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

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

On 6/14/2021 4:54 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.

> 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.