PRACtical: Subarray-Level Counter Update and Bank-Level Recovery Isolation for Efficient PRAC Rowhammer Mitigation

TL;DR

PRACtical enhances DRAM Rowhammer mitigation by optimizing counter updates and enabling bank-level recovery, significantly improving performance and energy efficiency while maintaining security guarantees.

Contribution

It introduces a centralized increment circuit for faster counter updates and bank-level recovery, reducing overhead and performance degradation in Rowhammer mitigation.

Findings

8% average performance improvement

19% energy reduction

less than 6% performance degradation under attack

Abstract

As DRAM density increases, Rowhammer becomes more severe due to heightened charge leakage, reducing the number of activations needed to induce bit flips. The DDR5 standard addresses this threat with in-DRAM per-row activation counters (PRAC) and the Alert Back-Off (ABO) signal to trigger mitigation. However, PRAC adds performance overhead by incrementing counters during the precharge phase, and recovery refreshes stalls the entire memory channel, even if only one bank is under attack. We propose PRACtical, a performance-optimized approach to PRAC+ABO that maintains the same security guarantees. First, we reduce counter update latency by introducing a centralized increment circuit, enabling overlap between counter updates and subsequent row activations in other subarrays. Second, we enhance the $RFM_{ab}$ mitigation by enabling bank-level granularity: instead of stalling the entire channel, only affected banks are paused. This is achieved through a DRAM-resident register that identifies attacked banks. PRACtical improves performance by 8% on average (up to 20%) over the state-of-the-art, reduces energy by 19%, and limits performance degradation from aggressive performance attacks to less than 6%, all while preserving Rowhammer protection.