As the shared last level cache (LLC) in multicore processors has been shown to be a critical resource for system performance, much work has been proposed for improving the quality of service (QoS) and throughput on LLC. Cache Allocation Technology (CAT) and Adaptive Cache Replacement Policies (ACRP) are two of the techniques that are featured in recent Intel processors. CAT implements way partitioning and provides the ability to control the cache space allocation among cores. ACRP works with multiple replacement policies and enables the cache to adapt to the cache replacement policy with less cache misses. In this paper, we first show an interesting finding that ACRP technique can violate the performance isolation provided by CAT. We find the cause for this problem is that the ACRP chooses the cache replacement policy upon the global information even although the cache space partitioning is being enabled by CAT. As the result, the cache/performance isolation can be impaired by the interference on cache replacement policy. To deal with this problem, we propose a low overhead method to predict the worst execution time degradation caused by the replacement policy adaptation. Thus, in the partitioned cache space, if the worst execution time estimated by our method is not beyond the response time required for this program, the QoS for this program can be quaranteed no matter how the cache replacement policies alternate.