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Some Notes about Hyper-Threading
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Robert Hyatt



Joined: 27 Feb 2006
Posts: 15816
Location: Birmingham, AL
PostPost subject: Re: Some Notes about Hyper-Threading    Posted: Mon Dec 12, 2011 6:42 pm Reply to topic Reply with quote
Werewolf wrote:
bob wrote:


No. The biggest HT gain comes from memory accesses. When you get a L1 cache miss and have to wait for 20 or so cycles or whatever for L2, or longer for L3, or MUCH longer for main memory, the other logical "core" can use the resources to continue since the first thread is "blocked" (much like what happens in a multiprogramming operating system when a process does I/O and others run while it is blocked.


Quote:

c) Chess demands 100% processing power of each core
d) Therefore HT will simply decrease performance for chess by 30%, for reasons stated above.


OK, I think I get it. But surely HT can't just _magically_ increase the performance of a core. If chess demands the core's full attention, and assuming the thread it uses is not blocked (which I'm assuming is the case) then trying to get a 2nd thread to do something on the same core would surely be like a riding a bike and trying to play the piano at the same time.

If that statement is wrong I've misunderstood things!


Here's the problem. If the CPU needs something from memory, it takes a variable number of clock cycles. Say 4 clocks for a L1 hit. Maybe 20 for a L2 hit. More for a L3 hit. And thousands of clock cycles if we have to go to real memory. That tends to stall a core since it can't proceed if all pending instructions depend on data values coming in from memory. During those "pauses" the other logical core can use that core's resources to work on a second instruction stream.

Very much as a modern operating system "interleaves" the execution of two processes as they block waiting on I/O. The more such "blocking" happens, the better that second logical core looks. If you are running completely out of L1 cache, you will barely see any HT speedup. If you don't fit in L1, and depend more on L2 or L3 or even main memory, then HT can help more...

Even though you "think" a core is 100% busy, it spends a significant amount of time waiting on data from cache and/or main memory...
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Subject Author Date/Time
Some Notes about Hyper-Threading Sedat Canbaz Fri Dec 09, 2011 3:50 pm
      Re: Some Notes about Hyper-Threading Ernest Bonnem Sat Dec 10, 2011 12:31 am
            Re: Some Notes about Hyper-Threading Sedat Canbaz Sat Dec 10, 2011 2:43 am
                  Re: Some Notes about Hyper-Threading Robert Hyatt Sat Dec 10, 2011 4:06 pm
                        Re: Some Notes about Hyper-Threading enrico fagiuoli Sat Dec 10, 2011 11:26 pm
                        Re: Some Notes about Hyper-Threading Ernest Bonnem Mon Dec 12, 2011 8:07 pm
                        Re: Some Notes about Hyper-Threading H.G.Muller Tue Jun 26, 2012 4:30 pm
                              Re: Some Notes about Hyper-Threading Daniel Shawul Tue Jun 26, 2012 11:35 pm
                                    Re: Some Notes about Hyper-Threading H.G.Muller Wed Jun 27, 2012 5:40 am
                                          Re: Some Notes about Hyper-Threading Daniel Shawul Wed Jun 27, 2012 10:23 am
                                                Re: Some Notes about Hyper-Threading H.G.Muller Wed Jun 27, 2012 10:57 am
                                                      Re: Some Notes about Hyper-Threading Daniel Shawul Wed Jun 27, 2012 11:33 am
                  Re: Some Notes about Hyper-Threading Ernest Bonnem Sun Dec 11, 2011 1:19 am
                        Re: Some Notes about Hyper-Threading M ANSARI Sun Dec 11, 2011 6:06 am
                              Re: Some Notes about Hyper-Threading Sedat Canbaz Mon Dec 12, 2011 11:14 am
                        Re: Some Notes about Hyper-Threading Sedat Canbaz Sun Dec 11, 2011 8:31 am
                              Re: Some Notes about Hyper-Threading Ernest Bonnem Sun Dec 11, 2011 6:00 pm
                                    Re: Some Notes about Hyper-Threading Vincent Diepeveen Sun Dec 11, 2011 7:12 pm
                                          Re: Some Notes about Hyper-Threading Vincent Diepeveen Sun Dec 11, 2011 7:35 pm
                                                Re: Some Notes about Hyper-Threading Sedat Canbaz Mon Dec 12, 2011 2:03 am
                                                      Re: Some Notes about Hyper-Threading Vincent Lejeune Mon Dec 12, 2011 8:46 am
                                                            Re: Some Notes about Hyper-Threading Sedat Canbaz Mon Dec 12, 2011 12:04 pm
                                                                  Re: Some Notes about Hyper-Threading Vincent Lejeune Mon Dec 12, 2011 12:32 pm
                                    Re: Some Notes about Hyper-Threading Sedat Canbaz Mon Dec 12, 2011 2:37 am
                                          Re: Some Notes about Hyper-Threading Ernest Bonnem Mon Dec 12, 2011 5:24 pm
                                                Re: Some Notes about Hyper-Threading Sedat Canbaz Mon Dec 12, 2011 6:14 pm
                                                      Re: Some Notes about Hyper-Threading Robert Hyatt Mon Dec 12, 2011 6:51 pm
                                                            Re: Some Notes about Hyper-Threading Sedat Canbaz Mon Dec 12, 2011 7:27 pm
                                                                  Re: Some Notes about Hyper-Threading Robert Hyatt Tue Dec 13, 2011 5:01 pm
                                                      Re: Some Notes about Hyper-Threading Vincent Lejeune Tue Dec 13, 2011 2:10 am
                                                            Re: Some Notes about Hyper-Threading Sedat Canbaz Tue Dec 13, 2011 8:35 am
                                                            Re: Some Notes about Hyper-Threading Sujay Jagannathan Wed Jun 27, 2012 12:42 pm
                                                      Re: Some Notes about Hyper-Threading Vincent Lejeune Tue Jun 26, 2012 4:00 pm
                                                            Re: Some Notes about Hyper-Threading Sedat Canbaz Tue Jun 26, 2012 4:56 pm
                                                                  Re: Some Notes about Hyper-Threading Vincent Lejeune Wed Jun 27, 2012 7:45 am
                                                                  Re: Some Notes about Hyper-Threading Vincent Lejeune Wed Jun 27, 2012 10:22 pm
                                                Re: Some Notes about Hyper-Threading Sedat Canbaz Mon Dec 12, 2011 7:51 pm
      Re: Some Notes about Hyper-Threading Rodolfo Leoni Sat Dec 10, 2011 10:45 am
            Re: Some Notes about Hyper-Threading Sedat Canbaz Sat Dec 10, 2011 12:25 pm
                  Re: Some Notes about Hyper-Threading Albert Silver Sat Dec 10, 2011 12:33 pm
                        Re: Some Notes about Hyper-Threading Vincent Diepeveen Sat Dec 10, 2011 7:52 pm
                              Re: Some Notes about Hyper-Threading Sedat Canbaz Sat Dec 10, 2011 10:23 pm
                                    Re: Some Notes about Hyper-Threading Russell Murray Sat Dec 10, 2011 11:58 pm
                                          Re: Some Notes about Hyper-Threading Wylie Garvin Sun Dec 11, 2011 12:23 am
                                                Re: Some Notes about Hyper-Threading Robert Hyatt Sun Dec 11, 2011 12:38 am
                                    Re: Some Notes about Hyper-Threading Vincent Diepeveen Sun Dec 11, 2011 4:25 pm
                  Re: Some Notes about Hyper-Threading Robert Hyatt Sat Dec 10, 2011 4:04 pm
                        Re: Some Notes about Hyper-Threading Rodolfo Leoni Sat Dec 10, 2011 8:53 pm
                              Re: Some Notes about Hyper-Threading Robert Hyatt Sun Dec 11, 2011 12:36 am
                                    Re: Some Notes about Hyper-Threading Rodolfo Leoni Sun Dec 11, 2011 2:19 pm
                                          Re: Some Notes about Hyper-Threading Robert Hyatt Sun Dec 11, 2011 3:49 pm
                                    Re: Some Notes about Hyper-Threading Sujay Jagannathan Wed Jun 27, 2012 12:36 pm
                              Re: Some Notes about Hyper-Threading Robert Flesher Sun Dec 11, 2011 7:00 pm
                                    Re: Some Notes about Hyper-Threading Vincent Diepeveen Sun Dec 11, 2011 7:27 pm
                              Re: Some Notes about Hyper-Threading Lonnie Cook Wed Jun 27, 2012 12:44 pm
                        Re: Some Notes about Hyper-Threading Carl Bicknell Mon Dec 12, 2011 9:06 am
                              Re: Some Notes about Hyper-Threading Robert Hyatt Mon Dec 12, 2011 1:31 pm
                                    Re: Some Notes about Hyper-Threading Carl Bicknell Mon Dec 12, 2011 4:00 pm
                                          Re: Some Notes about Hyper-Threading Robert Hyatt Mon Dec 12, 2011 4:31 pm
                                                Re: Some Notes about Hyper-Threading Carl Bicknell Mon Dec 12, 2011 4:54 pm
                                                      Re: Some Notes about Hyper-Threading Robert Hyatt Mon Dec 12, 2011 6:42 pm
                                                            Re: Some Notes about Hyper-Threading Robert Flesher Tue Dec 13, 2011 4:25 am
                                    Re: Some Notes about Hyper-Threading Vincent Diepeveen Tue Dec 13, 2011 6:46 pm
                                          Re: Some Notes about Hyper-Threading Robert Hyatt Wed Dec 14, 2011 4:30 am
                                                Re: Some Notes about Hyper-Threading Sedat Canbaz Wed Dec 14, 2011 1:01 pm
                                                      Re: Some Notes about Hyper-Threading Robert Hyatt Wed Dec 14, 2011 7:35 pm
                                                            Re: Some Notes about Hyper-Threading Sedat Canbaz Wed Dec 14, 2011 8:22 pm
                                                      Re: Some Notes about Hyper-Threading Sedat Canbaz Wed Dec 14, 2011 7:39 pm
                                                            Re: Some Notes about Hyper-Threading Robert Hyatt Wed Dec 14, 2011 10:43 pm
                                                                  Re: Some Notes about Hyper-Threading Sedat Canbaz Thu Dec 15, 2011 2:21 am
                                                                        Re: Some Notes about Hyper-Threading Robert Hyatt Fri Dec 16, 2011 5:31 pm
                                                                              Re: Some Notes about Hyper-Threading Sedat Canbaz Fri Dec 16, 2011 6:55 pm
                                                                                    Re: Some Notes about Hyper-Threading Robert Hyatt Sat Dec 17, 2011 3:52 pm
                                                                  Re: Some Notes about Hyper-Threading Sedat Canbaz Thu Dec 15, 2011 2:34 am
                                                                        Re: Some Notes about Hyper-Threading Sedat Canbaz Thu Dec 15, 2011 3:13 am
                                                                              Re: Some Notes about Hyper-Threading Sedat Canbaz Fri Dec 16, 2011 11:27 am
                                                                                    Re: Some Notes about Hyper-Threading Sedat Canbaz Fri Dec 16, 2011 12:19 pm
                                                                                    Re: Some Notes about Hyper-Threading Robert Hyatt Fri Dec 16, 2011 11:25 pm
                                                                                          Re: Some Notes about Hyper-Threading Sedat Canbaz Sat Dec 17, 2011 12:26 am
                                                                                          Re: Some Notes about Hyper-Threading Sedat Canbaz Sat Dec 17, 2011 1:27 am
                                                                                          Re: Some Notes about Hyper-Threading Robert Hyatt Sat Dec 17, 2011 4:50 am
                                                                                          Re: Some Notes about Hyper-Threading Robert Hyatt Sat Dec 17, 2011 4:49 am
                                                                                          Re: Some Notes about Hyper-Threading Sedat Canbaz Sat Dec 17, 2011 10:11 am
                                                                                          Re: Some Notes about Hyper-Threading Robert Hyatt Sat Dec 17, 2011 3:54 pm
                                                                              Re: Some Notes about Hyper-Threading Robert Hyatt Fri Dec 16, 2011 11:23 pm
                                                                                    Re: Some Notes about Hyper-Threading Sedat Canbaz Sat Dec 17, 2011 12:09 am
                                                                                          Re: Some Notes about Hyper-Threading Robert Hyatt Sat Dec 17, 2011 4:52 am
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