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

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

On 6/18/2021 11:41 AM, 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.
> 
> I probably I owe a better explanation to what I meant in first mail.
> Otherwise it sounds confusing.
> I'll try to write a more detailed one in next few days.
> 
>>> 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?
> 
> When we'll have just:
> 1. rte_eth_dev (hidden in .c)
> 2. rte_eth_dev_burst (visible)
> 
> And no specific public struct/array for callbacks - they will be hidden in rte_eth_dev.
> 

If we can hide them, agree this is better.

>>
>> 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.
> 
> I understand that, and as I said above - I think it is a good step forward.
> Though even better would be to hide rte_eth_dev_cb too.
> 
>>
>>> 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.
> 
> Cool, please keep me in a loop.
> I'll try to free some cycles for 21.11 to get involved and help (if needed off-course).

That would be great, thanks.