When customers deploy applications on Azure App Service Plans (ASP), they often wonder, “What’s eating my memory?” It’s a valid question, as memory usage in a shared environment like ASP can be affected by a multitude of factors beyond just the customer’s application. I’ll break down the various components that impact memory usage, using a P2v3 (16GB) instance as an example. These concepts apply across App Service Plans, but most customers are only able to see a portion of what’s consuming their memory due to system-level restrictions.
Here’s a guide to help customers better understand the various factors at play in their memory usage. It’s important to note that customers typically have access to the MemoryWorkingSet, which measures the memory actively used by their application processes, but Azure’s MemoryPercentage includes additional memory overhead for system and platform services. These services are crucial to the overall performance, stability, and security of the app environment, though they are managed by Azure and remain invisible to the customer.
Operating System and Platform Overhead
The underlying Windows operating system (OS) in the ASP environment consumes memory to manage services like networking, file systems, and security. This also includes IIS (Internet Information Services), which serves as the web server for the customer’s apps. Additionally, the Azure platform itself runs background processes to monitor app health, auto-scale resources, and manage diagnostics.
- Why it matters: This overhead can account for about 3-4% of total memory usage. While these services are necessary for the environment to run smoothly, customers do not have visibility into this memory usage.
- Customer impact: The customer doesn’t have access to system-level memory usage, which may lead to confusion when they see higher memory consumption than expected.
.NET Garbage Collection (GC) and Fragmentation
.NET applications, especially in 64-bit environments, use Garbage Collection (GC) to manage memory. This process can lead to memory fragmentation or reserved memory that hasn’t been released back to the system. Even if this memory isn’t actively being used, it’s still counted in the total memory consumption.
- Why it matters: Memory fragmentation and GC behavior can contribute to 2-3% of memory usage. Additionally, .NET may delay releasing memory in long-running applications, leading to inflated memory numbers.
- Customer impact: Customers won’t have granular access to GC cycles or memory fragmentation details, which means that memory may appear higher than what their app actively requires.
Memory Committed but Not Actively Used
Memory allocation in Windows can include committed memory—memory reserved for the application but not actively being used. Even though the memory is not actively part of the process’s working set, it’s considered allocated by the system and is counted in the memory percentage reported by Azure
- Why it matters: Committed memory can contribute another 2-3% to the total memory usage reported by Azure, even though it’s not part of the app’s current active memory.
- Customer impact: The customer won’t have direct access to the breakdown of committed vs. working memory, which can lead to confusion when the reported memory percentage appears higher than the total used by the app processes.
File Caching and I/O Buffering
Windows aggressively caches file I/O and network data to improve performance. These file system caches and network buffers take up memory and are counted toward total memory usage by Azure, but they aren’t attributed to the customer’s app processes. For example, file read/write operations performed by the app may result in disk caches that Azure includes in the total memory.
- Why it matters: File caching and I/O buffering can account for 1-2% of memory usage, even though customers don’t have access to these details. This is an essential part of optimizing the system but can skew memory metrics.
- Customer impact: If the app performs a lot of I/O operations, memory usage may appear higher due to caching, but the customer won’t have insight into this usage.
Background Services and Monitoring Tools
Azure runs several background processes for monitoring, scaling, diagnostics, and security, such as App Insights, Antimalware, and auto-heal services. These services consume memory and are accounted for in the total memory consumption of the ASP, but they aren’t related to the customer’s app processes directly.
- Why it matters: These background services could take up to 1-2% of memory but are invisible to the customer. Since these services are necessary for maintaining the app environment and ensuring uptime, memory usage from these services is part of the overall memory footprint.
- Customer impact: The customer won’t see these processes in their own app-level metrics, which might lead to confusion when trying to understand memory usage. However, it’s important to note that these services are essential for platform stability and performance monitoring.
Summing Up the Potential Overhead
For a P2v3 App Service Plan with 16GB of RAM, let’s estimate the potential overhead:
- System/Platform Overhead: 480MB – 640MB
- GC and Fragmentation: 320MB – 480MB
- Committed but Not Used Memory: 320MB
- File Caching/I/O Buffers: 160MB – 320MB
- Background Processes: 100MB – 200MB
This results in a total potential overhead of:
- Lower bound: 1.38GB
- Upper bound: 1.96GB
Why the Gap Exists
When customers review the MemoryWorkingSet for their app, they are only seeing the memory actively used by the application. On the other hand, Azure’s MemoryPercentage includes memory consumed by the app as well as the OS, platform services, background tasks, and caching mechanisms. These elements are essential to the efficient operation of the App Service Plan, even though they are outside the customer’s control and insight.
This explains why there is potentially a 5-7% gap between the customer’s observed memory usage and what Azure reports. This discrepancy is expected due to the system overhead, .NET’s reserved memory, file caching, and background services—all of which are necessary to maintain a stable, performant environment.
It’s important to understand that the “extra” memory usage reported by Azure is part of the shared hosting environment, where platform management processes ensure performance, security, and scalability. While the customer cannot directly control or observe this usage, it’s necessary to maintain a healthy, reliable, and scalable app environment in Azure. The 5-7% gap in memory usage is normal and expected and is the cost of running applications in a robust, managed service platform like Azure’s App Service Plan.
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