Editing Simultaneous multithreading (section)

== Disadvantages ==
Depending on the design and architecture of the processor, simultaneous multithreading can decrease performance if any of the shared resources are bottlenecks for performance.<ref>{{cite web|title=Replay: Unknown Features of the NetBurst Core. Page 15|url=http://www.xbitlabs.com/articles/cpu/display/replay_15.html#sect0|website=Replay: Unknown Features of the NetBurst Core.|publisher=xbitlabs.com|access-date=24 April 2011|url-status=dead|archive-url=https://web.archive.org/web/20110514180659/http://www.xbitlabs.com/articles/cpu/display/replay_15.html#sect0|archive-date=14 May 2011}}</ref> Critics argue that it is a considerable burden to put on software developers that they have to test whether simultaneous multithreading is good or bad for their application in various situations and insert extra logic to turn it off if it decreases performance. Current operating systems lack convenient [[Application programming interface|API]] calls for this purpose and for preventing processes with different priority from taking resources from each other.<ref>[http://agner.org/optimize/blog/read.php?i=6&v=t How good is hyperthreading?]</ref>

There is also a security concern with certain simultaneous multithreading implementations. Intel's hyperthreading in [[NetBurst]]-based processors has a vulnerability through which it is possible for one application to steal a [[Key (cryptography)|cryptographic key]] from another application running in the same processor by monitoring its cache use.<ref>[http://www.daemonology.net/hyperthreading-considered-harmful/ Hyper-Threading Considered Harmful]</ref> There are also sophisticated machine learning exploits to HT implementation that were explained at [[Black Hat Briefings|Black Hat 2018]].<ref>[https://www.blackhat.com/us-18/briefings/schedule/#tlbleed-when-protecting-your-cpu-caches-is-not-enough-10149/ TLBleed: When Protecting Your CPU Caches is Not Enough]</ref>