On-demand-fork: a microsecond fork for memory-intensive and latency-sensitive applications

Fork has long been the process creation system call for Unix. At its inception, fork was hailed as an efficient system call due to its use of copy-on-write on memory shared between parent and child processes. However, application memory demand has increased drastically since the early days and the cost incurred by fork to simply set up virtual memory (e.g., copy page tables) is now a concern, even for applications that only require hundreds of MBs of memory. In practice, fork performance already holds back system efficiency and latency across a range of uses cases that fork large processes, such as fault-tolerant systems, serverless frameworks, and testing frameworks.

This paper proposes On-demand-fork, a fast implementation of the fork system call specifically designed for applications with large memory footprints. On-demand-fork relies on the observation that copy-on-write can be generalized to page tables, even on commodity hardware. On-demandfork executes faster than the traditional fork implementation by additionally sharing page tables between parent and child at fork time and selectively copying page tables in small chunks, on-demand, when handling page faults. Ondemand-fork is a drop-in replacement for fork that requires no changes to applications or hardware.

We evaluated On-demand-fork on a range of microbenchmarks and real-world workloads. On-demand-fork significantly reduces the fork invocation time and has improved scalability. For processes with 1 GB of allocated memory, On-demand-fork has a 65× performance advantage over Fork. We also evaluated On-demand-fork on testing, fuzzing, and snapshotting workloads of well-known applications, obtaining execution throughput improvements between 59% and 226% and up to 99% invocation latency reduction.