Performance Impact of Spectre and Meltdown Mitigations

Over the past week or so, there has been a lot of talk about the big, bad vulnerabilities called Spectre and Meltdown. For many, getting the patches out and the problem mitigated is the highest priority. However, there are many out there that are worried about the impact on performance with these latest patches.

Terry Myerson has put up a blog post addressing these worries. For those with CPU’s older than a couple years old, you may have a noticeable impact on system performance. For those with newer CPU’s, there will be an impact, but it will be much less noticeable, if at all.

Here is the summary of what we have found so far:

• With Windows 10 on newer silicon (2016-era PCs with Skylake, Kabylake or newer CPU), benchmarks show single-digit slowdowns, but we don’t expect most users to notice a change because these percentages are reflected in milliseconds.
• With Windows 10 on older silicon (2015-era PCs with Haswell or older CPU), some benchmarks show more significant slowdowns, and we expect that some users will notice a decrease in system performance.
• With Windows 8 and Windows 7 on older silicon (2015-era PCs with Haswell or older CPU), we expect most users to notice a decrease in system performance.
• Windows Server on any silicon, especially in any IO-intensive application, shows a more significant performance impact when you enable the mitigations to isolate untrusted code within a Windows Server instance. This is why you want to be careful to evaluate the risk of untrusted code for each Windows Server instance, and balance the security versus performance tradeoff for your environment.

For context, on newer CPUs such as on Skylake and beyond, Intel has refined the instructions used to disable branch speculation to be more specific to indirect branches, reducing the overall performance penalty of the Spectre mitigation. Older versions of Windows have a larger performance impact because Windows 7 and Windows 8 have more user-kernel transitions because of legacy design decisions, such as all font rendering taking place in the kernel. We will publish data on benchmark performance in the weeks ahead.