Struct Exec 

pub struct Exec<'p> { /* private fields */ }

budget slots of a Pool, or the calling thread alone.

An Exec is the only way kernels start parallel work. It is a token:

• It is neither Clone nor Copy, and every parallel entry consumes it, so a kernel cannot run two phases at full width at the same time. Use reborrow to run several phases in sequence.
• Only Exec::new mints a token, and it needs the &Pool; a token never hands its pool back out, so a kernel cannot mint a fresh full-budget token.
• fork divides the budget among its tasks (shares differ by at most one and sum to the budget), and fork_sized hands out explicit shares that sum to at most the budget, so the number of tasks running at once under one root token is at most its budget at every nesting depth.

A parent waiting for its children keeps no slot.

Placement

Serial, budget one, or one task: the work runs inline on the calling thread and the scheduler is never called, so no global pool is started. The caller is a worker of the pool: the tasks run as a join tree on the caller, nothing is installed. The caller is a plain thread or a worker of a different pool: the pool is entered once and the caller waits; the tasks run on the target pool. No thread-local state is involved at any point.

Examples

use std::sync::atomic::{AtomicUsize, Ordering};
use threadteam::{Exec, Pool};

let pool = Pool::builder().threads(2).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let exec = Exec::new(&pool, 2)?;
let sum = AtomicUsize::new(0);
exec.for_ranges(100, 1.try_into().unwrap(), |r| {
    sum.fetch_add(r.sum::<usize>(), Ordering::Relaxed);
});
assert_eq!(sum.into_inner(), 4950);

A token cannot be duplicated:

fn needs_clone<T: Clone>() {}
needs_clone::<threadteam::Exec<'static>>();

fn needs_copy<T: Copy>() {}
needs_copy::<threadteam::Exec<'static>>();

let exec = threadteam::Exec::serial();
exec.fork(1, |_, _| {});
exec.fork(1, |_, _| {}); // `fork` consumed the token

Implementations

impl Exec<'static>

pub const fn serial() -> Exec<'static>

A token that runs everything inline on the calling thread.

It never touches a scheduler, so using it cannot start any pool.

Examples

use threadteam::Exec;

let exec = Exec::serial();
assert!(exec.is_serial());
let here = std::thread::current().id();
exec.fork(3, |_, _| assert_eq!(std::thread::current().id(), here));

impl<'p> Exec<'p>

pub fn new(pool: &'p Pool, budget: usize) -> Result<Exec<'p>, Error>

A token of budget slots of pool, clamped to the pool’s width.

This is the only place a budget is clamped.

Errors

Error::ZeroBudget when budget is zero.

Examples

use threadteam::{Error, Exec, Pool};

let pool = Pool::builder().threads(2).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let exec = Exec::new(&pool, 64)?;
assert_eq!(exec.budget(), pool.width());
assert_eq!(Exec::new(&pool, 0).unwrap_err(), Error::ZeroBudget);

pub fn budget(&self) -> NonZero<usize>

The number of slots this token stands for.

Examples

assert_eq!(threadteam::Exec::serial().budget().get(), 1);

pub fn dispatch_cost(&self, width: NonZero<usize>) -> Duration

The cost of one parallel section of width participants started now on this token: the steady state (sections back to back, shares that carry work). Duration::MAX when no section is possible: a serial token, or width < 2. width is clamped to the budget.

The kind of token is decided at call time:

• the token has team authority (it comes from a Team, or is a child of such a token): DispatchKind::Team;
• a plain token whose caller is a worker of its pool: DispatchKind::InPool;
• a plain token whose caller is outside the pool: DispatchKind::Outside.

The numbers come from the pool’s DispatchCosts (see PoolBuilder::dispatch_costs). The call allocates nothing and reads no thread-local state of this crate. The answer depends on the calling thread, so ask it on the thread that will dispatch.

Examples

use std::num::NonZeroUsize;
use std::time::Duration;
use threadteam::Exec;

let two = NonZeroUsize::new(2).unwrap();
assert_eq!(Exec::serial().dispatch_cost(two), Duration::MAX);

pub fn pays( &self, estimate: Duration, sections: u32, width: NonZero<usize>, ) -> bool

Whether a region of serial time estimate that opens sections parallel sections of width participants pays on this token: estimate >= PAYS * sections * dispatch_cost(width) (see pays_at). False for a serial token.

Examples

use std::time::Duration;
use threadteam::Exec;

let two = 2.try_into().unwrap();
assert!(!Exec::serial().pays(Duration::from_secs(1), 1, two));

pub fn width_for(&self, estimate: Duration, sections: u32) -> NonZero<usize>

The width in 1..=budget that minimises sections * D(k) + estimate / k for this token, counting only widths at which the region pays; 1 when none does (see width_for_by).

Examples

use std::time::Duration;
use threadteam::Exec;

assert_eq!(Exec::serial().width_for(Duration::from_secs(1), 1).get(), 1);

pub fn is_serial(&self) -> bool

Whether no parallelism is possible: the budget is one slot.

Kernels use this to take their serial path directly.

Examples

assert!(threadteam::Exec::serial().is_serial());

pub fn carries_team(&self) -> bool

Whether this token carries team authority: it was minted by a Team or is a child of such a token. Its sections are team sections (DispatchKind::Team) and team returns the carried (sub)team at once, without acquisition, so a consumer that wants a TeamExec (faer’s Par::Team) asks this first and keeps external for plain tokens, whose team would reserve members.

Examples

use threadteam::{Exec, Pool};

assert!(!Exec::serial().carries_team());
let pool = Pool::builder().threads(2).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
assert!(!Exec::new(&pool, 2)?.carries_team());
if let Ok(team) = pool.into_team(2) {
    assert!(team.exec(2).carries_team());
}

pub fn limit(self, max: NonZero<usize>) -> Exec<'p>

Narrows the token to at most max slots.

The result has budget min(budget, max) and the same pool; the dropped slots are simply not used, so slots are conserved. Callers narrow before broadcast or external to match a team size chosen by their own work model. Co-scheduling eligibility is unchanged by limit: a root token stays eligible for broadcast, a child produced by fork stays ineligible.

Examples

use std::num::NonZeroUsize;
use threadteam::{Exec, Pool};

let pool = Pool::builder().threads(4).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let exec = Exec::new(&pool, 4)?.limit(NonZeroUsize::new(2).unwrap());
assert!(exec.budget().get() <= 2);

pub fn reborrow(&mut self) -> Exec<'_>

A token with the same pool and budget, borrowed from self.

Parallel entries consume the token, but a kernel often runs several parallel phases in sequence. Each phase takes a reborrow. The parent is exclusively borrowed and unusable while the child lives, so the slots are held by one token at a time: this conserves slots exactly as handing the token over would.

Examples

use threadteam::Exec;

let mut exec = Exec::serial();
exec.reborrow().fork(2, |_, _| {});
exec.reborrow().fork(2, |_, _| {});
exec.fork(2, |_, _| {});

pub fn fork<F>(self, jobs: usize, f: F)

where F: Fn(usize, Exec<'_>) + Sync,

Runs jobs jobs on min(jobs, budget) tasks; job i gets a child token.

The budget is split into one share per task, shares differing by at most one and summing to the budget. A job’s child token carries its task’s share and may be passed to any kernel, which can fork again inside it. When jobs exceeds the budget the tasks take job indices from a shared counter, so the load balances; otherwise job i runs on task i.

One task, a serial token, or jobs <= 1 runs inline on the calling thread without any scheduler call. Returns when every job has finished.

Examples

use std::sync::atomic::{AtomicUsize, Ordering};
use threadteam::{Exec, Pool};

let pool = Pool::builder().threads(2).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let hits: Vec<AtomicUsize> = (0..5).map(|_| AtomicUsize::new(0)).collect();
Exec::new(&pool, 2)?.fork(5, |i, _child| {
    hits[i].fetch_add(1, Ordering::Relaxed);
});
assert!(hits.iter().all(|h| h.load(Ordering::Relaxed) == 1));

pub fn fork_sized<F>(self, shares: &[usize], f: F) -> Result<(), Error>

where F: Fn(usize, Exec<'_>) + Sync,

Runs one child per entry of shares; child i gets a token of exactly shares[i] slots.

For work of uneven size: give a large item several slots and the small ones one slot each. Child i always runs job i (there is no shared counter), and join, panic propagation and placement are those of fork, which is the even special case of this call. One child, or a serial token, runs inline on the calling thread without any scheduler call. An empty shares does nothing. Returns when every child has finished.

Leftover slots. When the shares sum to less than the budget, the remaining slots are given to no child and stay idle for the call: no task runs on them and, on a team token, their reserved members stay parked. They remain the parent’s: fork through reborrow and the parent token has its full budget again after the join.

Errors

Nothing runs and the token is consumed when

• a share is zero: Error::ZeroBudget;
• the shares sum to more than the budget: Error::SharesExceedBudget, naming the budget and the sum. Shares are never clamped.

Panics

Re-raises a panic from any child after every child has finished.

Examples

use threadteam::{Error, Exec, Pool};

let pool = Pool::builder().threads(4).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let mut exec = Exec::new(&pool, 4)?;
let budget = exec.budget().get(); // 1 without the `parallel` feature
if budget == 4 {
    // One large item on three slots, one small item on the last slot.
    exec.reborrow().fork_sized(&[3, 1], |i, child| {
        assert_eq!(child.budget().get(), [3, 1][i]);
    })?;
}
let err = exec.fork_sized(&[budget, 1], |_, _| {}).unwrap_err();
assert_eq!(err, Error::SharesExceedBudget { budget, sum: budget + 1 });

pub fn for_ranges<F>(self, len: usize, grain: NonZero<usize>, f: F)

where F: Fn(Range<usize>) + Sync,

Data-parallel loop over 0..len in contiguous ranges.

Uses at most budget tasks; every range is at least grain long except possibly the last, and together the ranges cover 0..len exactly once. When one task suffices (len <= grain, a serial token) f(0..len) runs inline on the calling thread without any scheduler call. len == 0 does nothing.

Examples

use std::sync::atomic::{AtomicUsize, Ordering};
use threadteam::{Exec, Pool};

let pool = Pool::builder().threads(2).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let covered = AtomicUsize::new(0);
Exec::new(&pool, 2)?.for_ranges(10, 4.try_into().unwrap(), |r| {
    covered.fetch_add(r.len(), Ordering::Relaxed);
});
assert_eq!(covered.into_inner(), 10);

pub fn broadcast<F>(self, f: F) -> Result<(), Refused>

where F: Fn(usize) + Sync,

Co-scheduling for barrier-bearing kernels: runs f(i) for every i in 0..budget with all participants running concurrently, or refuses.

A serial or budget-one token runs f(0) inline and returns Ok. Otherwise the pool is entered once and every participant runs on its own worker, so f may block on a barrier shared by the budget participants.

Errors

Nothing runs and Refused is returned when co-scheduling cannot be guaranteed:

• Refused::ForkChild: the token was produced by fork (or derived from such a token by reborrow or limit); its slots may share workers with siblings. Roots from Exec::new, and their reborrows and limits, stay eligible.

Callers must handle Refused by falling back to a barrier-free path.

• Refused::CallerInPool: the calling thread is a worker of the token’s pool and would occupy a slot.

Guarantee and limits

Concurrent broadcasts on one pool are queued on every worker in the same order, so two of them cannot each hold half the workers. The participants are the pool’s workers 0..budget; if unrelated tasks that never finish occupy those workers, the broadcast waits for them. Keep barrier kernels on a pool whose workers are not held by blocking work of another token.

Examples

use std::sync::Barrier;
use threadteam::{Exec, Pool};

let pool = Pool::builder().threads(2).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let exec = Exec::new(&pool, 2)?;
let barrier = Barrier::new(exec.budget().get());
exec.broadcast(|_i| {
    barrier.wait(); // every participant is running, or this call refused
}).expect("called from a plain thread on a root token");

pub fn can_broadcast(&self) -> bool

Whether broadcast would run right now, without consuming the token.

True exactly when broadcast would not return Refused: the token is not derived from fork and the calling thread is not a worker of its pool. A serial or budget-one token returns true, because broadcast then runs f(0) inline. The answer depends on the calling thread, so ask it on the thread that will broadcast.

Use it before building an expensive setup that only a broadcast needs, instead of paying for the setup and then being refused.

Examples

use threadteam::{Exec, Pool};

let pool = Pool::builder().threads(2).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let exec = Exec::new(&pool, 2)?;
assert!(exec.can_broadcast());
exec.fork(1, |_, child| {
    let predicted = child.can_broadcast();
    assert_eq!(child.broadcast(|_| {}).is_ok(), predicted);
});
assert!(Exec::serial().can_broadcast());

pub fn external<R>(self, f: impl FnOnce(NonZero<usize>) -> R + Send) -> R

where R: Send,

Hands the budget as a plain width to a library outside this stack.

This is the only place a bare width exists. The token is consumed, the pool is entered once (inline when the token is serial, has a budget of one, or the caller already is a worker of the pool) and f receives the budget, for example to build faer::Par::rayon(width) or to set a vendor BLAS thread count. Whatever the library schedules with that width runs on this pool, never on an ambient one. How the library divides the width is its own responsibility. Without the parallel feature, f runs inline and receives a width of one.

Placement

The token’s reserved members are lent for the call: each restores its own affinity mask and serves the pool’s jobs until the call returns. The calling thread does the same when it is itself a pinned participant of this pool (for example a member that leads a fork child), so the foreign work never keeps a team’s single-CPU placement. Both re-pin at the next pin section, that is at the next root team scope.

Examples

use threadteam::Exec;

let width = Exec::serial().external(|width| width.get());
assert_eq!(width, 1);

pub fn team<R>( &mut self, f: impl for<'team> FnOnce(TeamExec<'team>) -> R, ) -> Result<R, Refused>

Reserves budget - 1 pool members for the scope of f and hands f the exclusive TeamExec authority over them and the calling thread.

The calling thread is participant 0 (the leader); the members are pool workers that stay reserved, spinning and serving no Rayon job, until f returns (or unwinds). A serial or budget-one token yields a team of one without touching the pool.

Acquisition never waits for a Rayon job that is already running. Idle members of an earlier team (see WaitPolicy) are claimed with a compare-and-swap; missing ones are requested from the pool, and the call waits at most the pool’s acquisition timeout (PoolBuilder::acquire_timeout) for idle workers to wake. If not all members are there by then, every tentative claim is released and nothing runs. Before f is called, every participant has acknowledged on its own thread.

The token stays with the caller (this takes &mut self), so a refused acquisition leaves it available for a barrier-free fallback such as fork. It is conserved as usual: while f runs the token is borrowed.

This call opens a root team scope: it reads the calling thread’s affinity mask, which decides two things. With PoolBuilder::pin_participants on it places every member on its own CPU of that mask for the scope (the leader is never pinned). Whether or not pinning is on, it records whether the scope is oversubscribed (more participants than CPUs in the mask), which decides how the members park and how a joining leader waits. See the design record.

Errors

Refused::Unavailable when the members could not be reserved.

Examples

use threadteam::{Exec, Pool, Refused};

let pool = Pool::builder().threads(4).acquire_timeout(std::time::Duration::from_secs(10)).build()?;
let mut exec = Exec::new(&pool, 4)?;
match exec.team(|team| team.size().get()) {
    Ok(n) => assert!(n >= 1),
    Err(Refused::Unavailable) => exec.fork(4, |_, _| {}), // barrier-free fallback
    Err(_) => unreachable!(),
}

pub fn team_with<R>( &mut self, over: WaitOverride, f: impl for<'team> FnOnce(TeamExec<'team>) -> R, ) -> Result<R, Refused>

team with a scoped override of the waiting policy.

The override applies to this operation only and is inherited by the children of the team; the pool’s defaults are not changed. It governs the idle interval after this reservation ends and never weakens the reservation itself.

Errors

Refused::Unavailable when the members could not be reserved.

Trait Implementations

impl Debug for Exec<'_>

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.