Re: [dpdk-dev] [PATCH] examples/timer: fix incorrect time interval

[Thomas Monjalon <thomas@monjalon.net>]

06/05/2021 04:06, Chengchang Tang:
> 
> On 2021/5/6 5:37, Thomas Monjalon wrote:
> > 15/04/2021 09:12, Min Hu (Connor):
> >> From: Chengchang Tang <tangchengchang@huawei.com>
> >>
> >> Timer sample example assumes that the frequency of the timer is about
> >> 2Ghz to control the period of calling rte_timer_manage(). But this
> >> assumption is easy to fail. For example. the frequency of tsc on ARM64
> >> is much less than 2Ghz.
> >>
> >> This patch uses the frequency of the current timer to calculate the
> >> correct time interval to ensure consistent result on all platforms.
> >>
> >> In addition, the rte_rdtsc() is replaced with the more recommended
> >> rte_get_timer_cycles function in this patch.
> >>
> >> Fixes: af75078fece3 ("first public release")
> >> Cc: stable@dpdk.org
> >>
> >> Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
> >> Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
> > [...]
> >>  		/*
> >> -		 * Call the timer handler on each core: as we don't
> >> -		 * need a very precise timer, so only call
> >> -		 * rte_timer_manage() every ~10ms (at 2Ghz). In a real
> >> -		 * application, this will enhance performances as
> >> -		 * reading the HPET timer is not efficient.
> >> +		 * Call the timer handler on each core: as we don't need a
> >> +		 * very precise timer, so only call rte_timer_manage()
> >> +		 * every ~10ms. since rte_eal_hpet_init() has not been
> >> +		 * called, the rte_rdtsc() will be used at runtime.
> > 
> > I don't understand this last sentence.
> > 
> 
> This is explaining why we can use rte_get_timer_cycles() instead of rte_rdtsc().
> In this example, we call tsc to improve its performance. So, we invoked rte_rdtsc()
> here. Now the function rte_get_timer_cycles() encapsulates these counters. It will
> invoke the corresponding counter according to the user's initialization of the counter.

That's very confusing. Better to drop.

> 
> >> +		 * In a real application, this will enhance performances
> >> +		 * as reading the HPET timer is not efficient.
> >>  		 */
> >> -		cur_tsc = rte_rdtsc();
> >> +		cur_tsc = rte_get_timer_cycles();