On 6/17/2021 4:17 PM, Morten Brørup wrote:
>> From: Ananyev, Konstantin [mailto:konstantin.anan...@intel.com]
>> Sent: Thursday, 17 June 2021 16.59
>>
>>>>>>
>>>>>> 14/06/2021 15:15, Bruce Richardson:
>>>>>>> On Mon, Jun 14, 2021 at 02:22:42PM +0200, Morten Brørup wrote:
>>>>>>>>> From: dev [mailto:dev-boun...@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).
>> 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.
>> Do you plan to start working on it?
>>
>> Konstantin
>
> If you do proceed with this, be very careful. E.g. the inlined rx/tx burst
> functions should not touch more cache lines than they do today - especially
> if there are many active ports. The inlined rx/tx burst functions are very
> simple, so thorough code review (and possibly also of the resulting assembly)
> is appropriate. Simple performance testing might not detect if more cache
> lines are accessed than before the modifications.
>
> Don't get me wrong... I do consider this an improvement of the ethdev
> library; I'm only asking you to take extra care!
>
ack
If we split as above, I think device specific data 'struct rte_eth_dev_data'
should be part of 1 (rte_eth_dev). Which means Rx/Tx inline functions access
additional cache line.
To prevent this, what about duplicating 'data' in 2 (rte_eth_dev_burst)? We have
enough space for it to fit into single cache line, currently it is:
struct rte_eth_dev {
eth_rx_burst_t rx_pkt_burst; /* 0 8 */
eth_tx_burst_t tx_pkt_burst; /* 8 8 */
eth_tx_prep_t tx_pkt_prepare; /* 16 8 */
eth_rx_queue_count_t rx_queue_count; /* 24 8 */
eth_rx_descriptor_done_t rx_descriptor_done; /* 32 8 */
eth_rx_descriptor_status_t rx_descriptor_status; /* 40 8 */
eth_tx_descriptor_status_t tx_descriptor_status; /* 48 8 */
struct rte_eth_dev_data * data; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
'rx_descriptor_done' is deprecated and will be removed;
