Rightsize memory-optimized instances safely by first installing an in-guest agent to read true memory used, because cloud providers do not report memory utilization by default. Size against sustained peak working set, not average, and separate reclaimable page cache from the real working set so you do not mistake healthy caching for a shortage. Leave headroom so peak sits around 70 to 80 percent of the new memory, because memory has no graceful degradation: a shortage is an out-of-memory kill, not just slowness. Change in a maintenance window with the prior size ready as rollback, consider a newer-generation or Graviton family such as R8g for better price performance, then monitor a full business cycle before locking in the baseline.
Last updated: June 2026
Rightsizing a memory-optimized instance means matching its memory and CPU to the workload's real demand instead of the size someone picked on day one. Memory-optimized families, the AWS R and X series, Azure E and M series, and Google Cloud M series, carry a high ratio of memory to vCPU and a high hourly price, so an oversized one wastes more per hour than any general-purpose box. This is the Cut step of FinOps: rightsize and schedule before you commit, so reservations land on a lean baseline rather than locking in waste.
This article is part of our rightsizing and waste elimination cluster. For the full picture start with the complete guide to cloud rightsizing and waste elimination, the pillar this piece links up to. Rightsizing oversized compute and clearing stale storage go together; see also how to clean up old container images and registries.
Why can't I see memory utilization by default on cloud instances?
Cloud providers cannot see inside the guest operating system, so the metrics they collect by default at the hypervisor, CPU, disk, and network, do not include memory utilization. To rightsize memory you must install an in-guest agent that reports memory used from inside the VM, such as the CloudWatch agent on AWS or the equivalent Azure and Google Cloud monitoring agents; the CloudWatch memory metrics documentation covers the AWS setup. Without that agent you are sizing blind, which is exactly why memory-optimized rightsizing fails more often than CPU rightsizing: people downsize against a metric that was never measuring memory at all.
How do I rightsize a memory-optimized instance, step by step?
Follow these six steps to shrink a memory-optimized instance without triggering an out-of-memory failure.
- Install a memory metric. Deploy the in-guest agent that reports real memory used, because the default hypervisor metrics do not show it. Everything downstream depends on having a true memory signal.
- Read peak, not average. Look at sustained peak memory and working set over a representative window that includes month-end, backups, and batch jobs, not just the rolling average, which hides the spikes that cause failures.
- Separate cache from working set. Distinguish reclaimable page cache from the true working set. A database that fills memory with cache is healthy, not starved, and reclaimable memory should not count against your headroom.
- Pick the right family and size. Choose the family and size whose memory comfortably covers peak working set plus headroom. Consider a newer-generation or Graviton family for better price performance rather than only stepping down within the same family.
- Change in a maintenance window with rollback. Resize behind a load balancer or in a rolling fashion during a maintenance window, with the prior size ready as an immediate rollback if memory pressure appears.
- Watch and lock it in. Monitor memory and out-of-memory events for a full business cycle, then record the new baseline and set an alert so future drift is caught before it becomes waste again.
Sitting on oversized memory instances you are afraid to touch?
Our cost audit installs the right memory metrics, finds the safely shrinkable instances, executes the change with rollback ready, and locks in the new baseline. On the performance model, you pay only from realized savings. No savings, no fee.
Book a cloud cost audit →How much memory headroom should I leave when rightsizing?
Leave enough headroom that sustained peak working set sits well below total memory, commonly targeting peak around 70 to 80 percent of the new instance's memory. The right margin depends on how spiky and how reclaimable the workload's memory is: a steady service can run tighter than one with sharp batch peaks. The reason memory headroom matters more than CPU headroom is that memory has no graceful degradation. When CPU saturates, work slows down and recovers; when memory runs out, the Linux out-of-memory killer terminates the process, so under-provisioning trades a small monthly saving for an outage. Headroom is insurance priced in dollars against an incident priced in downtime.
Which memory-optimized family should I choose, and should I move to Graviton?
Choose the family whose memory-to-vCPU ratio and price match the workload, and treat a Graviton move as a separate, testable step. The current AWS memory-optimized lineup, set out on the EC2 memory-optimized instances page, spans the R family for general memory-intensive work, including Graviton-based R8g and Intel R8i and R7i, and the X family such as X2idn and X2iedn for the highest memory-to-vCPU ratios, plus High Memory instances for the largest in-memory databases. Moving to a Graviton family such as R8g can improve price performance, but only once you confirm your software runs on Arm; most managed runtimes and many container images do, yet some native dependencies still need an Arm build. Verify on the Arm family first, and keep the architecture switch separate from the size change so each is independently checkable.
What is the difference between rightsizing and downsizing here?
Downsizing means moving to a smaller instance; rightsizing means matching the instance to the workload, which is sometimes smaller, sometimes a different family, and occasionally larger. A memory-optimized instance running at high CPU but low memory is mis-provisioned, not oversized, and the fix is a general-purpose or compute family rather than a smaller R instance. Treating every overprovisioned box as a downsizing problem misses these family mismatches, which are common when a workload landed on a memory family by habit. For the full distinction, see what is the difference between rightsizing and downsizing.
The Cloud Waste Audit Framework includes the memory-metric setup, the headroom thresholds, and the safe-resize runbook with rollback steps. It is the downloadable companion to this article.
Frequently asked questions
Why can't I see memory utilization by default on cloud instances?
Cloud providers cannot see inside the guest operating system, so the default metrics they collect at the hypervisor, such as CPU and network, do not include memory utilization. To rightsize memory you must install an in-guest agent, such as the CloudWatch agent or the Azure and Google Cloud monitoring agents, that reports memory used from inside the VM. Without it you are guessing, which is why memory rightsizing is riskier than CPU rightsizing.
How much memory headroom should I leave when rightsizing?
Leave enough headroom that sustained peak working set sits well below total memory, commonly targeting peak usage around 70 to 80 percent of the new instance memory. The exact margin depends on how spiky the workload is and how reclaimable its memory is. Memory has no graceful degradation: when a CPU is saturated work slows down, but when memory runs out the kernel kills the process, so the cost of under-provisioning is an outage, not just latency.
Should I move to a Graviton memory-optimized family when rightsizing?
Moving to a Graviton-based memory-optimized family such as R8g can improve price performance, and the change pairs naturally with a rightsizing pass. The condition is that your software runs on Arm, which most managed runtimes and many container images now do. Test on the Arm family before committing, because some native dependencies still need an Arm build, and treat the architecture switch and the size change as separate, verifiable steps.
What is the difference between rightsizing and downsizing a memory-optimized instance?
Downsizing simply means moving to a smaller instance, while rightsizing means matching the instance to the workload, which sometimes means a smaller size, sometimes a different family, and occasionally a larger size. A memory-optimized instance pinned at high CPU but low memory is not rightsized by downsizing; it may need a general-purpose or compute family instead. Rightsizing is the decision; downsizing is one possible outcome of it.
The short version
Rightsize memory-optimized instances safely by measuring real memory with an in-guest agent, sizing against peak working set with cache excluded, leaving headroom so peak stays near 70 to 80 percent, and changing with rollback ready. The stakes are higher than CPU rightsizing because a memory shortfall is an out-of-memory kill, not slowness, so method matters. When you want the oversized instances found and shrunk safely with the savings locked in, that is exactly what our rightsizing and waste elimination service delivers.
Written by Morten Andersen and reviewed by Fredrik Filipsson, applying the See, Cut, Lock, Run method. Independent and vendor neutral.
Cloud pricing and service behavior change frequently. Verify the specifics in this guide against the providers’ own current documentation and the FinOps Foundation: FinOps Foundation Framework ↗ and FinOps Rate Optimization capability ↗. This article also reflects Cloud Cost Room’s hands-on, vendor-neutral engagement experience.