From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id 56364A0C4B; Thu, 17 Jun 2021 17:46:45 +0200 (CEST) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id CA35D4067A; Thu, 17 Jun 2021 17:46:44 +0200 (CEST) Received: from mga17.intel.com (mga17.intel.com [192.55.52.151]) by mails.dpdk.org (Postfix) with ESMTP id 54F7540150 for ; Thu, 17 Jun 2021 17:46:43 +0200 (CEST) IronPort-SDR: cI2BybJCGXW0/+oRn75SL+UiMAB7i8BDy6dzHas2ZLM22O/nq1oxv/STRSB0fqFMBnUih4RpIo jx64Yfpir2dw== X-IronPort-AV: E=McAfee;i="6200,9189,10018"; a="186772147" X-IronPort-AV: E=Sophos;i="5.83,280,1616482800"; d="scan'208";a="186772147" Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Jun 2021 08:45:02 -0700 IronPort-SDR: x66H8rnaGt+ombANrr7t0v3zy9M6sKJBOouG0OUuZ6rfETm6SuVCQGWV4G12H80K5SWtWt22kX uG7cLZLjyiQA== X-IronPort-AV: E=Sophos;i="5.83,280,1616482800"; d="scan'208";a="554438049" Received: from fyigit-mobl1.ger.corp.intel.com (HELO [10.213.201.111]) ([10.213.201.111]) by orsmga004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Jun 2021 08:44:59 -0700 To: "Ananyev, Konstantin" , Thomas Monjalon , "Richardson, Bruce" Cc: =?UTF-8?Q?Morten_Br=c3=b8rup?= , "dev@dpdk.org" , "olivier.matz@6wind.com" , "andrew.rybchenko@oktetlabs.ru" , "honnappa.nagarahalli@arm.com" , "jerinj@marvell.com" , "gakhil@marvell.com" References: <20210614105839.3379790-1-thomas@monjalon.net> <98CBD80474FA8B44BF855DF32C47DC35C6184E@smartserver.smartshare.dk> <2004320.XGyPsaEoyj@thomas> From: Ferruh Yigit X-User: ferruhy Message-ID: <0bb118ba-2658-a7d7-ad8f-bf27f62849f7@intel.com> Date: Thu, 17 Jun 2021 16:44:55 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Subject: Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" 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 > > > >