From mboxrd@z Thu Jan  1 00:00:00 1970
From: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>
Subject: Re: [PATCH] eal: roundup tsc frequency when estimating it
Date: Fri, 30 Nov 2018 07:17:00 +0000
Message-ID: <20181130071641.GA11625@ltp-pvn>
References: <20181129083138.23029-1-pbhagavatula@caviumnetworks.com>
 <20181129132152.754ce410@xeon-e3>
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Cc: "bruce.richardson@intel.com" <bruce.richardson@intel.com>, "dev@dpdk.org"
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To: Stephen Hemminger <stephen@networkplumber.org>, "Jacob,  Jerin"
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Hi Stephen,

On Thu, Nov 29, 2018 at 01:21:52PM -0800, Stephen Hemminger wrote:
> On Thu, 29 Nov 2018 08:32:03 +0000
> Pavan Nikhilesh <pbhagavatula@caviumnetworks.com> wrote:
>
> > When estimating tsc frequency using sleep/gettime round it up to the
> > nearest multiple of 10Mhz for more accuracy.
> >
> > Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>
>
> Rounding reduces accuracy.
>
> Why is this code being used?  Shouldn't get_tsc_freq_arch return a
> correct value?

This patch doesn't modify get_tsc_freq_arch(), it basically gives a more
accurate freq reading when we rely on sleep(1) i.e. only when
get_tsc_freq_arch() returns 0.

example:

static uint64_t
estimate_tsc_freq(void)
{
        RTE_LOG(WARNING, EAL, "WARNING: TSC frequency estimated roughly"
                " - clock timings may be less accurate.\n");
        /* assume that the sleep(1) will sleep for 1 second */
        uint64_t start =3D rte_rdtsc();
        sleep(1);
        return rte_rdtsc() - start;
}

This will not give the accurate cyc/sec in most cases, rounding it to 10Mhz=
 wil
do the job.

In case of ARM64 if we enable RTE_ARM_EAL_RDTSC_USE_PMU, get_tsc_freq_arch(=
)
will return 0 as there is no instruction to determine the clk of PMU.

>
> How well does the rdmsr() logic work in VM?
> It looks like Hyper-V has special MSR's for TSC frequency determination.

Maybe bruce can give a more accurate answer to this as it is x86 specific.

Thanks,
Pavan.