Linux clear buffer cache

How to clear the buffer/pagecache (disk cache) under Linux

Are you facing a performance issue and you suspect it might be related to cache usage? High cache usage should not normally cause performance issues, but it might be the root cause in some rare cases.

What is Memory Cache

In order to speed operations and reduce disk I/O, the kernel usually does as much caching as it has memory By design, pages containing cached data can be repurposed on-demand for other uses (e.g., apps) Repurposing memory for use in this way is no slower than claiming pristine untouched pages.

What is the purpose of /proc/sys/vm/drop_caches

Writing to /proc/sys/vm/drop_caches allows one to request the kernel immediately drop as much clean cached data as possible. This will usually result in some memory becoming more obviously available; however, under normal circumstances, this should not be necessary.

How to clear the Memory Cache using /proc/sys/vm/drop_caches

Writing the appropriate value to the file /proc/sys/vm/drop_caches causes the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.

1. In order to clear PageCache only run:

2. In order to clear dentries (Also called as Directory Cache) and inodes run:

3. In order to clear PageCache, dentries and inodes run:

Running sync writes out dirty pages to disks. Normally dirty pages are the memory in use, so they are not available for freeing. So, running sync can help the ensuing drop operations to free more memory.

Page cache is memory held after reading files. Linux kernel prefers to keep unused page cache assuming files being read once will most likely to be read again in the near future, hence avoiding the performance impact on disk IO.

dentry and inode_cache are memory held after reading directory/file attributes, such as open() and stat(). dentry is common across all file systems, but inode_cache is on a per-file-system basis. Linux kernel prefers to keep this information assuming it will be needed again in the near future, hence avoiding disk IO.

How to clear the Memory Cache using sysctl

You can also Trigger cache-dropping by using sysctl -w vm.drop_caches=[number] command.

1. To free pagecache, dentries and inodes, use the below command.

2. To free dentries and inodes only, use the below command.

3. To free the pagecache only, use the below command.

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How to Drop/Flush/Clear Cache Memory or RAM in Linux (RedHat/CentOS 7/8) in 6 Best Steps

Table of Contents

In this article, I will take you through different steps to drop/flush/clear cache memory in Linux. As you might be aware Linux has very robust Memory Management System but still if you need to clear cache memory due to certain reasons then you need to do it manually.

Someday if you ran into a problem where you find that updated data is not visible or accessible from Page Cache then you might need to clear cache memory once and check if it helps. This is required because once the cache memory is cleared then System has to access the files from Disk and hence you will get the updated data. The only downside is that clearing Cache memory will slow down your systems atleast until the cache re-build takes place.

What is Cache

Cache are usually a small reserve amount of memory used generally for faster access of disk files and directories. This Cache is generally called as Page Cache in Linux.

How Page Cache Works in Linux

Page cache is the main Linux disk Cache used in Linux. System will usually add a page based on User read process request. If the requested page is not available in the Cache then the page will be added to the disk and will be available as long as it is needed. This increases the performance of Input Output Read Operations. The only criteria is that Cache should be enough memory available. Page Cache also needs to be in sync with the disk files as new changes in the file should be synced with Cached data or it will marked as dirty and eventually will be removed from the Page Cache.

How to Check Linux Cache Memory

You might be aware of free command in Linux command line to check the current memory usage of the System. Using this command, you can check the total memory, used memory, cache or buffer memory, available memory etc as can be observed from below output. As you can see from below output, 137 MB is the current buffer/Cache memory.

How to Clear Cache Memory in Linux

Example 1: How to Clear Page Cache Only

If you want to clear your disk cache then you need to run echo 1 > /proc/sys/vm/drop_caches after running sync command as shown below.

Example 2: How to Clear Page Cache Every day through crontab

You can also set a small script in crontab to clear cache memory every day as shown below. Please note that clearing cache memory everyday might slow down your system. Hence this needs to be carefully setup.

Example 3: How to Clear dentries and inodes Only

If you want to clear your dentries and inodes then you need to run echo 2 > /proc/sys/vm/drop_caches after running sync command as shown below.

Example 4: How to Clear dentries and inodes every day through crontab

You can also set a small script in crontab to clear page cache every day as shown below. Please note that clearing cache memory everyday might slow down your system. Hence this needs to be carefully setup.

Example 5: How to Clear Page Cache, dentries and inodes

If you want to clear your all disk cache, dentries and inodes then you need to run echo 3 > /proc/sys/vm/drop_caches after running sync command as shown below.

Example 6: How to Clear Page Cache, dentries and inodes every day through crontab

You can also set a small script in crontab to clear cache memory every day as shown below. Please note that clearing cache memory everyday might slow down your system. Hence this needs to be carefully setup.

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How to Clear RAM Memory Cache, Buffer and Swap Space on Linux

In this article, we will see How to Clear RAM Memory Cache, Buffer, and Swap Space on Linux. In every system we do come across caches that have unwanted files and can harm our system, the same thing happens in Linux cache and if you want to clear the cache and free some memory then Linux has many commands to do that.

To Clear Cache in Linux:

In all the Linux systems we have three options to clear cache without interrupting any services or processes.

Example 1: To Clear PageCache only

Syntax :

The command # free -h will give us the status of the memory

drop_caches is used a clean cache without killing any application, you can run the # free -h command to see the difference between used and free memory before and after clearing the cache

Example 2: To Clear dentries and inodes

Syntax:

Example 3: To Clear PageCache, dentries and inodes

Syntax:

Now using Linux Kernel, to free Buffer and Cache in Linux we will Create a shell script to auto clear RAM cache daily, through a cron scheduler task., the command vim script.sh is used to create a shell script “script.sh”

Now in script, you have to add the below syntax:

Now to set run permission, to clear ram cache, you have to call the script whenever required, setting a cron to clear RAM caches every day for 3 hours.

Example 4: To Clear Swap Space in Linux

You can clear the swap space by running the below command

Syntax :

You can run the # free -h command to see the difference between used and free memory before and after clearing the swap space

Add the above command to a cron script, Here we are going to combine this two different command into one single command, to form a proper script which will help us to clear Swap Space and RAM Cache.

echo 3 > /proc/sys/vm/drop_caches & & swapoff -a & & swapon -a & & printf ‘\n%s\n’ ‘ ‘ Ram-cache and the swap get cleared’

Now Ram cache and swap will be cleared you can run # free -h command to see

After running the command you will get output like this

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How to Clear RAM Memory Cache, Buffer and Swap Space on Linux

Like any other operating system, GNU/Linux has implemented memory management efficiently and even more than that. But if any process is eating away your memory and you want to clear it, Linux provides a way to flush or clear ram cache.

How to Clear Cache in Linux?

Every Linux System has three options to clear cache without interrupting any processes or services.

1. Clear PageCache only.

2. Clear dentries and inodes.

3. Clear pagecache, dentries, and inodes.

Explanation of the above command.

sync will flush the file system buffer. Command Separated by “;” run sequentially. The shell waits for each command to terminate before executing the next command in the sequence. As mentioned in the kernel documentation, writing to drop_cache will clean cache without killing any application/service, command echo is doing the job of writing to file.

If you have to clear the disk cache, the first command is safest in enterprise and production as “. echo 1 > ….” will clear the PageCache only. It is not recommended to use the third option above “. echo 3 >” in production until you know what you are doing, as it will clear pagecache, dentries, and inodes.

Is it a good idea to free Buffer and Cache in Linux that might be used by Linux Kernel?

Free Buffer and Cache in Linux

When you are applying various settings and want to check, if it is actually implemented specially on the I/O-extensive benchmark, then you may need to clear the buffer cache. You can drop cache as explained above without rebooting the System i.e., no downtime required.

Linux is designed in such a way that it looks into the disk cache before looking onto the disk. If it finds the resource in the cache, then the request doesn’t reach the disk. If we clean the cache, the disk cache will be less useful as the OS will look for the resource on the disk.

Moreover, it will also slow the system for a few seconds while the cache is cleaned and every resource required by OS is loaded again in the disk cache.

Now we will be creating a shell script to auto clear RAM cache daily at 2 am via a cron scheduler task. Create a shell script clearcache.sh and add the following lines.

Set execute permission on the clearcache.sh file.

Now you may call the script whenever you are required to clear the ram cache.

Now set a cron to clear RAM cache every day at 2 am. Open crontab for editing.

Append the below line, save and exit to run it at 2 am daily.

For more details on how to cron a job, you may like to check our article on 11 Cron Scheduling Jobs.

Is it a good idea to auto clear the RAM cache on the production server?

Clear RAM Cache on Linux Production Server?

No! it is not. Think of a situation when you have scheduled the script to clear ram cache every day at 2 am. Every day at 2 am the script is executed and it flushes your RAM cache. One day for whatsoever reason may be more than expected users are online on your website and seeking resources from your server.

At the same time, the scheduled script runs and clears everything in the cache. Now all the users are fetching data from the disk. It will result in a server crash and corrupt the database. So clear ram-cache only when required, and known your footsteps, else you are a Cargo Cult System Administrator.

How to Clear Swap Space in Linux?

If you want to clear Swap space, you may like to run the below command.

Also, you may add the above command to a cron script above, after understanding all the associated risks.

Now we will be combining both above commands into one single command to make a proper script to clear RAM Cache and Swap Space.

After testing both the above commands, we will run the command “free -h” before and after running the script and will check the cache.

That’s all for now, if you liked the article, don’t forget to provide us with your valuable feedback in the comments to let us know, what you think is a good idea to clear ram cache and buffer in production and Enterprise?

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How to Clear Memory Cache in Linux

Sometimes the system goes out of memory due to huge RAM is used by cached objects. In that cases, either you need to increase physical memory in the system or add more swap space. You can also instruct kernel to clear RAM memory cache on system by adding a number in /proc/sys/vm/drop_caches file.

It is safe but not recommended to clear the memory cache on a Linux system. Clearing the Memory cache in Linux systems slows down the system performance as reading files from memory is much faster than persistent disk. Since it discards cached objects from memory, it may cost a significant amount of I/O and CPU to recreate the dropped objects. This tutorial will help you to clear the memory cache on Linux/Unix system via the command line.

How to Clear Memory Cache on Linux

There are three options available to clear the memory cache in Linux. Choose one of the below options to flush the Linux system cache memory as per your requirements.

• Clear PageCache, dentries and inodes in cache memory. In short it will clear all the memory cache:
• Clear dentries and inodes only in cache memory
• Clear page cache only in cache memory

Here the first command sync is used to synchronize all the in-memory cache files to the persistent storage. The next command is separated with a “;”. Once the first command is completed, the next command will be triggered to clear cache memory.

Scheduleng the Clear Memory Cache with Crontab

You can also schedule a corn job to clear the cache on a regular basis. Schedule the following in system crontab to automatically flush cache memory at a regular interval.

Open a terminal and execute ‘crontab -e’ command to edit crontab:

Append below entry to the file:

The above cron will execute on every hour and flushes the memory cache on your system.

On the production servers, it is not recommended to schedule a clear cache command. It can lead to data corruption or data loss. So beware before running the above command in a production environment.

How to find Cached Memory in Linux

Use free command to find out cache memory uses by Linux system. The output of the free command is like below

Here the last column is showing cached memory (12953 MB) on Linux system. The -m option is used to show output MB’s.

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