Cosy: Develop in User-Land, Run in Kernel-Mode

User applications that move a lot of data across the user-kernel boundary suffer from a serious performance penalty. We provide a framework, Compound System Calls (Cosy), to enhance the performance of such userlevel applications. Cosy provides a user-friendly mechanism to execute the data-intensive code segment of the application in the kernel. This is achieved by aggregating the data-intensive system calls and the intermediate code into a compound. This compound is executed in the kernel, avoiding redundant data copies. A Cosy version of GCC makes the formation of a Cosy compound simple. Cosy-GCC automatically converts user-defined code segments into compounds. To ensure the security of the kernel, we use a combination of static and dynamic checks. We limit the execution time of the application in the kernel by using a modified preemptible kernel. Kernel data integrity is assured by performing necessary dynamic checks. Static checks are enforced by Cosy-GCC. To study the performance benefits of our Cosy prototype, we instrumented applications such asgrep andls. These application showed an improvement of 20–80%. Our current work focuses on faster and secure execution of entire programs in the kernel without source code modification.