John R Pierce <pierce at hogranch.com> writes: > On 10/1/2017 8:38 AM, hw wrote: >> HP says that what they call "NUMA split mode" should be disabled in the >> BIOS of the Z800 workstation when running Linux. They are reasoning >> that Linux kernels do not support this feature and even might not boot >> if it´s enabled. > > hmm, that workstation is a dual Xeon 56xx (Westmere-EP, derived from > Nehalem), new in 2010 > >> Since it apparently was years ago since they made this statement, I´m >> wondering if I should still leave this feature disabled or not. More >> recent kernels might support it, and it´s supposed to improve >> performance. >> >> Could someone explain what this feature actually is or does, and if >> Centos kernels support it? > > > On these sorts of dual socket hardware architectures, half of the > memory is directly attached to each CPU, and the two CPUs are linked > with a QPI bus. All the memory appears in one unified address space, > but the memory belonging to the 'other' CPU has a little higher > latency to access since it has to go across the QPI. In non-NUMA > mode, this is ignored, and all memory is treated as equal from the OS > perspective. in NUMA mode, an attempt is made to keep process memory > on one CPU's memory, and to prefer scheduling those processes on the > cores of that CPU. This can get messy, say you have a process running > on core 0 (in cpu0) which allocates a big block of shared memory, then > spawns 8 worker threads which all run concurrently and use this same > shared working memory space. there's only 4 or 6 cores on each of > the two CPUs, so either these worker threads have to wait for an > available core on the same CPU as the memory allocation, or some of > them end up running across the QPI bus anyways. > > I believe Linux, even RHEL 6, does support NUMA configurations, but > its very questionable if a random typical workload would actually gain > much from it, and it adds significant overhead in keeping track of all > this. Is it possible that you are confusing enabling/disabling NUMA with NUMA split mode? It is possible to disable/enable NUMA, and when NUMA is enabled, you can also enable the mysterious NUMA split mode. I´m trying to download the PDF you pointed me to, but the download is stalled. I´m running Centos 7.4, but perhaps there´s an explanation in the PDF that might tell me what NUMA split mode is supposed to be. So far, I found out that KSM is disabled by default and would probably be a disadvantage here, so I´m using numad and probably gain something from most, if not all, things using local memory instead of going across nodes. This will need some further investigation, though. -- "Didn't work" is an error.