The linux file system cache (Page Cache) is used to make IO operations faster. Under certain circumstances an administrator or developer might want to manually clear the cache. In this article we will explain how the Linux File System cache works. Then we will demonstrate how to monitor the cache usage and how to clear the cache. We will do some simple performance experiments to verify the cache is working as expected and that the cache flush and clear procedure is also working as expected.
How Linux File System Cache Works
The kernel reserves a certain amount of system memory for caching the file system disk accesses in order to make overall performance faster. The cache in linux is called the Page Cache. The size of the page cache is configurable with generous defaults enabled to cache large amounts of disk blocks. The max size of the cache and the policies of when to evict data from the cache are adjustable with kernel parameters. The linux cache approach is called a write-back cache. This means if data is written to disk it is written to memory into the cache and marked as dirty in the cache until it is synchronized to disk. The kernel maintains internal data structures to optimize which data to evict from cache when more space is needed in the cache.
During Linux read system calls, the kernel will check if the data requested is stored in blocks of data in the cache, that would be a successful cache hit and the data will be returned from the cache without doing any IO to the disk system. For a cache miss the data will be fetched from IO system and the cache updated based on the caching policies as this same data is likely to be requested again.
When certain thresholds of memory usage are reached background tasks will start writing dirty data to disk to ensure it is clearing the memory cache. These can have an impact on performance of memory and CPU intensive applications and require tuning by administrators and or developers.
Using Free command to view Cache Usage
We can use the free command from the command line in order to analyze the system memory and the amount of memory allocated to caching. See command below:
\# free -m
Proc Sys VM Drop Caches Command
The linux kernel provides an interface to drop the cache let’s try out these commands and see the impact on the free setting.
The linux file system cache (Page Cache) is used to make IO operations faster. Under certain circumstances an administrator or developer might want to manually clear the cache. In this article we will explain how the Linux File System cache works. Then we will demonstrate how to monitor the cache usage and how to clear the cache. We will do some simple performance experiments to verify the cache is working as expected and that the cache flush and clear procedure is also working as expected.
How Linux File System Cache Works
The kernel reserves a certain amount of system memory for caching the file system disk accesses in order to make overall performance faster. The cache in linux is called the Page Cache. The size of the page cache is configurable with generous defaults enabled to cache large amounts of disk blocks. The max size of the cache and the policies of when to evict data from the cache are adjustable with kernel parameters. The linux cache approach is called a write-back cache. This means if data is written to disk it is written to memory into the cache and marked as dirty in the cache until it is synchronized to disk. The kernel maintains internal data structures to optimize which data to evict from cache when more space is needed in the cache.
During Linux read system calls, the kernel will check if the data requested is stored in blocks of data in the cache, that would be a successful cache hit and the data will be returned from the cache without doing any IO to the disk system. For a cache miss the data will be fetched from IO system and the cache updated based on the caching policies as this same data is likely to be requested again.
When certain thresholds of memory usage are reached background tasks will start writing dirty data to disk to ensure it is clearing the memory cache. These can have an impact on performance of memory and CPU intensive applications and require tuning by administrators and or developers.
Using Free command to view Cache Usage
We can use the free command from the command line in order to analyze the system memory and the amount of memory allocated to caching. See command below:
Proc Sys VM Drop Caches Command
The linux kernel provides an interface to drop the cache let’s try out these commands and see the impact on the free setting.