JD wrote: > > On 09/25/2010 12:35 PM, James Wilkinson wrote: > >> Michael Miles wrote: >> >>> Thank's for the clear up. My question is with Hyperthreading that is if >>> each core does double duty so to speak by looking after two threads >>> would it not do basically the same work as one core full bore on one thread. >>> Is there a speed difference (faster, slower) >>> >> Good question. The answer is âit depends, but itâs usually fasterâ. >> >> Reasons why it can be faster: >> * Most modern processors can despatch up to three or four instructions >> at a time (IF the front end can identify enough instructions that >> logically can be run at the same time), but will have six to ten >> execution units to actually run the instructionsÂ. Therefore, one >> thread might be able to make use of execution units the other thread >> isnât using. >> >> * Compared to CPU speed, it takes a seriously long time to get data >> from main memory. If one thread is waiting for data to arrive, the >> other one can make full use of the processor. >> >> * Most modern CPUs do out-of-order execution, which means they can >> often find things to do while waiting for data to come from (L2/L3) >> cache. Thatâs not guaranteed, though, so the other thread might get >> more resources to play with. >> >> On the other hand, Atom isnât out-of-order, and canât do anything >> while itâs waiting for data from Level 2 cache. So the other thread >> has full run of the core. >> >> Why it can be slower: >> * The cache memory is having to look after two sets of data, not just >> one, which means thereâll be a lot more cache misses. The worst case >> example would be something like two threads, each of which are >> regularly hitting a different 6K of data, on a Pentium 4 with only 8K >> Level 1 data cache. Each thread will be constantly replacing the >> otherâs data, meaning each thread is continually having to wait for >> data from Level 2 cache. >> >> This effect was especially noticeable on Pentium 4-based CPUs: a lot of >> high-end benchmarks would be run with SMT turned off. >> >> Hope this helps, >> >> James. >> >> Â The instruction units are specialised: if a thread is 100% integer, >> the FPU units wonât be of any use to it. >> >> > Correct James. The clobbering of the cache by 2 different threads > does not depend on whether or not the cpu is hyperthreaded. > Any two threads can achieve this clobering on any cpu, and it is > often the case. > The only situation where hyperthreading will show noticeable > improvement of execution speed is where the threads are all > children of the same process and are well behaved and work > almost entirely on the parent process' data space, with proper > synchronization. However, if the parent data space and text > space is larger than the cache, then the sibling threads can > still cause cache refill every time a sibling accesses a different > data space than other siblings. Ditto with the instruction cache. > Different threads have a different set of instructions. > > My basic attitude is forget hyperthreading. IMHO it is largely > a hype! > > > Thanks for the explanation!!! One more question that I am a bit confused with If I run Hardware Lister (lshw) it tells me my Phenom 2 965 is Hyperthreaded product: AMD Phenom(tm) II X4 965 Processor vendor: Advanced Micro Devices [AMD] bus info: cpu@0 version: AMD Phenom(tm) II X4 965 Processor serial: To Be Filled By O.E.M. slot: AM2 size: 3600MHz capacity: 3600MHz width: 64 bits clock: 200MHz capabilities: mathematical co-processor, FPU exceptions reporting, wp, virtual mode extensions, debugging extensions, page size extensions, time stamp counter, model-specific registers, 4GB+ memory addressing (Physical Address Extension), machine check exceptions, compare and exchange 8-byte, on-chip advanced programmable interrupt controller (APIC), memory type range registers, page global enable, machine check architecture, conditional move instruction, page attribute table, 36-bit page size extensions, clflush, multimedia extensions (MMX), fast floating point save/restore, streaming SIMD extensions (SSE), streaming SIMD extensions (SSE2), HyperThreading, fast system calls, no-execute bit (NX), multimedia extensions (MMXExt), fxsr_opt, pdpe1gb, rdtscp, 64bits extensions (x86-64), multimedia extensions (3DNow!Ext), multimedia extensions (3DNow!), constant_tsc, rep_good, nonstop_tsc, extd_apicid, pni, monitor, cx16, popcnt, lahf_lm, cmp_legacy, svm, extapic, cr8_legacy, abm, sse4a, misalignsse, 3dnowprefetch, osvw, ibs, skinit, wdt So is this true and can it be turned on? Michael -- users mailing list users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe or change subscription options: https://admin.fedoraproject.org/mailman/listinfo/users Guidelines: http://fedoraproject.org/wiki/Mailing_list_guidelines