Configuration

The io_context_options struct provides runtime tuning knobs for the I/O context and its backend scheduler. The defaults listed in the table below are the struct field defaults — the values you get from a freshly default-constructed io_context_options.

The inline budget defaults in the table are not what an io_context constructed from io_context_options runs with on a multi-core machine. When the effective concurrency_hint exceeds 1 and the inline budgets are still at their struct defaults, that constructor overrides them to (0, 0, 0). That disables the inline fast path; see the note under Inline Completion Budget. Pass an explicit io_context_options with non-default budgets to keep the fast path enabled in multi-threaded mode.

 
#include <boost/corosio/io_context.hpp>

corosio::io_context_options opts;
opts.max_events_per_poll = 256;
opts.inline_budget_max   = 32;

corosio::io_context ioc(opts);

Both io_context and native_io_context accept options:

#include <boost/corosio/native/native_io_context.hpp>

corosio::io_context_options opts;
opts.max_events_per_poll = 512;

corosio::native_io_context<corosio::epoll> ioc(opts);

Available Options

Option Default Backends Description

max_events_per_poll

128

epoll, kqueue

Number of events fetched per reactor poll call. Larger values reduce syscall frequency under high load; smaller values improve fairness between connections.

inline_budget_initial

2

epoll, kqueue, select

Starting inline completion budget per handler chain. After a posted handler executes, the reactor grants this many speculative inline completions before forcing a re-queue.

inline_budget_max

16

epoll, kqueue, select

Hard ceiling on adaptive inline budget ramp-up. The budget doubles each cycle it is fully consumed, up to this limit.

unassisted_budget

4

epoll, kqueue, select

Inline budget when no other thread is running the event loop. Prevents a single-threaded context from starving connections.

thread_pool_size

1

POSIX (epoll, kqueue, select)

Number of worker threads in the shared thread pool used for blocking file I/O and DNS resolution. Ignored on IOCP where file I/O uses native overlapped I/O.

locking

locking_mode::safe

all

Locking-safety tier: safe (default, full thread safety), unsafe_io (per-descriptor I/O locks off), or unsafe (all locks off). Governs the thread-safety contract. See Locking Tiers (locking) for the tiers and their restrictions.

enable_sqpoll

false

io_uring

Enable IORING_SETUP_SQPOLL. The kernel busy-polls the submission ring, so submitting becomes a userspace-only memory store and the io_uring_enter syscall leaves the submit path. Most useful for sustained traffic.

sq_thread_idle_ms

0

io_uring

Idle timeout of the SQ-poll kernel thread, in milliseconds. After that many milliseconds without submissions the thread sleeps, and the next submit re-wakes it. 0 selects the kernel default (1 s). Ignored unless enable_sqpoll is true.

sq_thread_cpu

-1

io_uring

CPU to pin the SQ-poll kernel thread to; -1 leaves it unpinned for the kernel scheduler to place. Ignored unless enable_sqpoll is true.

Options that do not apply to the active backend are silently ignored. Three options are validated on some I/O backends. On POSIX, a thread_pool_size less than 1 causes construction to throw std::invalid_argument. On the epoll, kqueue, and select I/O backends, construction throws std::out_of_range when max_events_per_poll is outside [1, INT_MAX] or inline_budget_max exceeds INT_MAX.

Tuning Guidelines

Event Buffer Size (max_events_per_poll)

The event buffer controls how many I/O events are fetched in a single epoll_wait() or kevent() call.

  • High-throughput streaming (few connections, high bandwidth): increase to 256-512 to reduce syscall overhead.

  • Many idle connections (chat servers, WebSocket hubs): keep at 128 or lower for better fairness.

Inline Completion Budget

The inline budget controls how many I/O completions the reactor completes speculatively within a single handler chain before forcing a re-queue through the scheduler.

  • Streaming workloads (file transfer, video): inline_budget_max = 32 or higher reduces context switches.

  • Request-response workloads (HTTP, RPC): keep at 16 to prevent one connection from monopolizing a thread.

  • Single-threaded contexts: unassisted_budget caps the budget when only one thread is running the event loop, preserving fairness.

  • Disable the fast path entirely: set all three options to 0 to force a re-queue on every completion. This is useful as a baseline, or when a workload is dominated by cross-thread work-stealing.

The struct defaults are (2, 16, 4), which is what a default-constructed context on a multi-core machine carries. Constructing an io_context with concurrency_hint > 1 while all three budget fields still hold those defaults overrides them to (0, 0, 0). That disables the inline fast path. Multi-thread workloads benefit from cross-thread work-stealing, which "post-everything" mode enables. Setting any budget field to a non-default value disables the override.

Thread Pool Size (thread_pool_size)

On POSIX platforms, DNS resolution uses a shared thread pool. On the epoll, kqueue, and select I/O backends, file I/O (stream_file, random_access_file) uses it too.

  • Concurrent file operations: increase to match expected parallelism (e.g. 4 for four concurrent file reads). The whole set starts at once on the first file or resolver call. A larger pool therefore makes that one call more expensive and no other.

  • No file I/O: leave at 1. The pool is created with the context, but its workers start on the first file or resolver operation, so a pool nothing uses costs nothing.

Locking Tiers (locking)

The locking option selects which internal locks the scheduler and reactor elide, trading thread-safety guarantees for reduced synchronization overhead. It governs the thread-safety contract, independently of the concurrency_hint (see Concurrency hint and locking tier).

Tier Behavior

locking_mode::safe (default)

Full thread safety. All locks enabled; any thread may use the context.

locking_mode::unsafe_io

Disables only the per-descriptor I/O locks; scheduler locking stays on. A single thread must run and drive the context, but DNS resolution and POSIX file I/O remain available (they rely on scheduler locking).

locking_mode::unsafe

Disables all locking. Eliminates 15-25% of overhead on the post-and-dispatch hot path. Imposes the full restrictions below.

 
corosio::io_context_options opts;
opts.locking = corosio::locking_mode::unsafe;

corosio::io_context ioc(opts);
ioc.run(); // only one thread may call this
The unsafe and unsafe_io tiers impose hard restrictions. Violating them is undefined behavior.

  • Only one thread may call run() (or any run/poll variant).

  • Posting work from another thread is undefined behavior.

  • Signal sets should not be shared across contexts.

  • Delay and timeout cancellation via stop_token from another thread is not permitted. The cancel path posts into the scheduler, and these tiers permit no cross-thread posting. Cross-thread cancellation requires the safe tier.

The unsafe tier additionally makes:

  • DNS resolution on POSIX return operation_not_supported.

  • File I/O (stream_file, random_access_file) on the epoll, kqueue, and select I/O backends return operation_not_supported on open().

  • win_object_handle return operation_not_supported on assign(). The Windows thread pool completes its waits from a foreign thread.

These stay available under unsafe_io because they rely on scheduler locking, which that tier keeps enabled.

These tiers correspond to Boost.Asio’s SAFE / UNSAFE_IO / UNSAFE concurrency-hint constants, exposed here as a dedicated option separate from the concurrency_hint.

Concurrency hint and locking tier

The concurrency_hint is a performance tuning value indicating how many threads are expected to call run(). It tunes the reactor inline-completion budget defaults and, on IOCP, the completion-port concurrency. The unsafe_io and unsafe tiers are single-threaded, so their effective hint for that tuning is 1 regardless of the value passed.

Edit this Page