Как очистить cache linux

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.

# sync; echo 1 > /proc/sys/vm/drop_caches

2. Clear dentries and inodes.

# sync; echo 2 > /proc/sys/vm/drop_caches

3. Clear pagecache, dentries, and inodes.

# sync; echo 3 > /proc/sys/vm/drop_caches

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.

#!/bin/bash # Note, we are using "echo 3", but it is not recommended in production instead use "echo 1" echo "echo 3 > /proc/sys/vm/drop_caches"

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.

0 2 * * * /path/to/clearcache.sh

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.

# echo 3 > /proc/sys/vm/drop_caches && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared' OR $ su -c "echo 3 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root

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 Cache on Linux

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:

What we see from the free command above is that there is 7.5 GB of RAM on this system. Of this only 209 MB is used and 6.5 MB is free. 667 MB is used in the buffer cache. Now let’s try to increase that number by running a command to generate a file of 1 Gigabyte and reading the file. The command below will generate approximately 100MB of random data and then append 10 copies of the file together into one large_file.

# dd if=/dev/random of=/root/data_file count=1400000
# for i in `seq 1 10`; do echo $i; cat data_file >> large_file; done

Now we will make sure to read this 1 Gig file and then check the free command again:

We can see the buffer cache usage has gone up from 667 to 1735 Megabytes a roughly 1 Gigabyte increase in the usage of the buffer cache.

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.

We can see above that the majority of the buffer cache allocation was freed with this command.

Experimental Verification that Drop Caches Works

Can we do a performance validation of using the cache to read the file? Let’s read the file and write it back to /dev/null in order to test how long it takes to read the file from disk. We will time it with the time command. We do this command immediately after clearing the cache with the commands above.

It took 8.4 seconds to read the file. Let’s read it again now that the file should be in the filesystem cache and see how long it takes now.

Boom! It took only .2 seconds compared to 8.4 seconds to read it when the file was not cached. To verify let’s repeat this again by first clearing the cache and then reading the file 2 times.

It worked perfectly as expected. 8.5 seconds for the non-cached read and .2 seconds for the cached read.

Conclusion

The page cache is automatically enabled on Linux systems and will transparently make IO faster by storing recently used data in the cache. If you want to manually clear the cache that can be done easily by sending an echo command to the /proc filesystem indicating to the kernel to drop the cache and free the memory used for the cache. The instructions for running the command were shown above in this article and the experimental validation of the cache behavior before and after flushing were also shown.

About the author

Linux Wolfman

Linux Wolfman is interested in Operating Systems, File Systems, Databases and Analytics and always watching for new technologies and trends. Reach me by tweeting to @linuxhint and ask for the Wolfman.

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