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

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

On 28-Mar-18 9:55 AM, Tan, Jianfeng wrote:
> 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
> 

Hi all,

First of all, @Thomas, this is not a "new library" - it's part of EAL. 
We're going to be removing a few threads from EAL as it is because of 
IPC (Jianfeng has already submitted patches for those), so i don't think 
it's such a big deal to have two IPC threads instead of one. I'm open to 
suggestions on how to make this work without a second thread, but i 
don't see it.

We've discussed possibility of using rte_service internally, but decided 
against it for reasons already outlined by Harry - it's not a suitable 
mechanism for this kind of thing, not as it is.

Using interrupt thread for this _will_ work, however this will require a 
a lot more changes, as currently alarm API allocates everything through 
rte_malloc, while we want to use IPC for rte_malloc (which would make it 
a circular dependency). So it'll probably be more API and more 
complexity for dealing with malloc vs rte_malloc allocations. Hence the 
least-bad approach taken here: a new thread.

-- 
Thanks,
Anatoly