On 29 Sep 2010, at 12:49, John Baldwin wrote: > On Tuesday, September 28, 2010 6:24:32 pm Robert N. M. Watson wrote: >> >> On 28 Sep 2010, at 19:40, Sean Bruno wrote: >> >>>> If you go fully dynamic you should use mp_maxid + 1 rather than maxcpus. >>> >>> I assume that mp_maxid is the new kern.smp.maxcpus? Can you inject some >>> history here so I can understand why one is "better" than the other? >> >> So, unlike maxcpus, mp_maxid is in theory susceptible to races in a brave >> new world in which we support hotplug CPUs -- a brave new world we're > not yet ready for, however. If you do use mp_maxid, be aware you need to add > one to get the size of the array -- maxcpus is the number of CPUs that > can be used, whereas mp_maxid is the highest CPU ID (counting from 0) that is > used. > > Hmm, I'm not sure that is true. My feeling on mp_maxid is that it is the > largest supported CPUID. Platforms that support hotplug would need to set > mp_maxid such that it has room for hotpluggable CPUs. You don't want to > go reallocate the UMA datastructures for every zone when a CPU is hotplugged > for example.
Yes, we'll have to break one (or even both) of two current assumptions with the move to hotplug: contiguous in-use CPU IDs and mp_maxid representing the greatest possible CPU ID as a constant value. The former is guaranteed, but I wonder about the latter. On face value, you might say "oh, we know how many sockets there are", but if course, we don't know how many threads will arrive when a package is inserted. For many subsystems, DPCPU will present a more than adequate answer for avoiding resizing, although not for low-level systems (perhaps such as UMA?). Likewise, on virtual machine platforms where hotplug actually might reflect a longer-term scheduling choice by the admin/hypervisor (i.e., resource partitioning), it may be harder to reason about what the future holds. Robert_______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
