[symfony/cache] Big time shifting for items with small TTL #31573
Description
Description
Recently we've faced problem with memcached expiry time shifting by +-1 second. This problem corresponds with fact that there is no resolution clock in memcached. So if I set cache with expire time 3 seconds, it will be there at least 2. But when I use symfony/cache wrapper, I as a rule get result with less than 2 seconds. And I don't know why yet. One of the reason may be long execution stack, another is that in Memcached there are two ways to set expiration time: in seconds less than seconds in 30 days, and in timestamp. In symfony/cache wrapper, expiry always sets as time() + expiry_seconds (second option) here: symfony/cache/CacheItem.php:97.
How to reproduce
put following code inside test.php:
<?php
require_once 'vendor/autoload.php';
use Symfony\Component\Cache\Adapter\MemcachedAdapter;
$m = new Memcached();
$m->addServer('localhost', 11211);
$m->set('test', true, (int)$argv[1]);
$time = 0;
while ($m->get('test')) {
usleep(100 * 1000);
$time += 0.1;
var_dump("Memcached: $time");
}
$cacheClient = MemcachedAdapter::createConnection(['memcached://localhost:11211']);
$cache = new MemcachedAdapter($cacheClient);
$item = $cache->getItem('test');
$item->expiresAfter((int)$argv[1]);
$cache->save($item);
$time = 0;
while ($cache->getItem('test')->isHit()) {
usleep(100 * 1000);
$time += 0.1;
var_dump("Symfony: $time");
}docker run -p 11211:11211 -d memcached
docker run -it --rm --network=host -v $PWD:/src -w /src chialab/php:7.3 php test.php 3
Example output
string(14) "Memcached: 0.1"
string(14) "Memcached: 0.2"
string(14) "Memcached: 0.3"
string(14) "Memcached: 0.4"
...
string(14) "Memcached: 2.6"
string(14) "Memcached: 2.7"
string(14) "Memcached: 2.8"
string(12) "Symfony: 0.1"
string(12) "Symfony: 0.2"
string(12) "Symfony: 0.3"
...
string(12) "Symfony: 1.7"
string(12) "Symfony: 1.8"
You can run it hundred times and through symfony/cache, cache will not leave at least 2 seconds.