At first, some background on this. I have a long-running script that maintains persistence on some connections, so if connection was reset, it retries with an exponential backoff. Sometimes it runs without issues and sometimes it eats a lot of memory and even becomes unresponsive and is getting killed by watchdog. Apart from maintaining connections, my script just reads incoming data and writes it into a file, so there are no memory leaks in my code.

After a research, I found that timed out connections produce a lot of garbage, that can be cleaned only with GC. Here is a test scenario:

$loop = React\EventLoop\Factory::create();
$loop->addPeriodicTimer(1, function () use ($loop) {
    $connector = new \React\Socket\Connector($loop, array(
        'timeout' => 0.5,
        'dns' => false,
        'tls' => false,
        'unix' => false,
    ));
    // Use host and port combination that results in connection being timed out.
    $connector->connect('89.123.45.67:3128')
        ->then(function (\React\Socket\ConnectionInterface $stream) {
            $stream->close();
            printf("[+] %.3fMB\n", memory_get_usage() / 1024 / 1024);
        })->otherwise(function (\Exception $e) {
            printf("[-] %.3fMB (%s)\n", memory_get_usage() / 1024 / 1024, $e->getMessage());
        });
});
$loop->addTimer(60, function () use ($loop) {
    printf(
        "%.3fMB => %d => %.3fMB\n",
        memory_get_usage() / 1024 / 1024,
        gc_collect_cycles(),
        memory_get_usage() / 1024 / 1024);
    $loop->stop();
});
$loop->run();

[-] 0.755MB (Timed out after 0.5 seconds)
[-] 0.786MB (Timed out after 0.5 seconds)
[-] 0.810MB (Timed out after 0.5 seconds)
...
[-] 2.114MB (Timed out after 0.5 seconds)
[-] 2.138MB (Timed out after 0.5 seconds)
[-] 2.162MB (Timed out after 0.5 seconds)
2.187MB => 9145 => 0.775MB

After doing more research, I found that is caused by promise cancellation: the more ->then() you have on cancellable promise, the more garbage it will leave after cancellation. So I tweaked code a bit and got this result:

[-] 0.749MB (Timed out after 0.5 seconds)
[-] 0.771MB (Timed out after 0.5 seconds)
[-] 0.787MB (Timed out after 0.5 seconds)
...
[-] 1.636MB (Timed out after 0.5 seconds)
[-] 1.651MB (Timed out after 0.5 seconds)
[-] 1.667MB (Timed out after 0.5 seconds)
1.684MB => 6195 => 0.761MB

Now we have 30% less garbage to collect. There is still a room for improvement, but it needs to be done with the Promise component.