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 C2150A0C4D; Thu, 17 Jun 2021 15:08:50 +0200 (CEST) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id AFE274069C; Thu, 17 Jun 2021 15:08:50 +0200 (CEST) Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by mails.dpdk.org (Postfix) with ESMTP id 15CF240150 for ; Thu, 17 Jun 2021 15:08:48 +0200 (CEST) IronPort-SDR: pzQhzVvCoDbIm1g2utXO0LiimpDdeysWNUlB2uBn+AwsBC57LVMuQHD2FjSp/f1cTAKa6PYFgv EXtsp4zCneUw== X-IronPort-AV: E=McAfee;i="6200,9189,10017"; a="227878745" X-IronPort-AV: E=Sophos;i="5.83,280,1616482800"; d="scan'208";a="227878745" Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Jun 2021 06:08:23 -0700 IronPort-SDR: ZBmOr4j8EP8NfWNDWOX1+Gtet2w5tHMhDWMsbtZ/ag7CkWjrlreT9ryCGDdNjZvrC/Pum7VhKD xQ6P2xj+eGOg== X-IronPort-AV: E=Sophos;i="5.83,280,1616482800"; d="scan'208";a="554393757" 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 06:08:20 -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: Date: Thu, 17 Jun 2021 14:08:16 +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 6/14/2021 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.