On Fri, 2025-02-21 at 13:52 +0000, Matthew Wilcox wrote: > I don't think we should add a command line option (ie blame the > sysadmin > for getting it wrong). Instead, we should figure out the right > number. > Is it half the number of threads per socket? A quarter? 90%? It's > bootup, the threads aren't really doing anything else. But we > should figure it out, not the sysadmin. I don't think we will find a number that delivers the best performance on every system out there. With the two systems we tested, we already see some differences. The Skylake servers have 36 threads per socket and deliver the best performance when we use 8 threads which is 22%. Using more threads decreases the performance. On Cascade Lake with 48 threads per socket, we see the best performance when using 32 threads which is 66%. Using more threads also decreases the performance here (not included in the table obove). The performance benefits of using more than 8 threads are very marginal though. I'm completely open to change the default so something that makes more sense. From the experiments we did so far, 25% of the threads per node deliver a reasonable good performance. We could still keep the parameter for sysadmins that want to micro-optimize the bootup time though.
