This paper presents a new multicore operating system with a fault tolerance mechanism called Shimos2, which runs an instance of the kernel on each CPU core. It allows administrators to designate applications as requiring "high availability and reliability," without any modifications to an application's source code. Shimos2 contains a checkpoint/restart module, and it saves the running status of the designated applications periodically to the kernel's private memory area. Furthermore, a timer daemon and a monitor daemon are employed to detect kernels that are not working as a result of the host kernel being dead or hanging due to transient hardware faults. Once the host kernel is not working anymore, the stopped processes can be restarted on an idle kernel by automatically reloading the checkpointed image from where the last checkpoint was set. We have conducted experiments to evaluate Shimos2 from various aspects, including runtime overhead. The experimental results show that compared with Shimos2 without its fault tolerance mechanism, Shimos2 imposes less than 1.1% extra overhead on the designated applications themselves, and less than 1.2% extra overhead on other applications which have not been designated as "highly available." Compared to restarting the stopped processes on another virtual machine in KVM by using BLCR, Shimos2 can save around 18% of service downtime. In this paper, we propose a reliable multicore OS called Shimos2. The motivation behind the design and implementation of Shimos2 is grounded on two main trends. First, more and more cores are available on a single CPU die, but a large number of those cores are idle, usually because the number of cores exceeds the number of active tasks. Second, as software systems become more pervasive, the impact of the consequences of their failures becomes more significant. Work stoppages caused by the failure of a highly available application bring about keenly felt lost benefits. A white paper published by a solutions provider (6) shows that the average penalty for an hour of downtime for a brokerage service is more than 6.4 million US dollars. Therefore, these two trends offer a significant opportunity and also point out the direction fault tolerance solutions should take as an important factor in designing and implementing new operating systems for multicore systems.