Scalable and Effective Page-table and TLB management on NUMA Systems

Memory management operations that modify page-tables, typically performed during memory allocation/deallocation, are infamous for their poor performance in highly threaded applications, largely due to process-wide TLB shootdowns that the OS must issue due to the lack of hardware support for TLB coherence. We study these operations in NUMA settings, where we observe up to 40x overhead for basic operations such as munmap or mprotect. The overhead further increases if pagetable replication is used, where complete coherent copies of the page-tables are maintained across all NUMA nodes. While eager system-wide replication is extremely effective at localizing page-table reads during address translation, we find that it creates additional penalties upon any page-table changes due to the need to maintain all replicas coherent.

In this paper, we propose a novel page-table management mechanism, called Hydra, to enable transparent, on-demand, and partial page-table replication across NUMA nodes in order to perform address translation locally, while avoiding the overheads and scalability issues of system-wide full page-table replication. We then show that Hydra's precise knowledge of page-table sharers can be leveraged to significantly reduce the number of TLB shootdowns issued upon any memory-management operation. As a result, Hydra not only avoids replication-related slowdowns, but also provides significant speedup over the baseline on memory allocation/deallocation and access control operations. We implement Hydra in Linux on x86_64, evaluate it on 4-and 8-socket systems, and show that Hydra achieves the full benefits of eager page-table replication on a wide range of applications, while also achieving a 12% and 36% runtime improvement on Webserver and Memcached respectively due to a significant reduction in TLB shootdowns.