Stdlib: memory_reset_peak_usage() — peak counter not reset after free (VmMemory RSS, pairs #5108)

[PurHur]

Category

stdlib

Problem

memory_reset_peak_usage() is registered (#5539) and accepts zero arguments, but memory_get_peak_usage() does not drop after a large allocation is freed — unlike Zend. Issue #5108 still describes the function as missing; the remaining gap is behavioral parity of the peak counter reset.

Root cause: ext/standard/VmMemory.php tracks peak via RSS approximation (readRssBytes()). Freeing PHP variables does not shrink RSS, so resetPeakUsage() baselines peak at the still-high current RSS and get_peak_usage() reports reset_fail.

php-src reference

• php/php-src ext/standard/basic_functions.c — PHP_FUNCTION(memory_reset_peak_usage)
• php/php-src Zend/zend_alloc.c — zend_reset_peak_memory_usage()

Repro (failure today)

<?php
$before = memory_get_peak_usage();
$str = str_repeat('x', 1 << 20);
$peak = memory_get_peak_usage();
unset($str);
memory_reset_peak_usage();
$after = memory_get_peak_usage();
echo ($peak > $before ? "peak_grew\n" : "peak_flat\n");
echo ($after < $peak ? "reset_ok\n" : "reset_fail\n");

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php repro.php
php repro.php  # Zend in same container
'

Observed (VM, 2026-06-07): peak_grew + reset_fail.

Observed (Zend PHP 8.2 in container): peak_grew + reset_ok.

Also missing: bool $real_usage = false parameter and return value (tracked separately in #7247 MemoryUsage enum work).

Scope (PHP-in-PHP)

Path	Work
ext/standard/VmMemory.php	Track emalloc-style peak counters independently of RSS
ext/standard/JitMemory.php / lib/JIT/Builtin/MemoryRuntime.php	Mirror reset semantics in JIT/AOT
Tests	extend test/compliance/cases/stdlib/memory_reset_peak_usage.phpt

Implementation hints

• Increment peak on VM allocation paths / builtin helpers; do not use RSS alone for emalloc peak.
• memory_reset_peak_usage() should return bool per Zend when Stdlib: MemoryUsage enum — PHP 8.4 memory_get_usage()/memory_get_peak_usage() real_usage parameter missing (ext/standard/basic_functions.stub.php) #7247 lands; this issue focuses on counter semantics.

php-src-strict

Default CI/compliance must match Zend peak reset after unset(); no php-compiler-strict shortcut unless static proof shows no user-visible peak introspection.

Done when

• Repro prints peak_grew then reset_ok on VM (matches Zend container run)
• memory_get_peak_usage() after reset ≤ baseline taken immediately after reset call
• RSS-only path removed or demoted to $real_usage=true branch (Stdlib: MemoryUsage enum — PHP 8.4 memory_get_usage()/memory_get_peak_usage() real_usage parameter missing (ext/standard/basic_functions.stub.php) #7247)
• ./script/ci-fast.sh --filter memory_reset_peak_usage green
• Update Stdlib: memory_reset_peak_usage() — missing peak counter reset (ext/standard/basic_functions.c) #5108 comment: function exists; reset semantics fixed here

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter memory_reset_peak_usage'

Pairs

• Stdlib: memory_reset_peak_usage() — missing peak counter reset (ext/standard/basic_functions.c) #5108 (update: function exists, reset broken)
• Stdlib: memory_reset_peak_usage() — missing builtin (ext/standard/info.c) #5539, Bootstrap: memory_get_usage()/memory_get_peak_usage() — native counters without host Zend (basic_functions.c) #4862, php-in-php: VmMemory RSS path — remove host getrusage() fallback for bootstrap self-host (#4862, #3280) #7287 (VmMemory / RSS path)
• Stdlib: MemoryUsage enum — PHP 8.4 memory_get_usage()/memory_get_peak_usage() real_usage parameter missing (ext/standard/basic_functions.stub.php) #7247 ($real_usage / MemoryUsage enum)