Date: Wed, 29 Sep 2010 14:54:17 +0100 From: "Robert N. M. Watson" <rwatson@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: sbruno@freebsd.org, "freebsd-current@FreeBSD.org" <freebsd-current@freebsd.org>, Joshua Neal <jdneal@gmail.com> Subject: Re: MAXCPU preparations Message-ID: <07839A8A-5CC5-47C5-B098-89FE81CA2F3E@freebsd.org> In-Reply-To: <201009290749.22669.jhb@freebsd.org> References: <1285601161.7245.7.camel@home-yahoo> <1285699244.2454.63.camel@home-yahoo> <A0A7B3BB-806C-40EA-B8FE-344EF0C8A187@freebsd.org> <201009290749.22669.jhb@freebsd.org>
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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
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