Rowhammer defenses face an awkward choice. Precise tracking requires large associative tables in the memory controller, while probabilistic row sampling consumes DRAM bandwidth even when the server is not under attack. Microsoft’s Sigries, implemented in Azure Cobalt 200, combines both and changes mode at sub-bank granularity[1].

A small table acts as gatekeeper

In light mode, each sub-bank uses a Misra-Gries counter table with no more than a few dozen entries. Benign access streams and some attacks fit within that undersized table. When its spill behavior indicates that tracking is overwhelmed, only the affected sub-bank enters heavy mode and uses row sampling. Other sub-banks continue counting, so attack overhead remains spatially limited.

Conceptual hardware view of Sigries in a cloud SoC memory controller. Small per-sub-bank counters remain in light mode for ordinary traffic, while only an overwhelmed sub-bank switches to the sampling-based heavy mode. This is not an Azure Cobalt 200 photograph or manufacturing drawing. Original figure created for this article.

Trace-driven evaluation reports that every tested commodity workload stayed in light mode and added no DRAM bandwidth traffic. Area and power are described as minimal because the tables are small and use low-power state. Under attacks that overflow the table, sampling introduces a bounded cost rather than silently losing protection.

The erratum matters

The public paper includes an erratum. The original transition from heavy back to light mode could miss a row whose count remained dangerous across the boundary. The correction overlaps the two modes through another refresh interval before leaving heavy mode. This is not a footnote to hide: it illustrates why production security hardware needs formal verification, conservative recovery, and a patchable configuration contract.

What we take from it

Sigries moves the design target from the lowest average overhead to predictable behavior in both benign and adversarial regimes. Its most transferable idea is selective escalation: use cheap tracking as a gate, then spend bandwidth only where evidence demands it. The erratum also improves the paper’s operational value by showing the boundary condition that verification and deployment must preserve.

Source and attribution

This independent digest uses original wording and an original conceptual plate. It reproduces no paper figure, table, or prose. Microsoft Research publicly provides the paper with its erratum; the proceedings version is © IEEE 2026.