Why GAE memcache keeps reseting?

I am using memcache to store only integer value with a key created with a string value. I didn't define expiration value to let the values hang on as long as they can which google documations say. But the problem is oldest item stays there 4 hours at most. Usually less than 2 hours only. Suddenly everything goes zero. Here is the code:

Key mKey=null;
MemcacheService memcache = MemcacheServiceFactory.getMemcacheService();

    try {
        mKey=KeyFactory.createKey("device",deviceNo);
        if (memcache.get(mKey)==null) {
        Query query = mgr.createQuery("select d.insDate from TrialCheck d where d.DeviceId = :deviceNo");
        resultList.addAll(query.setParameter("deviceNo", deviceNo).getResultList());
            if (resultList.size()<1){
                resultList.add(0);
            } else {
                for (Integer d:resultList) {
                    memcache.put(mKey, d);
                }
            }
        } else {
            Integer installDate = (Integer) memcache.get(mKey);
            resultList.add(installDate);
        }

I read some posts saying that it is about instance going down but I couldn't see any correlation between the instance and memcache reset. Is this behaviour normal for shared memcache or the code has problem?