Performance Costs of FFI Calls
When you call native C functions from PHP using FFI, there are several sources of overhead that can affect your program's performance. The first is type conversion: PHP values (zval) must be converted to their C equivalents before the C function can use them, and the results must be converted back to PHP types. This process is not free, especially for complex types or large data structures. Next, libffi setup comes into play. Each FFI call requires libffi to construct a call frame, which involves preparing arguments, allocating memory, and managing the function's calling conventions. This setup is necessary for every call, regardless of how simple the function is. Finally, boundary crossing between PHP and C introduces additional context switching costs. Each time you cross the boundary, the PHP engine must pause its execution, hand over control to the C function, and then resume PHP execution after the call returns. All these steps together mean that FFI calls are not as fast as pure PHP function calls or native C code running in a PHP extension. For performance-critical code, especially in tight loops or small, frequent operations, FFI may introduce unacceptable overhead. FFI is most suitable for less frequent, heavier operations where the cost of crossing the language boundary is amortized over significant work performed in C.
benchmark_ffi_vs_php.php
1234567891011121314151617181920212223242526272829303132<?php // Benchmarking a C abs() function via FFI vs. a pure PHP equivalent // Load the standard C library and define the abs() function $ffi = FFI::cdef( "int abs(int x);", "libc.so.6" ); function php_abs(int $x): int { return $x < 0 ? -$x : $x; } $iterations = 1000000; $input = -12345; // Benchmark FFI call $start = microtime(true); for ($i = 0; $i < $iterations; $i++) { $ffi->abs($input); } $ffi_time = microtime(true) - $start; // Benchmark PHP call $start = microtime(true); for ($i = 0; $i < $iterations; $i++) { php_abs($input); } $php_time = microtime(true) - $start; echo "FFI abs() time: $ffi_time seconds\n"; echo "PHP abs() time: $php_time seconds\n";
A common mistake is to overuse FFI for small, frequent operations that could easily be implemented in pure PHP. Each FFI call—no matter how trivial—incurs the overhead of type conversion, call frame setup, and context switching. When such calls are made repeatedly in performance-critical code, the accumulated overhead can significantly degrade performance, making the program much slower than a pure PHP solution.
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Can you give examples of when using FFI is appropriate in PHP?
What are some alternatives to FFI for performance-critical code in PHP?
How can I measure the overhead introduced by FFI in my PHP application?
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When you call native C functions from PHP using FFI, there are several sources of overhead that can affect your program's performance. The first is type conversion: PHP values (zval) must be converted to their C equivalents before the C function can use them, and the results must be converted back to PHP types. This process is not free, especially for complex types or large data structures. Next, libffi setup comes into play. Each FFI call requires libffi to construct a call frame, which involves preparing arguments, allocating memory, and managing the function's calling conventions. This setup is necessary for every call, regardless of how simple the function is. Finally, boundary crossing between PHP and C introduces additional context switching costs. Each time you cross the boundary, the PHP engine must pause its execution, hand over control to the C function, and then resume PHP execution after the call returns. All these steps together mean that FFI calls are not as fast as pure PHP function calls or native C code running in a PHP extension. For performance-critical code, especially in tight loops or small, frequent operations, FFI may introduce unacceptable overhead. FFI is most suitable for less frequent, heavier operations where the cost of crossing the language boundary is amortized over significant work performed in C.
benchmark_ffi_vs_php.php
1234567891011121314151617181920212223242526272829303132<?php // Benchmarking a C abs() function via FFI vs. a pure PHP equivalent // Load the standard C library and define the abs() function $ffi = FFI::cdef( "int abs(int x);", "libc.so.6" ); function php_abs(int $x): int { return $x < 0 ? -$x : $x; } $iterations = 1000000; $input = -12345; // Benchmark FFI call $start = microtime(true); for ($i = 0; $i < $iterations; $i++) { $ffi->abs($input); } $ffi_time = microtime(true) - $start; // Benchmark PHP call $start = microtime(true); for ($i = 0; $i < $iterations; $i++) { php_abs($input); } $php_time = microtime(true) - $start; echo "FFI abs() time: $ffi_time seconds\n"; echo "PHP abs() time: $php_time seconds\n";
A common mistake is to overuse FFI for small, frequent operations that could easily be implemented in pure PHP. Each FFI call—no matter how trivial—incurs the overhead of type conversion, call frame setup, and context switching. When such calls are made repeatedly in performance-critical code, the accumulated overhead can significantly degrade performance, making the program much slower than a pure PHP solution.
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