How to flush cache in windows

How can I do a CPU cache flush in x86 Windows?

I am interested in forcing a CPU cache flush in Windows (for benchmarking reasons, I want to emulate starting with no data in CPU cache), preferably a basic C implementation or Win32 call.

Is there a known way to do this with a system call or even something as sneaky as doing say a large memcpy ?

Intel i686 platform (P4 and up is okay as well).

4 Answers 4

Fortunately, there is more than one way to explicitly flush the caches.

The instruction «wbinvd» writes back modified cache content and marks the caches empty. It executes a bus cycle to make external caches flush their data. Unfortunately, it is a privileged instruction. But if it is possible to run the test program under something like DOS, this is the way to go. This has the advantage of keeping the cache footprint of the «OS» very small.

Additionally, there is the «invd» instruction, which invalidates caches without flushing them back to main memory. This violates the coherency of main memory and cache, so you have to take care of that by yourself. Not really recommended.

For benchmarking purposes, the simplest solution is probably copying a large memory block to a region marked with WC (write combining) instead of WB. The memory mapped region of the graphics card is a good candidate, or you can mark a region as WC by yourself via the MTRR registers.

You can find some resources about benchmarking short routines at Test programs for measuring clock cycles and performance monitoring.

There are x86 assembly instructions to force the CPU to flush certain cache lines (such as CLFLUSH), but they are pretty obscure. CLFLUSH in particular only flushes a chosen address from L1 caches.

something as sneaky as doing say a large memcopy?

Yes, this is the simplest approach, and will make sure that the CPU flushes all levels of cache. Just exclude the cache flushing time from your benchmakrs and you should get a good idea how your program performs under cache pressure.

There is unfortunately no way to explicitly flush the cache. A few of your options are:

1.) Thrash the cache by doing some very large memory operations between iterations of the code you’re benchmarking.

2.) Enable Cache Disable in the x86 Control Registers and benchmark that. This will probably disable the instruction cache also, which may not be what you want.

3.) Implement the portion of your code your benchmarking (if it’s possible) using Non-Temporal instructions. Though, these are just hints to the processor about using the cache, it’s still free to do what it wants.

1 is probably the easiest and sufficient for your purposes.

Edit: Oops, I stand corrected there is an instruction to invalidate the x86 cache, see drhirsch’s answer

The x86 instruction WBINVD writes back and invalidates all caches. It is described as:

Writes back all modified cache lines in the processor’s internal cache to main memory and invalidates (flushes) the internal caches. The instruction then issues a special-function bus cycle that directs external caches to also write back modified data and another bus cycle to indicate that the external caches should be invalidated.

Importantly, the instruction can only be executed in ring0, i.e. the operating system. So your userland programs can’t simply use it. On Linux, you can write a kernel module that can execute that instruction on demand. Actually, someone already wrote such a kernel module: https://github.com/batmac/wbinvd

Luckily, the kernel module’s code is really tiny, so you can actually check it before loading code from strangers on the internet into your kernel. You can use that module (and trigger executing the WBINVD instruction) by reading /proc/wbinvd , for example via cat /proc/wbinvd .

However, I found that this instruction (or at least this kernel module) is really slow. On my i7-6700HQ I measured it to take 750µs! This number seems really high to me, so I might have made a mistake measuring this — please keep that in mind! Explanation of that instruction just say:

The amount of time or cycles for WBINVD to complete will vary due to size and other factors of different cache hierarchies.

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Flush DNS Cache (2020 How-To Guide)

Flush DNS to solve Internet connection-related issues

Flush DNS is a command which helps to clear outdated or corrupted DNS resolver cache. As your computer automatically caches IPs and other DNS results to make subsequent requests to the same hostname faster, it might also cache some old or bad results. As a consequence, you might not be able to communicate with hosts properly. In fact, it is one of the best methods to solve INET_E_RESOURCE_NOT_FOUND error. DNS Flush is also often used altogether with ip release renew commands.

No matter what operating system (Windows, Linux, Mac, Ubuntu) or browser you use, you can manually speed up the process and clear DNS Cache using simple steps provided below. To make the navigation easier, use Table of Contents to jump to guide for your operating system version.

The following instructions explain how to flush DNS on these operating systems:

• Windows 10, 8.1, 8, 7, Vista, 2000, NT, XP, 98;
• MacOS versions from 10.6 (Snow Leopard) to 10.15 (Catalina);
• Linux;
• Ubuntu;
• Chrome.

The method helps to solve various DNS-related issues.

How to Flush DNS Cache on Microsoft Windows

Windows 10

Flush DNS on Windows 10 is a simple command that needs to be entered in Windows Command Processor, better known as Command Prompt (CMD). You must, however, open CMD with Administrator’s privileges. Here’s what you need to do.

1. In Windows 10, use Windows search and type Command Prompt .
2. Right-click Command Prompt and choose Run as Administrator.
3. In User Account Control pop-up, click Yes.
4. In CMD, type ipconfig /flushdns and press Enter.

Follow these easy steps to complete the task on Windows 10.

Windows 8 / Windows 8.1

To flush DNS on Windows 8 or 8.1, ensure to use Command Prompt with Admin’s privileges. Follow these steps to clear your DNS cache.

1. In Windows 8/8.1, use Windows search and type Command Prompt .
2. Right-click Command Prompt and choose Run as Administrator.
3. In User Account Control pop-up, click Yes.
4. In CMD, type ipconfig /flushdns and press Enter.

Windows Vista / Windows 7

In order to flush DNS cache on Windows 7, you need to open Command Prompt as Administrator. Here’s how.

1. Click on Start >All Programs >Accessories and find Command Prompt.
2. Right-clickCommand Prompt and choose Run as Administrator.
3. In CMD, type ipconfig /flushdns and press Enter.

Windows XP / 2000 / NT / 98

If you’re looking on how to clear DNS cache on older Windows versions, look no further. All you need to do is:

1. Click on Start > Run > cmd.exe > OK.
2. Type ipconfig /flushdns and press Enter.

How to Flush DNS Cache on Mac OS

To clear DNS Cache on Mac OS X systems, follow the guidelines given below. Please pay attention to the commands shown for different Mac OS versions – not all of them are the same for every OS.

MacOS 10.15: Catalina

To flush DNS on MacOS Catalina, follow these steps.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal)
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

First Method to open Terminal in Mac.

MacOS 10.14: Mojave

Clearing DNS on MacOS Mojave is a quick and easy procedure if you follow these steps.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

MacOS 10.13: High Sierra

Flushing DNS on MacOS High Sierra can be done by following these guidelines.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

Second method to open terminal on Mac.

MacOS 10.12: Sierra

If you want to flush DNS on MacOS Sierra, follow these directions.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

This command flushes DNS cache in the majority of Mac OS X systems except Yosemite, Lion and previous versions.

Mac OS X 10.11: El Capitan

To flush DNS on MacOS X El Capitan, follow these steps.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

Mac OS X 10.10: Yosemite

In order to flush DNS on MacOS X Yosemite, carry out these steps.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo discoveryutil udnsflushcaches and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

This command clears the cache for most Yosemite OS X.

Mac OS X 10.9: Mavericks

To flush DNS on MacOS X Mavericks, follow these steps.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

Mac OS X 10.8: Mountain Lion

To clear DNS cache on MacOS X Mountain Lion, complete given instructions.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

Mac OS X 10.7: Lion

To flush DNS on MacOS X Lion, follow these steps.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo killall -HUP mDNSResponder and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

Mac OS X 10.6: Snow Leopard and older versions

To flush DNS on MacOS X Lion, follow these steps.

1. Open Terminal (Application › Utilities › Terminal or click Go > Utilities > Terminal).
2. Type sudo lookupd -flushcache and press Enter to run the command.
3. Enter your password (it might not appear on the screen as you type).
4. Press Enter. Close the Terminal.

Clear DNS Cache on Linux or Ubuntu

Linux

1. Launch Terminal by pressing Ctrl + Alt + T.
2. Linux offers various commands for DNS flushing. Here are they:
3. To clear nscd DNS cache: sudo /etc/init.d/nscd restart ;
4. To clear dnsmasq DNS cache: sudo /etc/init.d/dnsmasq restart ;
5. To clear BIND DNS cache, use one of the given commands:
   1. sudo /etc/init.d/named restart ;
   2. sudo rndc restart ;
   3. sudo rndc exec .

Linux commands for clearing domain name system cache.

Ubuntu

1. Launch Terminal by pressing Ctrl + Alt + T.
2. Type the following command: sudo /etc/init.d/dns-clean restart and press Enter.
   
3. Now, type sudo /etc/init.d/networking force-reload and press Enter.
   

Bonus: Flush DNS on Google Chrome

Google Chrome browser has an in-built caching system. In order to clear DNS cache from Chrome, complete these steps.

Open Chrome browser. Copy and paste the following line into the Chrome address bar, then press Enter.

chrome://net-internals/#dns

Click Clear host cache.

Now click on Sockets.

Click Flush socket pools.

If you want to clear DNS cache on browsers like Firefox or Safari, we suggest resetting them.

That’s it! We hope that our guide on how to flush DNS on Windows, Mac, Linux, Ubuntu or Chrome helped you to solve the problem.

This post was originally published on August 22, 2019 and updated on October 28, 2020.

Matt Corey is passionate about the latest tech news, gadgets and everything IT. Matt loves to criticize Windows and help people solve problems related to this operating system. When he’s not tinkering around with new gadgets he orders, he enjoys skydiving, as it is his favorite way to clear his mind and relax.

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