POSIX-style read() and write() have long been the standard interface for accessing file data. However, the data copy their semantics require imposes an unnecessary overhead on accessing files stored in persistent memories attached to the processor memory bus (PMEM). PMEM-aware file systems provide direct-access (DAX)-based mmap() to avoid the copy, but it forces the programmer to manage concurrency control and atomicity guarantees.
We propose a new IO interface, called SubZero IO, that avoids data movement overheads when accessing persistent memory-backed files while still interacting cleanly with legacy read() and write(). SubZero IO provides two new system calls – peek() and patch() – that allow for data access and modification without a copy. They can improve performance significantly, but they require substantial changes to how an application performs IO. To avoid this, we describe a PMEM-aware implementation of read() and write() that can transparently provide most of the benefits of peek() and patch() for some applications.
Measurements of simple benchmarks show that SubZero outperforms copy-based read() and write() by up to 2× and 6×, respectively. At the application level, peek() improves GET performance of the Apache Web Server by 3.8×, and patch() boosts the SET performance of Kyoto Cabinet up to 2.7×.
