Dynamic random access memory (DRAM) is a limited resource on all platforms and must be controlled or monitored to ensure utilization is kept in check.

Cisco NX-OS uses memory in the following three ways:

• Page cache —When you access files from persistent storage (CompactFlash), the kernel reads the data into the page cache, which means that when you access the data in the future, you can avoid the slow access times that are associated with disk storage. Cached pages can be released by the kernel if the memory is needed by other processes. Some file systems (tmpfs) exist purely in the page cache (for example, /dev/sh, /var/sysmgr, /var/tmp), which means that there is no persistent storage of this data and that when the data is removed from the page cache, it cannot be recovered. tmpfs-cached files release page-cached pages only when they are deleted.

• Kernel —The kernel needs memory to store its own text, data, and Kernel Loadable Modules (KLMs). KLMs are pieces of code that are loaded into the kernel (as opposed to being a separate user process). An example of kernel memory usage is when an inband port driver allocates memory to receive packets.

• User processes —This memory is used by Cisco NX-OS or Linux processes that are not integrated in the kernel (such as text, stack, heap, and so on).

When you are troubleshooting high memory utilization, you must first determine what type of utilization is high (process, page cache, or kernel). Once you have identified the type of utilization, you can use additional troubleshooting commands to help you figure out which component is causing this behavior.

If page cache and kernel issues have been ruled out, utilization might be high as a result of some user processes taking up too much memory or a high number of running processes (due to the number of features enabled).

Note	Cisco NX-OS defines memory limits for most processes (rlimit). If this rlimit is exceeded, sysmgr will crash the process, and a core file is usually generated. Processes close to their rlimit may not have a large impact on platform utilization but could become an issue if a crash occurs.

Cisco NX-OS has built-in kernel monitoring of memory usage to help avoid system hangs, process crashes, and other undesirable behavior. The platform manager periodically checks the memory utilization (relative to the total RAM present) and automatically generates an alert event if the utilization passes the configured threshold values. When an alert level is reached, the kernel attempts to free memory by releasing pages that are no longer needed (for example, the page cache of persistent files that are no longer being accessed), or if critical levels are reached, the kernel will kill the highest utilization process. Other Cisco NX-OS components have introduced memory alert handling, such as the Border Gateway Protocol's (BGP's) graceful low memory handling, that allows processes to adjust their behavior to keep memory utilization under control.