median): the cap holds them back",
            runs,
            job_named(job),
            cores(median)
        );
        let Some(slice) = context.cpus else {
            return CpuStep::Refused(format!("{held}; the slice's CPUs could not be read"));
        };
        let slice_cpus = u32::try_from(slice / 1000).unwrap_or(u32::MAX);
        let mut to = current.saturating_add(step).min(slice_cpus);
        let mut why = format!("it has all {slice_cpus} of the slice's CPUs");
        if let (Admission::Caps, Some(overcommit)) =
            (config.host.admission, config.host.cpu_overcommit)
        {
            let others = budget
                .cpus
                .saturating_sub(cap.saturating_mul(u64::from(profile.slots)));
            let room = cpu_budget(slice, overcommit).saturating_sub(others);
            let fit = room / 1000 / u64::from(slots.max(1));
            let fit = u32::try_from(fit).unwrap_or(u32::MAX);
            if fit < to {
                to = fit;
                why = format!(
                    "{} CPUs for each of its {slots} slots do not fit the CPU budget beside the other profiles'",
                    current.saturating_add(1)
                );
            }
        }
        return if to > current {
            CpuStep::To(Cpus { to, reason: held })
        } else {
            CpuStep::Refused(format!("{held}; {why}"))
        };
    }
    let runs: usize = jobs.values().map(Vec::len).sum();
    // A job whose runs at the cap have no measured average may be the one that needs it.
    if lowering == Lowering::Held
        || (runs as u64) < u64::from(config.sizing.min_jobs)
        || jobs.values().any(Vec::is_empty)
    {
        return CpuStep::Keep;
    }
    let Some((job, heaviest)) = jobs
        .iter()
        .map(|(job, averages)| (*job, spread(averages.clone()).p95))
        .max_by_key(|(_, p95)| *p95)
    else {
        return CpuStep::Keep;
    };
    let need = heaviest.saturating_mul(HEADROOM);
    let target = u32::try_from(need.div_ceil(1000))
        .unwrap_or(u32::MAX)
        .max(1);
    if target >= current {
        return CpuStep::Keep;
    }
    let to = target.max(current.saturating_sub(step)).max(1);
    let mut reason = format!(
        "{runs} runs at its {current} CPUs: the heaviest job, {}, averaged {} CPUs (p95); twice that is {target}",
        job_named(job),
        cores(heaviest)
    );
    if to != target {
        let _ = write!(
            reason,
            "; a step toward {target}, at most {}% a change",
            config.sizing.max_step
        );
    }
    CpuStep::To(Cpus { to, reason })
}