https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892045#M3818 Our tests show that the live HD capture performance is much worse with hyperthreading than without it on the new HP Z800 system. <BR /><BR />The new Nahalem processor has small individual L2 cache (512K per core) with an relatively large L3 cache shared by the four cores. Our threads all execute data-hungary tasks. Do you think the small L2 cache may cause too many misses with hyperthreading? Tue, 18 Aug 2009 17:58:34 GMT shyaki 2009-08-18T17:58:34Z https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892045#M3818 Our tests show that the live HD capture performance is much worse with hyperthreading than without it on the new HP Z800 system. <BR /><BR />The new Nahalem processor has small individual L2 cache (512K per core) with an relatively large L3 cache shared by the four cores. Our threads all execute data-hungary tasks. Do you think the small L2 cache may cause too many misses with hyperthreading? Tue, 18 Aug 2009 17:58:34 GMT https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892045#M3818 shyaki 2009-08-18T17:58:34Z https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892046#M3819 <DIV style="margin:0px;"> <DIV id="quote_reply" style="width: 100%; margin-top: 5px;"> <DIV style="margin-left:2px;margin-right:2px;">Quoting - <A href="https://community.intel.com/en-us/profile/262284">shyaki</A></DIV> <DIV style="background-color:#E5E5E5; padding:5px;border: 1px; border-style: inset;margin-left:2px;margin-right:2px;"><EM>Our tests show that the live HD capture performance is much worse with hyperthreading than without it on the new HP Z800 system. <BR /><BR />The new Nahalem processor has small individual L2 cache (512K per core) with an relatively large L3 cache shared by the four cores. Our threads all execute data-hungary tasks. Do you think the small L2 cache may cause too many misses with hyperthreading?</EM></DIV> </DIV> </DIV> It's certainly possible, if a thread of your application requires more than half the L1 or L2 cache, or 8 threads together require more than than the entire L3, that cache capacity problems would produce the effect you reported.<BR />If there is an advantage to specifying which threads share cores and associated caches, you would need that specification (KMP_AFFINITY for Intel OpenMP).<BR />Data intensive applications with good memory locality are likely not to benefit from HT, as you could use up the entire memory bandwidth with 1 thread per core.<BR /> Tue, 18 Aug 2009 18:13:55 GMT https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892046#M3819 TimP 2009-08-18T18:13:55Z https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892047#M3820 <DIV style="margin:0px;"></DIV> <BR />If you use vtune you should be able to see the cache behavior reported. If you are not a vtune user it is a good tool and makes use of the on chip hardware profiling to measure details of what is happening on the processor. You could compare the reports single and multi-threaded and see what is going on. Fri, 21 Aug 2009 00:27:39 GMT https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892047#M3820 Tom_Spyrou 2009-08-21T00:27:39Z https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892048#M3821 <DIV style="margin:0px;"> <DIV id="quote_reply" style="width: 100%; margin-top: 5px;"> <DIV style="margin-left:2px;margin-right:2px;">Quoting - <A href="https://community.intel.com/en-us/profile/262284">shyaki</A></DIV> <DIV style="background-color:#E5E5E5; padding:5px;border: 1px; border-style: inset;margin-left:2px;margin-right:2px;"><EM>Our tests show that the live HD capture performance is much worse with hyperthreading than without it on the new HP Z800 system. <BR /><BR />The new Nahalem processor has small individual L2 cache (512K per core) with an relatively large L3 cache shared by the four cores. Our threads all execute data-hungary tasks. Do you think the small L2 cache may cause too many misses with hyperthreading?</EM></DIV> </DIV> </DIV> <BR /> <DIV>I agree with the previous posts. You might find this guide usefulhttp://software.intel.com/en-us/articles/using-intel-vtune-performance-analyzer-to-optimize-software-on-intel-core-i7-processors/ to find out if cache misses harm your performance.</DIV> Fri, 21 Aug 2009 21:51:51 GMT https://community.intel.com/t5/Intel-Moderncode-for-Parallel/Performance-is-worse-with-hyperthreading-on-dual-xeon-w5580/m-p/892048#M3821 Roman_D_Intel 2009-08-21T21:51:51Z