Re: [dpdk-dev] [PATCH v3 2/3] eal: add synchronous multi-process communication

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 25-Jan-18 1:05 PM, Burakov, Anatoly wrote:
> On 25-Jan-18 1:00 PM, Ananyev, Konstantin wrote:
>>
>>
>>> -----Original Message-----
>>> From: Burakov, Anatoly
>>> Sent: Thursday, January 25, 2018 12:26 PM
>>> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>; Tan, Jianfeng 
>>> <jianfeng.tan@intel.com>; dev@dpdk.org
>>> Cc: Richardson, Bruce <bruce.richardson@intel.com>; thomas@monjalon.net
>>> Subject: Re: [PATCH v3 2/3] eal: add synchronous multi-process 
>>> communication
>>>
>>> On 25-Jan-18 12:19 PM, Ananyev, Konstantin wrote:
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: Burakov, Anatoly
>>>>> Sent: Thursday, January 25, 2018 12:00 PM
>>>>> To: Tan, Jianfeng <jianfeng.tan@intel.com>; dev@dpdk.org
>>>>> Cc: Richardson, Bruce <bruce.richardson@intel.com>; Ananyev, 
>>>>> Konstantin <konstantin.ananyev@intel.com>; thomas@monjalon.net
>>>>> Subject: Re: [PATCH v3 2/3] eal: add synchronous multi-process 
>>>>> communication
>>>>>
>>>>> On the overall patch,
>>>>>
>>>>> Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>
>>>>>
>>>>> For request(), returning number of replies received actually makes
>>>>> sense, because now we get use the value to read our replies, if we 
>>>>> were
>>>>> a primary process sending messages to secondary processes.
>>>>
>>>> Yes, I also think it is good to return number of sends.
>>>> Then caller can compare number of sended requests with number of
>>>> received replies and decide should it be considered a failure or no.
>>>>
>>>
>>> Well, OK, that might make sense. However, i think it would've be of more
>>> value to make the API consistent (0/-1 on success/failure) and put
>>> number of sent messages into the reply, like number of received. I.e.
>>> something like
>>>
>>> struct reply {
>>>      int nb_sent;
>>>      int nb_received;
>>> };
>>>
>>> We do it for the latter already, so why not the former?
>>
>> The question is what treat as success/failure?
>> Let say we sent 2 requests (of 3 possible), got back 1 response...
>> Should we consider it as success or failure?
>>
> 
> I think "failure" is "something went wrong", not "secondary processes 
> didn't respond". For example, invalid parameters, or our socket suddenly 
> being closed, or some other error that prevents us from sending requests 
> to secondaries.
> 
> As far as i can tell from the code, there's no way to know if the 
> secondary process is running other than by attempting to connect to it, 
> and get a response. So, failed connection should not be a failure 
> condition, because we can't know if we *can* connect to the process 
> until we do. Process may have ended, but socket files will still be 
> around, and there's nothing we can do about that. So i wouldn't consider 
> inability to send a message a failure condition.
> 

Just to clarify - i'm suggesting leaving this decision up to the user. 
If a user expects there to be "n" processes running, but only "m" 
responses were received, he could treat it as error. Another user might 
simply send periodical updates/polls to secondaries, for whatever reason 
(say, stats display), and won't really care if one of them just died, so 
there's no error for that user.

However, all of this has nothing to do with API. If we're able to send 
messages - it's not a failure. If we can't - it is. That's the part API 
should be concerned about, and that's what the return value should 
indicate, IMO.

-- 
Thanks,
Anatoly