Date: Sat, 15 Mar 2008 17:16:46 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Joseph Koshy <joseph.koshy@gmail.com> Cc: freebsd-arch@freebsd.org Subject: Re: [PATCH] hwpmc(4) changes to use 'mp_maxid' instead of 'mp_ncpus'. Message-ID: <20080315170411.A42065@fledge.watson.org> In-Reply-To: <84dead720803142243r6c8cc68dm325e7fb925189fd@mail.gmail.com> References: <20080313180805.GA83406@dragon.NUXI.org> <200803131516.12284.jhb@freebsd.org> <84dead720803132232k15c3aad7pe59875f0c84e0c27@mail.gmail.com> <200803141431.53846.jhb@freebsd.org> <84dead720803142243r6c8cc68dm325e7fb925189fd@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sat, 15 Mar 2008, Joseph Koshy wrote:
> Therefore we can use either a count (mp_ncpus) or a maximum id (mp_maxid) to
> represent {MACHINE-MAX}, but either one would do.
>
> However, x86 MD code uses both, with newer code seeming to prefer mp_maxid.
> So I am puzzled. There are far more uses of mp_ncpus there though.
I suspect that's because kernel code wants to index into a data structure
using the CPU ID, i.e., curcpu, but don't want to size the array at MAXCPU,
which will be an increasingly large compile-time constant over time. This
relies on the relative non-sparseness of CPU IDs to be of benefit, and
generally, this does hold. For example, on the HTT boxes, CPU IDs might be
0..3 with 0 and 2 being used, and that's still less than 16 or 32. However,
in some cases we size kernel arrays to MAXCPU, and sometimes to mp_maxid.
There's a reasonable argument that sizing arrays this way is a dubious
practice as you more ideally want to store per-CPU data hung off the percpu
block to avoid adjacent per-cpu data in the same cache line.
I ran into some similar concerns when trying to figure out how best to export
memory allocator statistics from the kernel. In the end what I concluded was
that I would export contiguous CPU data up to mp_maxid from the kernel, and
that userspace would try to avoid any compile-time knowledge of CPU limits so
that it doesn't matter if a kernel is compiled for UP (MAXCPU=1) or SMP
(MAXCPU=(n), where n is often 16, I believe). I do end up exporting data for
absent CPUs under mp_maxid.
Robert N M Watson
Computer Laboratory
University of Cambridge
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20080315170411.A42065>