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A system call is a controlled way for a program to ask the operating-system kernel to perform work it cannot do directly in user mode. It takes time because the processor must cross a protected boundary, the kernel must handle the request, and execution must return to the program. There is no single fixed cost: the machine, kernel configuration, security mitigations, and requested operation all affect it.
What a system call does
Linux describes a system call as an entry point into the kernel and the fundamental interface between applications and the operating system. Programs commonly reach that interface through a C library function such as read() or open(), rather than issuing a raw system call themselves. The library wrapper prepares the request and can handle details such as translating a kernel error into a return value of -1 with an explanatory errno. Linux man-pages: intro(2); Linux man-pages: syscalls(2)
A library function is not necessarily a one-to-one synonym for a system call. A wrapper may do additional work, and an application-level function may not enter the kernel at all. The system call is specifically the request across the application/kernel interface.
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What happens when a Linux program makes one
- The program calls an interface. Usually it invokes a C library wrapper for the desired operation.
- The request is prepared for the platform ABI. The wrapper supplies a system-call number and arguments in the locations prescribed by the processor architecture and calling convention.
- Execution enters the kernel. A processor-defined mechanism transfers control to privileged kernel code, which establishes the state needed to handle the request.
- The kernel dispatches and handles the request. The operation may be quick, or it may involve substantial work such as interacting with a file system or waiting for I/O.
- Return processing runs before user code resumes. Linux entry/exit paths can handle work such as tracing, auditing, signals, or task work before reversing the transition and returning to user mode. The exact path depends on architecture and configuration.
The instruction, argument registers, and ABI conventions differ across architectures. The Linux syscall(2) manual documents those differences; code that bypasses a library wrapper must account for the relevant architecture-specific interface and error conventions. Linux man-pages: syscall(2)
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Why crossing the boundary takes time
The transition is more than executing one special instruction. The processor transfers control through a protected mechanism, and low-level entry code establishes or preserves state so the kernel can safely handle the request. The kernel then does the requested work, processes applicable entry or return tasks, and restores the conditions for user code to continue. Linux kernel documentation notes that transitions between execution domains require state updates with strict ordering constraints. Linux kernel documentation: Entry/exit handling
Some overhead belongs to entering and leaving the kernel; other time belongs to the operation itself. A call that simply asks the kernel to perform a small operation has a different total duration from one that blocks while waiting for a device or file-system work. If the call blocks, scheduling and external I/O latency can dominate the boundary cost.
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Security mitigations can affect the path
On systems using Linux Page Table Isolation (PTI), the kernel documentation describes page-table register changes on applicable system-call, interrupt, and exception entry and exit paths. PCID support can make page-table switching cheaper. The documented impact depends on CPU features and configuration; the documentation says the loss of global pages has a very small performance effect in its described context, never exceeding 1%. That figure is about that PTI discussion, not a universal penalty for every system call. Linux kernel documentation: Page Table Isolation
How much overhead does a system call add?
There is no portable time-per-call figure established by the available evidence. An absolute measurement needs at least the processor and architecture, kernel version and configuration, mitigation state, measurement method, and the exact operation. A benchmark that isolates entry and return measures something different from an operation that performs kernel work, blocks, or waits for I/O.
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A 2022 USENIX Annual Technical Conference paper reported that, in its evaluation, standard system-call invocation entry and exit took 28 times as long as a function call and return; with PTI enabled, the ratio was 52 times. Those are relative results from the paper’s test setup, not nanoseconds and not a promise for current hardware or every workload. USENIX ATC ’22: Reducing system call overhead
Does a system call switch processes?
Not necessarily. A system call crosses from user execution into privileged kernel execution and back; that is not the same claim as saying the scheduler switches from one process to another on every call. A call may block and lead to scheduling, but blocking and a scheduler task switch are separate effects from the basic kernel-entry transition.
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Can programs reduce system-call overhead?
Optimization depends on what the program is doing. For I/O-heavy workloads, reducing unnecessary calls or combining small operations where the required semantics allow can reduce how often a program crosses the boundary. Batched or asynchronous interfaces such as io_uring can amortize entry/exit costs in suitable cases, but they have constraints and are not general replacements for arbitrary synchronous system calls. The USENIX paper discusses these approaches and their trade-offs. USENIX ATC ’22: Reducing system call overhead
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