From: Thomas Monjalon <thomas@monjalon.net>
To: Bruce Richardson <bruce.richardson@intel.com>
Cc: Jerin Jacob <jerin.jacob@caviumnetworks.com>,
Gowrishankar <gowrishankar.m@linux.vnet.ibm.com>,
dev@dpdk.org, Chao Zhu <chaozhu@linux.vnet.ibm.com>,
Konstantin Ananyev <konstantin.ananyev@intel.com>,
viktorin@rehivetech.com, jianbo.liu@linaro.org
Subject: Re: [dpdk-dev] [PATCH v2 5/5] eal/timer: honor architecture specific rdtsc hz function
Date: Thu, 12 Oct 2017 12:12:48 +0200 [thread overview]
Message-ID: <2940582.YtahPHvW5m@xps> (raw)
In-Reply-To: <20171012084847.GA46424@bricha3-MOBL3.ger.corp.intel.com>
12/10/2017 10:48, Bruce Richardson:
> On Wed, Oct 11, 2017 at 09:25:58PM +0200, Thomas Monjalon wrote:
> > 11/10/2017 20:57, Jerin Jacob:
> > > From: Thomas Monjalon <thomas@monjalon.net>
> > > > 22/09/2017 10:25, Gowrishankar:
> > > > > From: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > > >
> > > > > When calibrating the tsc frequency, first, probe the architecture specific
> > > > > rdtsc hz function. if not available, use the existing calibrate scheme
> > > > > to calibrate the tsc frequency.
> > > > >
> > > > > Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > >
> > > > I agree on the idea.
> > >
> > > OK
> > >
> > > > The namespace of cycles related function in DPDK is a real mess.
> > >
> > > Absolutely!!
> > >
> > > > I think we can choose better names in this series as a first step
> > > > to tidy this mess.
> > > > I will explain below.
> > > >
> > > > At first, we should avoid TSC and RDTSC which are Intel-only wording.
> > > > The generic word could be "cycles" (the word used in arch headers),
> > > > or "ticks".
> > > > We should also name the timer sources or their function in a generic way.
> > > > Examples: CPU cycles? fast counter? precise counter?
> > > >
> > > > Sometimes we use "hz", sometimes "freq".
> > > > It would better to keep one of them.
> > > >
> > > > > --- a/lib/librte_eal/common/eal_common_timer.c
> > > > > +++ b/lib/librte_eal/common/eal_common_timer.c
> > > > > @@ -80,8 +80,11 @@
> > > > > void
> > > > > set_tsc_freq(void)
> > > > > {
> > > > > - uint64_t freq = get_tsc_freq();
> > > > > + uint64_t freq;
> > > > >
> > > > > + freq = rte_rdtsc_arch_hz();
> > > >
> > > > This new function is arch-specific and exported as a new API.
> > >
> > > I thought of avoid exporting it. But then if the function is in
> > > lib/librte_eal/common/include/arch/../rte_cycles.h it is anyway exposed to
> > > application. i.e whatever files in lib/librte_eal/common/include/arch/../
> > > anyway exposed to application.
> >
> > Ah yes, you are right!
> >
> > > See last comment.
> > >
> > > >
> > > > > + if (!freq)
> > > > > + freq = get_tsc_freq();
> > > >
> > > > The function get_tsc_freq is guessing the freq with OS-specific method.
> > > >
> > > > > if (!freq)
> > > > > freq = estimate_tsc_freq();
> > > >
> > > > The function estimate_tsc_freq is doing an estimation based on sleep().
> > > >
> > > > At the end, the most accurate frequency is saved in eal_tsc_resolution_hz
> > > > and can be retrieved with rte_get_tsc_hz().
> > > > I don't understand why rte_rdtsc_arch_hz() is also exported to the apps.
> > > >
> > > > TSC and HPET timer sources are wrapped in rte_get_timer_hz() in the
> > > > Similarly we can get the current timer with rte_get_timer_cycles().
> > > > In the case of TSC, it calls rte_get_tsc_cycles() which is an alias
> > > > of rte_rdtsc().
> > > > Some code is still using directly rte_rdtsc().
> > > > There is also rte_rdtsc_precise which adds a memory barrier.
> > > >
> > > > The real question is what is the right abstraction for the application?
> > > > Do we want the fastest timer? the CPU timer? a precise timer?
> > > >
> > > > I would like to see a real discussion on this topic, in order of building
> > > > a new timer API which would alias the old one for some time.
> > >
> > > I guess, we may need to see to how abstract vmware TSC support also in
> > > proper way
> >
> > Yes
> >
> > > > If you don't want to bother with all these questions, I suggest to not
> > > > export the new function rte_rdtsc_arch_hz() and rename it to tsc_arch_hz.
> > >
> > > If I understand it correctly, You would like to create a header file
> > > in lib/librte_eal/common/include/arch/../ which should not be exported and change
> > > the name to tsc_arch_hz.
> >
> > I had not think about the way to do this.
> > What about having internal headers in lib/librte_eal/common/arch/ ?
> >
>
> Yes, this area needs cleanup, but I also think that this patchset (and
> follow-on patch for x86-specific implementation) does not make things
> significantly worse than they are now, while also giving significant
> benefits for users both in terms of improved clock accuracy and reduced
> startup time (due to not needing sleep). Therefore I'd like to see this
> merged into 17.11 as a definite improvement, even if it does not "fix" the
> bigger issues of poor naming etc. I think this is important enough an
> improvement that we need to see it in the LTS release.
I agree Bruce.
My initial point was to avoid exporting a new function with a wrong name.
It would be better to find a way to avoid this export.
If it cannot be done in 17.11 timeframe, we can at least add the
experimental tag.
next prev parent reply other threads:[~2017-10-12 10:12 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-22 8:25 [dpdk-dev] [PATCH v2 0/5] improve tsc frequency calibration Gowrishankar
2017-09-22 8:25 ` [dpdk-dev] [PATCH v2 1/5] eal/x86: define architecture specific rdtsc hz Gowrishankar
2017-09-22 8:25 ` [dpdk-dev] [PATCH v2 2/5] eal/ppc64: " Gowrishankar
2017-10-12 22:20 ` Thomas Monjalon
2017-09-22 8:25 ` [dpdk-dev] [PATCH v2 3/5] eal/armv7: " Gowrishankar
2017-09-22 8:25 ` [dpdk-dev] [PATCH v2 4/5] eal/armv8: " Gowrishankar
2017-09-22 8:25 ` [dpdk-dev] [PATCH v2 5/5] eal/timer: honor architecture specific rdtsc hz function Gowrishankar
2017-10-11 17:36 ` Thomas Monjalon
2017-10-11 18:57 ` Jerin Jacob
2017-10-11 19:25 ` Thomas Monjalon
2017-10-12 8:48 ` Bruce Richardson
2017-10-12 10:12 ` Thomas Monjalon [this message]
2017-10-12 10:25 ` Bruce Richardson
2017-10-12 10:16 ` Jerin Jacob
2017-10-13 0:02 ` [dpdk-dev] [PATCH 0/4] improve tsc frequency calibration Thomas Monjalon
2017-10-13 0:02 ` [dpdk-dev] [PATCH 1/4] timer: honor arch-specific TSC frequency query Thomas Monjalon
2017-10-13 0:02 ` [dpdk-dev] [PATCH 2/4] eal/armv8: implement arch-specific TSC freq query Thomas Monjalon
2017-10-13 0:02 ` [dpdk-dev] [PATCH 3/4] eal/ppc64: " Thomas Monjalon
2017-10-13 0:02 ` [dpdk-dev] [PATCH 4/4] eal/x86: " Thomas Monjalon
2017-10-13 3:47 ` Stephen Hemminger
2017-10-13 7:30 ` Thomas Monjalon
2017-10-13 15:13 ` Stephen Hemminger
2017-10-13 15:17 ` Bruce Richardson
2017-10-13 3:21 ` [dpdk-dev] [PATCH 0/4] improve tsc frequency calibration Jerin Jacob
2017-10-13 11:08 ` Thomas Monjalon
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