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: >>=20 >> On 28 Sep 2010, at 19:40, Sean Bruno wrote: >>=20 >>>> If you go fully dynamic you should use mp_maxid + 1 rather than = maxcpus. >>>=20 >>> 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? >>=20 >> 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=20 > 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=20 > can be used, whereas mp_maxid is the highest CPU ID (counting from 0) = that is used. >=20 > 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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