Memory overcommit is a strategy used by operating systems and hypervisors to allocate more memory to applications or virtual machines (VMs) than is physically available in the system. This technique leverages the fact that not all applications or VMs use their allocated memory at the same time, allowing for better utilization of system resources.
History and Development
The concept of memory overcommitment has its roots in the early days of computing when resources were extremely limited. However:
- In the late 1990s and early 2000s, as virtualization became more prevalent, virtualization platforms like Xen and KVM started to implement memory overcommitment to allow for more efficient use of server hardware.
- By 2006, VMware introduced ESXi with features like Transparent Page Sharing (TPS) to enhance memory overcommitment capabilities, making it possible to allocate more memory than physically present by sharing identical pages of memory among VMs.
How Memory Overcommit Works
Here's how memory overcommitment operates:
- Overcommit Ratio: An overcommit ratio is set, which dictates how much memory can be overcommitted. For example, a 1:2 ratio means for every 1 GB of physical memory, up to 2 GB can be allocated to VMs.
- Page Sharing: Systems use techniques like TPS or Kernel Same-page Merging (KSM) in Linux to identify and merge identical memory pages across different VMs, reducing the actual memory usage.
- Memory Ballooning: A balloon driver can be used to reclaim memory from a VM when it's not in use, allowing the host to reassign this memory to other VMs.
- Swapping: If memory pressure becomes too high, the system might swap out least-used pages to disk, although this can significantly degrade performance.
Benefits
- Resource Optimization: More VMs can run on the same physical hardware, leading to better hardware utilization and cost savings.
- Flexibility: Dynamic allocation of resources based on current demand rather than static allocation.
- Performance: When implemented correctly, it can lead to better performance by allowing the system to handle peak loads more efficiently.
Challenges and Risks
- Performance Degradation: Overcommitment can lead to performance issues if not managed well, especially when swapping to disk is required.
- System Stability: Excessive overcommitment can lead to system instability or crashes if physical memory is exhausted.
- Complexity: Managing memory overcommitment requires sophisticated algorithms and monitoring to prevent or mitigate negative impacts.
External Links
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