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Firefox engineers discover a Windows Defender bug that causes high CPU usage

bugzilla.mozilla.org

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Re: Firefox engineers discover a Windows Defender bug that causes high CPU usage

#4

Post by mconley »

TL;DR: Windows Defender had a bug that made certain system calls expensive on CPU cycles when Defender's Real-time Protection feature is enabled. After discovery, Mozilla reported this issue to Microsoft. Microsoft is releasing a patch that should result in lower CPU usage when using Firefox on sites like YouTube (a ~75% CPU usage reduction was noted when browsing YouTube in Firefox with the fixed version of Defender).

It seems like the HN submission form truncated the # from the end of the URL I linked to, which linked to the relevant comment. I'll try that here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1441918#c82

and

https://bugzilla.mozilla.org/show_bug.cgi?id=1441918#c91

Re: Firefox engineers discover a Windows Defender bug that causes high CPU usage

#5

Post by ape4 »

boredumb wrote: post #3
bdcp wrote: post #2

[flagged]

Firefox engineers discovered a Windows Defender bug that causes high CPU usage.

"This problem has two sides: Microsoft was doing a lot of useless computations upon each event; and we are generating a lot of events. The combination is explosive. Now that Microsoft has done their part of the job (comment 82), we need to reduce our dependency to VirtualProtect. Bug 1822650 in particular will help with that."

Re: Firefox engineers discover a Windows Defender bug that causes high CPU usage

#8

Post by dang »

mconley wrote: post #4

TL;DR: Windows Defender had a bug that made certain system calls expensive on CPU cycles when Defender's Real-time Protection feature is enabled. After discovery, Mozilla reported this issue to Microsoft. Microsoft is releasing a patch that should result in lower CPU usage when using Firefox on sites like YouTube (a ~75% CPU usage reduction was noted when browsing YouTube in Firefox with the fixed version of Defender…

Ok, I've put that back in the URL above. Thanks.

Re: Firefox engineers discover a Windows Defender bug that causes high CPU usage

#9

Post by nvrspyx »

bdcp wrote: post #2

[flagged]

It was also fixed with a definition update in Windows Defender some time last month, so you probably have the update since these happen in the background and don't require any restart. You can check by going to:

    C:\ProgramData\Microsoft\Windows Defender\Definition Updates\{BUNCH-OF-NUMBERS}
Right click `mpengine.dll`, choose Properties, click Details tab, and check to see if Product Version is >= 1.1.20200.3. Mine is 1.1.20200.4 and was updated in mid/late March. If the version is less than 1.1.20200.3, you can manually trigger a definitions update in Windows Defender under Virus & Threat Protection.

Re: Firefox engineers discover a Windows Defender bug that causes high CPU usage

#10

Post by consumer451 »

Random thought:

I am not sure what the at-scale energy use reduction of this bug fix will be, but...

If I had a pile of money I would consider creating a special bug bounty style program for energy use reduction.

This might be a very efficient way to reduce carbon output from personal and data center computing.

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