On Wednesday 27 August 2008 06:01, Mike Travis wrote: > Dave Jones wrote: > ... > > > But yes, for this to be even remotely feasible, there has to be a > > negligable performance cost associated with it, which right now, we > > clearly don't have. Given that the number of people running 4096 CPU > > boxes even in a few years time will still be tiny, punishing the common > > case is obviously absurd. > > > > Dave > > I did do some fairly extensive benchmarking between configs of NR_CPUS = > 128 and 4096 and most performance hits were in the neighborhood of < 5% on > systems with 8 cpus and 4GB of memory (our most common test system). 5% is a pretty nasty performance hit... what sort of benchmarks are we talking about here? I just made some pretty crazy changes to the VM to get "only" around 5 or so % performance improvement in some workloads. What places are making heavy use of cpumasks that causes such a slowdown? Hopefully callers can mostly be improved so they don't need to use cpumasks for common cases. Until then, it would be kind of sad for a distro to ship a generic x86 kernel and lose 5% performance because it is set to 4096 CPUs... But if I misunderstand and you're talking about specific microbenchmarks to find the worst case for huge cpumasks, then I take that back. > [But > changing cpumask_t's to be pointers instead of values will likely increase > this.] I've tried to be very sensitive to this issue with all my previous > changes, so convincing the distros to set NR_CPUS=4096 would be as painless > for them as possible. ;-) > > Btw, huge count cpu systems I don't think are that far away. I believe the > nextgen Larabbee chips will be geared towards HPC applications [instead of > just GFX apps], and putting 4 of these chips on a motherboard would add up > to 512 cpu threads (1024 if they support hyperthreading.) It would be quite interesting if they make them cache coherent / MP capable. Will they be? -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
