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Current network intrusion detection systems are lack of controllability, manifested as significant packet loss due to the long-term resources occupation by a single flow. The reasons can be classified into two kinds. The first kind is known as normal reasons, that is, the processing of mass arriving packets of a large flow can not be limited to a determinable period of time and thus makes other flows starved. The second kind, in which the CPU is trapped in a dead-loop like state due to processing some packets with particular content of a flow, is considered as abnormal reasons. In fact, it is a kind of software crashes. In this paper, we discuss the innate defects of traditional packet-driven NIDS, and implement a flow-driven framework which can achieve fine-grained controllability. An Active Two-threshold scheme based on ideal Exit-Point (ATEP) is proposed in order to diminish data preserving overhead during flow switches and to detect crash in time. A quick crash recovery mechanism is also given which can recover the trapped thread from 90% crashes in 0.2 ms. The experimental results show that our flow-driven framework with ATEP scheme can achieve higher throughput and less packet loss ratio than the uncontrollable packet-driven systems with less than 1% of extra CPU overhead. What's more, in the case of crash occurrence, the ATEP scheme is still able to maintain rather steady throughput without sudden decrease.