Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 6/17/2021 3:58 PM, Ananyev, Konstantin wrote:
> 
> 
>>>>>
>>>>> 14/06/2021 15:15, Bruce Richardson:
>>>>>> On Mon, Jun 14, 2021 at 02:22:42PM +0200, Morten Brørup wrote:
>>>>>>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Thomas Monjalon
>>>>>>>> Sent: Monday, 14 June 2021 12.59
>>>>>>>>
>>>>>>>> Performance of access in a fixed-size array is very good
>>>>>>>> because of cache locality
>>>>>>>> and because there is a single pointer to dereference.
>>>>>>>> The only drawback is the lack of flexibility:
>>>>>>>> the size of such an array cannot be increase at runtime.
>>>>>>>>
>>>>>>>> An approach to this problem is to allocate the array at runtime,
>>>>>>>> being as efficient as static arrays, but still limited to a maximum.
>>>>>>>>
>>>>>>>> That's why the API rte_parray is introduced,
>>>>>>>> allowing to declare an array of pointer which can be resized
>>>>>>>> dynamically
>>>>>>>> and automatically at runtime while keeping a good read performance.
>>>>>>>>
>>>>>>>> After resize, the previous array is kept until the next resize
>>>>>>>> to avoid crashs during a read without any lock.
>>>>>>>>
>>>>>>>> Each element is a pointer to a memory chunk dynamically allocated.
>>>>>>>> This is not good for cache locality but it allows to keep the same
>>>>>>>> memory per element, no matter how the array is resized.
>>>>>>>> Cache locality could be improved with mempools.
>>>>>>>> The other drawback is having to dereference one more pointer
>>>>>>>> to read an element.
>>>>>>>>
>>>>>>>> There is not much locks, so the API is for internal use only.
>>>>>>>> This API may be used to completely remove some compilation-time
>>>>>>>> maximums.
>>>>>>>
>>>>>>> I get the purpose and overall intention of this library.
>>>>>>>
>>>>>>> I probably already mentioned that I prefer "embedded style programming" with fixed size arrays, rather than runtime configurability.
>>>> It's
>>>>> my personal opinion, and the DPDK Tech Board clearly prefers reducing the amount of compile time configurability, so there is no way
>> for
>>>>> me to stop this progress, and I do not intend to oppose to this library. :-)
>>>>>>>
>>>>>>> This library is likely to become a core library of DPDK, so I think it is important getting it right. Could you please mention a few
>>>> examples
>>>>> where you think this internal library should be used, and where it should not be used. Then it is easier to discuss if the border line
>> between
>>>>> control path and data plane is correct. E.g. this library is not intended to be used for dynamically sized packet queues that grow and
>> shrink
>>>> in
>>>>> the fast path.
>>>>>>>
>>>>>>> If the library becomes a core DPDK library, it should probably be public instead of internal. E.g. if the library is used to make
>>>>> RTE_MAX_ETHPORTS dynamic instead of compile time fixed, then some applications might also need dynamically sized arrays for their
>>>>> application specific per-port runtime data, and this library could serve that purpose too.
>>>>>>>
>>>>>>
>>>>>> Thanks Thomas for starting this discussion and Morten for follow-up.
>>>>>>
>>>>>> My thinking is as follows, and I'm particularly keeping in mind the cases
>>>>>> of e.g. RTE_MAX_ETHPORTS, as a leading candidate here.
>>>>>>
>>>>>> While I dislike the hard-coded limits in DPDK, I'm also not convinced that
>>>>>> we should switch away from the flat arrays or that we need fully dynamic
>>>>>> arrays that grow/shrink at runtime for ethdevs. I would suggest a half-way
>>>>>> house here, where we keep the ethdevs as an array, but one allocated/sized
>>>>>> at runtime rather than statically. This would allow us to have a
>>>>>> compile-time default value, but, for use cases that need it, allow use of a
>>>>>> flag e.g.  "max-ethdevs" to change the size of the parameter given to the
>>>>>> malloc call for the array.  This max limit could then be provided to apps
>>>>>> too if they want to match any array sizes. [Alternatively those apps could
>>>>>> check the provided size and error out if the size has been increased beyond
>>>>>> what the app is designed to use?]. There would be no extra dereferences per
>>>>>> rx/tx burst call in this scenario so performance should be the same as
>>>>>> before (potentially better if array is in hugepage memory, I suppose).
>>>>>
>>>>> I think we need some benchmarks to decide what is the best tradeoff.
>>>>> I spent time on this implementation, but sorry I won't have time for benchmarks.
>>>>> Volunteers?
>>>>
>>>> I had only a quick look at your approach so far.
>>>> But from what I can read, in MT environment your suggestion will require
>>>> extra synchronization for each read-write access to such parray element (lock, rcu, ...).
>>>> I think what Bruce suggests will be much ligther, easier to implement and less error prone.
>>>> At least for rte_ethdevs[] and friends.
>>>> Konstantin
>>>
>>> One more thought here - if we are talking about rte_ethdev[] in particular, I think  we can:
>>> 1. move public function pointers (rx_pkt_burst(), etc.) from rte_ethdev into a separate flat array.
>>> We can keep it public to still use inline functions for 'fast' calls rte_eth_rx_burst(), etc. to avoid
>>> any regressions.
>>> That could still be flat array with max_size specified at application startup.
>>> 2. Hide rest of rte_ethdev struct in .c.
>>> That will allow us to change the struct itself and the whole rte_ethdev[] table in a way we like
>>> (flat array, vector, hash, linked list) without ABI/API breakages.
>>>
>>> Yes, it would require all PMDs to change prototype for pkt_rx_burst() function
>>> (to accept port_id, queue_id instead of queue pointer), but the change is mechanical one.
>>> Probably some macro can be provided to simplify it.
>>>
>>
>> We are already planning some tasks for ABI stability for v21.11, I think
>> splitting 'struct rte_eth_dev' can be part of that task, it enables hiding more
>> internal data.
> 
> Ok, sounds good.
> 
>>
>>> The only significant complication I can foresee with implementing that approach -
>>> we'll need a an array of 'fast' function pointers per queue, not per device as we have now
>>> (to avoid extra indirection for callback implementation).
>>> Though as a bonus we'll have ability to use different RX/TX funcions per queue.
>>>
>>
>> What do you think split Rx/Tx callback into its own struct too?
>>
>> Overall 'rte_eth_dev' can be split into three as:
>> 1. rte_eth_dev
>> 2. rte_eth_dev_burst
>> 3. rte_eth_dev_cb
>>
>> And we can hide 1 from applications even with the inline functions.
> 
> As discussed off-line, I think:
> it is possible.
> My absolute preference would be to have just 1/2 (with CB hidden).

How can we hide the callbacks since they are used by inline burst functions.

> But even with 1/2/3 in place I think it would be  a good step forward.
> Probably worth to start with 1/2/3 first and then see how difficult it
> would be to switch to 1/2.

What do you mean by switch to 1/2?

If we keep having inline functions, and split struct as above three structs, we
can only hide 1, and 2/3 will be still visible to apps because of inline
functions. This way we will be able to hide more still having same performance.

> Do you plan to start working on it?
> 

We are gathering the list of the tasks for the ABI stability, most probably they
will be worked on during v21.11. I can take this one.

> Konstantin
> 
> 
> 
>