a) You have to be on the same machine, and it is probably most effective if you are on the same core, since it is a cache-based probe. If the attacker does not share the same cache as the victim, this attack will not work.
b) If there are more that just the attacker and the victim on the same machine (core), it is very likely to add a lot of noise to the side channel (attack) signal since it will be busy "dirtying" the cache that the side channel attack is being carried out on. Very likely this will prevent the attack from being successful.
b) If there are more that just the attacker and the victim on the same machine (core), it is very likely to add a lot of noise to the side channel (attack) signal since it will be busy "dirtying" the cache that the side channel attack is being carried out on. Very likely this will prevent the attack from being successful.