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

[Chengchang Tang <tangchengchang@huawei.com>]

On 2021/5/6 16:08, Thomas Monjalon wrote:
> 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.
> 
OK, I will remove this sentence in the next version.
>>
>>>> +		 * 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();
> 
> 
> 
> 
> .
>