Date: Thu, 12 Sep 1996 23:39:32 -0700 From: erich@uruk.org To: Terry Lambert <terry@lambert.org>, smp@csn.net (Steve Passe) Cc: peter@spinner.dialix.com, rv@groa.uct.ac.za, freebsd-smp@freebsd.org Subject: Re: Intel XXpress - some SMP benchmarks Message-ID: <199609130639.XAA14603@uruk.org> In-Reply-To: Your message of "Thu, 12 Sep 1996 14:42:50 PDT." <199609122142.OAA07599@phaeton.artisoft.com>
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Terry Lambert <terry@lambert.org> writes:
> > if we have sparse tables, ie 16 entries (most of which will be empty), code
> > like the following becomes very inefficient:
> >
> > #ifdef SMP
> > for (j = i = 0; i < SPARSE_TABLE_SIZE; i++) {
> > if (p == SMPcurproc[i])
> > j++;
> > }
> > ...
>
> Use it like a hash:
>
> #ifdef SPARSE
> int cpuid[ MAX_CPUID]; /* filled out by probe sequence*/
> #define CPUID(x) (cpuid[ x])
> #else /* !SPARSE*/
> #define CPUID(x) (x)
> #endif /* !SPARSE*/
>
> Obviously, this would save one indirection per reference, which shouldn't
> be too frequent anyway.
I use a virtual CPU mapping scheme which is:
-- BSP -> virtual number 0
-- others mapped consecutively from there (doesn't really matter which
order, but I use APIC id order).
Well, the way I'm doing it now in the code I'm writing is something
like:
int apic_to_virtual[MAX_APIC_ID];
int virtual_to_apic[MAX_CPUS];
#define CPUNUM(x) (cpunum[x])
#define CUR_CPUNUM() (cpunum[cur_apicid()])
The APIC register reference takes so long that the array reference is
simply absorbed in the overhead. I always get the virtual number (and
possibly the APIC id, if necessary), then operate from there.
Walking a sparse array takes up enough other overhead (checking for
whether you're done as well as not using non-existent entries)
that translating back to the actual APIC ID is OK. If anything it
simplifies the code to walk only consecutive numbers.
--
Erich Stefan Boleyn \_ E-mail (preferred): <erich@uruk.org>
Mad Genius wanna-be, CyberMuffin \__ (finger me for other stats)
Web: http://www.uruk.org/~erich/ Motto: "I'll live forever or die trying"
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