OPcache memory exhaustion and interned strings buffer limits

Answer

When only the interned strings buffer is exhausted, OPcache does not trigger a full cache restart. It stops interning new strings; subsequent identical strings are stored separately in the request‑specific memory, which can increase overall memory usage but leaves the bytecode cache intact.

Confirmed facts

• The general OPcache memory pool (opcache.memory_consumption) exhaustion causes a restart (or opcache_reset()) to free space for new bytecode.
• The interned strings buffer (opcache.interned_strings_buffer) is a separate sub‑pool used only for sharing identical string constants across scripts.
• When this sub‑pool is full, PHP emits a warning like “Interned strings buffer overflow” and ceases to intern new strings; existing interned strings remain usable.
• No automatic restart is performed for this condition.

How to verify the exhaustion state

1. Call opcache_get_status(true) (the true argument includes detailed statistics).
2. Inspect the interned_strings_usage section:

$status = opcache_get_status(true);
$usage = $status['interned_strings_usage'];
if ($usage['used_memory'] >= $usage['buffer_size']) {
    // interned strings buffer is exhausted
}

3. Optionally compare memory_usage['used_memory'] with opcache.memory_consumption to ensure the general pool still has free space.
4. Check PHP error log for the “Interned strings buffer overflow” warning; its presence confirms the condition.

Next step (if needed)

If you need to distinguish between a true restart‑triggering memory exhaustion and an interned‑strings‑only issue, provide the output of opcache_get_status() showing both memory_usage and interned_strings_usage values.