{"id":2786,"date":"2024-12-03T04:06:56","date_gmt":"2024-12-03T04:06:56","guid":{"rendered":"https:\/\/codeinsightacademy.com\/blog\/?p=2786"},"modified":"2024-12-05T04:25:38","modified_gmt":"2024-12-05T04:25:38","slug":"how-to-clear-ram-clear-cache","status":"publish","type":"post","link":"https:\/\/codeinsightacademy.com\/blog\/linux\/how-to-clear-ram-clear-cache\/","title":{"rendered":"How to clear RAM – Clear Cache – Drop Cache"},"content":{"rendered":"\n

The “1, 2, 3” cache clear method in Linux refers to clearing different types of caches managed by the operating system. Each cache type is managed differently and cleared by writing a specific value to the file \/proc\/sys\/vm\/drop_caches<\/code>.<\/p>\n\n\n\n

Cache Types:<\/h3>\n\n\n\n
  1. PageCache (Value: 1<\/code>)<\/strong>:
    • PageCache stores file contents that are cached in memory to reduce disk I\/O.<\/li><\/ul><\/li>
    • Dentries and Inodes (Value: 2<\/code>)<\/strong>:
      • Dentry is a cache of directory entries, and inodes store file metadata.<\/li><\/ul><\/li>
      • PageCache + Dentries + Inodes (Value: 3<\/code>)<\/strong>:
        • Clears both PageCache and Dentry\/inode caches.<\/li><\/ul><\/li><\/ol>\n\n\n\n

          Steps to Clear Cache with Examples:<\/h3>\n\n\n\n

          1. Check Current Memory Usage<\/strong><\/h4>\n\n\n\n

          Use the free -h<\/code> command to view current memory and cached memory:<\/p>\n\n\n\n

          free -h\n<\/pre>\n\n\n\n
          Look under the “buff\/cache” column to see the cache usage.<\/p>\n\n\n\n
          2. Clear Cache<\/strong><\/h4>\n\n\n\n
          Use the following commands to clear caches. Root privileges are required, so prefix with sudo<\/code>:<\/p>\n\n\n\n
          Clear PageCache:<\/h5>\n\n\n\n
          sudo sh -c \"echo 1 > \/proc\/sys\/vm\/drop_caches\"\n<\/pre>\n\n\n\n
          Clear Dentry and Inode Cache:<\/h5>\n\n\n\n
          sudo sh -c \"echo 2 > \/proc\/sys\/vm\/drop_caches\"\n<\/pre>\n\n\n\n
          Clear Both PageCache and Dentry\/Inode Cache:<\/h5>\n\n\n\n
          sudo sh -c \"echo 3 > \/proc\/sys\/vm\/drop_caches\"\n<\/pre>\n\n\n\n
          3. Check Memory Usage Again<\/strong><\/h4>\n\n\n\n
          Run the free -h<\/code> command again to verify that the cache has been cleared.<\/p>\n\n\n\n
          Example Workflow:<\/h3>\n\n\n\n
          1. Check Memory Before Clearing:<\/strong> free -h<\/code> Output: total used free shared buff\/cache available Mem: 15Gi 4.0Gi 2.0Gi 0.5Gi 9Gi 10Gi Swap: 2Gi 0B 2Gi<\/code><\/li>
          2. Clear All Caches (PageCache, Dentries, and Inodes):<\/strong> sudo sh -c \"echo 3 > \/proc\/sys\/vm\/drop_caches\"<\/code><\/li>
          3. Check Memory After Clearing:<\/strong> free -h<\/code> Output: total used free shared buff\/cache available Mem: 15Gi 4.0Gi 7.0Gi 0.5Gi 4Gi 11Gi Swap: 2Gi 0B 2Gi<\/code><\/li><\/ol>\n\n\n\n
            Notice how the “buff\/cache” column decreases.<\/p>\n\n\n\n
            Best Practices<\/h3>\n\n\n\n
            • Clearing caches isn’t typically necessary because Linux uses unused memory to cache files and automatically clears it when needed.<\/li>
            • Use this method only for debugging or specific needs (e.g., testing performance under no cache conditions).<\/li><\/ul>\n\n\n\n
              Would you like to try this on your system now? Let me know if you need additional help!<\/p>\n\n\n\n
              Let’s break down PageCache<\/strong>, Dentries<\/strong>, and Inodes<\/strong> with simple explanations and commands you can use on your Ubuntu system to see how they work.<\/p>\n\n\n\n
              \n\n\n\n
              1. PageCache<\/strong><\/h3>\n\n\n\n
              • What it is:<\/strong>
                • PageCache stores file data read from disk in memory to speed up future reads.<\/li>
                • If a file is accessed again, the system reads it from memory instead of the slower disk.<\/li><\/ul><\/li>
                • Example:<\/strong>
                  1. Create a Large File:<\/strong> dd if=\/dev\/zero of=testfile bs=1M count=100<\/code> This creates a file testfile<\/code> of 100 MB size.<\/li>
                  2. Clear Cache:<\/strong> sudo sh -c \"echo 3 > \/proc\/sys\/vm\/drop_caches\"<\/code><\/li>
                  3. Read the File (Populates PageCache):<\/strong> time cat testfile > \/dev\/null<\/code> The first read is slow because it loads data from disk.<\/li>
                  4. Read the File Again (From PageCache):<\/strong> time cat testfile > \/dev\/null<\/code> The second read is faster as it fetches data from memory (PageCache).<\/li>
                  5. Verify Cache Usage:<\/strong> Check memory usage using: free -h<\/code><\/li><\/ol><\/li><\/ul>\n\n\n\n
                    \n\n\n\n
                    2. Dentries<\/strong><\/h3>\n\n\n\n
                    • What it is:<\/strong>
                      • A dentry<\/strong> (directory entry) cache stores metadata about directories, such as filenames and their locations in the filesystem.<\/li>
                      • This speeds up directory traversals and file lookups.<\/li><\/ul><\/li>
                      • Example:<\/strong>
                        1. Clear Cache:<\/strong> sudo sh -c \"echo 3 > \/proc\/sys\/vm\/drop_caches\"<\/code><\/li>
                        2. List a Large Directory (Populates Dentry Cache):<\/strong> ls -R \/usr > \/dev\/null<\/code><\/li>
                        3. List the Directory Again (From Dentry Cache):<\/strong> time ls -R \/usr > \/dev\/null<\/code> The second run will be faster because the directory structure is cached in memory.<\/li><\/ol><\/li><\/ul>\n\n\n\n
                          \n\n\n\n
                          3. Inodes<\/strong><\/h3>\n\n\n\n