Date: Wed, 16 Mar 2011 05:38:32 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Jung-uk Kim <jkim@FreeBSD.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org Subject: Re: svn commit: r219672 - in head: share/man/man9 sys/i386/include Message-ID: <20110316051750.M2847@besplex.bde.org> In-Reply-To: <201103151714.p2FHEQdF049456@svn.freebsd.org> References: <201103151714.p2FHEQdF049456@svn.freebsd.org>
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On Tue, 15 Mar 2011, Jung-uk Kim wrote:
> Log:
> Unconditionally use binuptime(9) for get_cyclecount(9) on i386. Since this
> function is almost exclusively used for random harvesting, there is no need
> for micro-optimization. Adjust the manual page accordingly.
That's what I said when it was being committed, but it isn't clear that
there is _no_ need for micro-optimization in random harvesting. IIRC,
random harvesting originally was too active so it benefited more from
micro-optimizations.
The timecounter is fast enough if the timecounter hardware is the TSC,
since TSC reads used to take 12 cycles on Athlons and now take 40+,
and timecounter software overhead only adds about 30 cycles to this.
But now the timecounter harware is even more rarely the TSC, and most
timecounter hardware is very slow -- about 3000 cycles for APCI-"fast"
and 9000 for ACPI and 15000 for i8254 at 3 GHz.
> Modified: head/sys/i386/include/cpu.h
> ==============================================================================
> --- head/sys/i386/include/cpu.h Tue Mar 15 16:50:17 2011 (r219671)
> +++ head/sys/i386/include/cpu.h Tue Mar 15 17:14:26 2011 (r219672)
> @@ -70,15 +70,10 @@ void swi_vm(void *);
> static __inline uint64_t
> get_cyclecount(void)
> {
> -#if defined(I486_CPU) || defined(KLD_MODULE)
> struct bintime bt;
>
> - if (!tsc_present) {
> - binuptime(&bt);
> - return ((uint64_t)bt.sec << 56 | bt.frac >> 8);
> - }
> -#endif
> - return (rdtsc());
> + binuptime(&bt);
> + return ((uint64_t)bt.sec << 56 | bt.frac >> 8);
> }
You should pessimize all arches to use binuptime() to get enough test
coverage to see if anyone notices. Then get_cyclecount() can be removed.
The correct function to use for fast and possibly-wrong times is
clock_binuptime(CLOCK_FASTEST, tsp), where CLOCK_FASTEST maps to CLOCK_TSC
on x86 if there is a TSC and nothing faster.
Bruce
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