Date: Mon, 27 Sep 2010 14:08:51 -0700 From: Sean Bruno <seanbru@yahoo-inc.com> To: Attilio Rao <attilio@freebsd.org> Cc: "sbruno@freebsd.org" <sbruno@freebsd.org>, "current@freebsd.org" <current@freebsd.org> Subject: Re: MAXCPU preparations Message-ID: <1285621731.7245.167.camel@home-yahoo> In-Reply-To: <AANLkTini=hUrGq3dMcksf45E1q8MPpdhnkhr-gA0sW5-@mail.gmail.com> References: <1285601161.7245.7.camel@home-yahoo> <4CA0BE08.50408@freebsd.org> <1285604516.7245.16.camel@home-yahoo> <AANLkTini=hUrGq3dMcksf45E1q8MPpdhnkhr-gA0sW5-@mail.gmail.com>
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On Mon, 2010-09-27 at 12:41 -0500, Attilio Rao wrote:
> 2010/9/27 Sean Bruno <seanbru@yahoo-inc.com>:
> > On Mon, 2010-09-27 at 08:53 -0700, Julian Elischer wrote:
> >> On 9/27/10 8:26 AM, Sean Bruno wrote:
> >> > Does this look like an appropriate modification to libmemstat?
> >> >
> >> > Sean
> >> >
> >> >
> >> > ==== //depot/yahoo/ybsd_7/src/lib/libmemstat/memstat.h#4
> >> > - /home/seanbru/ybsd_7/src/lib/libmemstat/memstat.h ====
> >> > @@ -28,12 +28,13 @@
> >> >
> >> > #ifndef _MEMSTAT_H_
> >> > #define _MEMSTAT_H_
> >> > +#include<sys/param.h>
> >> >
> >> > /*
> >> > * Number of CPU slots in library-internal data structures. This
> >> > should be
> >> > * at least the value of MAXCPU from param.h.
> >> > */
> >> > -#define MEMSTAT_MAXCPU 64
> >> > +#define MEMSTAT_MAXCPU MAXCPU /* defined in
> >> > sys/${ARCH}/include/param.h */
> >> >
> >>
> >>
> >> wouldn't it be better to do a sysctlbyname() and use the real value
> >> for the system?
> >>
> >
> > That was my initial thought (as prodded by scottl and peter).
> >
> > If it is made dynamic, could this be opening a race condition where the
> > call to sysctlbyname() returns a count of CPUS that is in turn changed
> > by the offlining of a CPU? Or am I thinking to much about this?
>
> We still can't support CPU hotplugging so the easy answer is 'don't
> bother about variadic CPUs number'.
> I don't really know what libmemstat is willing to do with that macro
> (and I don't have time to look at it now) maybe you could shade a
> light about what's its usage? Does it really needs to know MAXCPUS or
> just wants a large enough value to fill anything?
>
> Thanks,
> Attilio
>
>
Give or take, MEMSTAT_MAXCPU is used to allocate an array of per cpu
stats (see lib/libmemstat/memstat_internal.h::struct memory_type).
If the number of probed CPUs is more that this value, the library
returns an error.
Sean
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