Re: [dpdk-dev] [PATCH v6 2/2] eal: add asynchronous request API to DPDK IPC

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

Hi Thomas and Harry,


On 3/28/2018 4:22 PM, Van Haaren, Harry wrote:
>> From: Thomas Monjalon [mailto:thomas@monjalon.net]
>> Sent: Wednesday, March 28, 2018 8:30 AM
>> To: Tan, Jianfeng <jianfeng.tan@intel.com>
>> Cc: Burakov, Anatoly <anatoly.burakov@intel.com>; dev@dpdk.org; Ananyev,
>> Konstantin <konstantin.ananyev@intel.com>; Van Haaren, Harry
>> <harry.van.haaren@intel.com>
>> Subject: Re: [dpdk-dev] [PATCH v6 2/2] eal: add asynchronous request API to
>> DPDK IPC
>>
>> 28/03/2018 04:08, Tan, Jianfeng:
>>> Hi Thomas ,
>>>
>>> From: Thomas Monjalon [mailto:thomas@monjalon.net]
>>>> 27/03/2018 15:59, Anatoly Burakov:
>>>>> Under the hood, we create a separate thread to deal with replies to
>>>>> asynchronous requests, that will just wait to be notified by the
>>>>> main thread, or woken up on a timer.
>>>> I really don't like that a library is creating a thread.
>>>> We don't even know where the thread is created (which core).
>>>> Can it be a rte_service? or in the interrupt thread?
>>> Agree that we'd better not adding so many threads in a library.
>>>
>>> I was considering to merge all the threads into the interrupt thread,
>> however, we don't have an interrupt thread in freebsd. Further, we don't
>> implement alarm API in freebsd. That's why I tend to current implementation,
>> and optimize it later.
>>
>> I would prefer we improve the current code now instead of polluting more
>> with more uncontrolled threads.
>>
>>> For rte_service, it may be not a good idea to reply on it as it needs
>> explicit API calls to setup.
>>
>> I don't see the issue of the explicit API.
>> The IPC is a new service.

My concern is that not every DPDK application sets up rte_service, but 
IPC will be used for very fundamental functions, like memory allocation. 
We could not possibly ask all DPDK applications to add rte_service now.

And also take Harry's comments below into consideration, most likely, we 
will move these threads into interrupt thread now by adding

> Although I do like to see new services, if we want to enable "core" dpdk functionality with Services, we need a proper designed solution for that. Service cores is not intended for "occasional" work - there is no method to block and sleep on a specific service until work becomes available, so this would imply a busy-polling. Using a service (hence busy polling) for rte_malloc()-based memory mapping requests is inefficient, and total overkill :)
>
> For this patch I suggest to use some blocking-read capable mechanism.

The problem here is that we add too many threads; blocking-read does not 
decrease # of threads.

>
> The above said, in the longer term it would be good to have a design that allows new file-descriptors to be added to a "dpdk core" thread, which performs occasional lengthy work if the FD has data available.

Interrupt thread vs rte_service, which is the direction to go? We 
actually have some others threads, in vhost and even virtio-user; we can 
also avoid those threads if we have a clear direction.

Thanks,
Jianfeng