Multithreading (computer architecture): Difference between revisions

Revision as of 09:44, 1 November 2018

Revision as of 16:46, 1 November 2018

Line 22:

Multiple threads can interfere with each other when sharing hardware resources such as caches or [[translation lookaside buffer]]s (TLBs). As a result, execution times of a single thread are not improved and can be degraded, even when only one thread is executing, due to lower frequencies or additional pipeline stages that are necessary to accommodate thread-switching hardware.

Overall efficiency varies; Intel claims up to 3130% improvement with its [[Hyper-Threading Technology]],<ref>{{cite web|url=http://cache-www.intel.com/cd/00/00/01/77/17705_htt_user_guide.pdf|title=Intel Hyper-Threading Technology, Technical User's Guide|archive-url=https://web.archive.org/web/20100821074918/http://cache-www.intel.com/cd/00/00/01/77/17705_htt_user_guide.pdf|archive-date=2010-08-21}}</ref> while a synthetic program just performing a loop of non-optimized dependent floating-point operations actually gains a 100% speed improvement when run in parallel. On the other hand, hand-tuned [[assembly language]] programs using [[MMX (instruction set)|MMX]] or [[AltiVec]] extensions and performing data prefetches (as a good video encoder might) do not suffer from cache misses or idle computing resources. Such programs therefore do not benefit from hardware multithreading and can indeed see degraded performance due to contention for shared resources.

From the software standpoint, hardware support for multithreading is more visible to software, requiring more changes to both application programs and operating systems than multiprocessing. Hardware techniques used to support [[thread (computer science)|multithreading]] often parallel the software techniques used for [[multitasking of computer programs|computer multitasking]]. Thread scheduling is also a major problem in multithreading.