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 872CBA0C46; Fri, 18 Jun 2021 12:28:20 +0200 (CEST) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 17F0F40150; Fri, 18 Jun 2021 12:28:20 +0200 (CEST) Received: from mga12.intel.com (mga12.intel.com [192.55.52.136]) by mails.dpdk.org (Postfix) with ESMTP id AB82440142 for ; Fri, 18 Jun 2021 12:28:17 +0200 (CEST) IronPort-SDR: T3FZJ59J3gR6UdXgw3XSfdnZbMjzTch85fwLSpkifM1J4wcCBN588+kb7GPpC0xj8fQjuI5BAE ZM/xe6yT4s9g== X-IronPort-AV: E=McAfee;i="6200,9189,10018"; a="186224248" X-IronPort-AV: E=Sophos;i="5.83,283,1616482800"; d="scan'208";a="186224248" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Jun 2021 03:28:15 -0700 IronPort-SDR: H2o9UTflRaa+CAh3PV5YHATGqujO0i/2PoHSIq8Yvtfge9qejb3wr7HaZBq4WtBj3B4xhSMMAh Zw9Lh+ttd14w== X-IronPort-AV: E=Sophos;i="5.83,283,1616482800"; d="scan'208";a="422180835" Received: from fyigit-mobl1.ger.corp.intel.com (HELO [10.213.219.119]) ([10.213.219.119]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Jun 2021 03:28:13 -0700 To: "Ananyev, Konstantin" , =?UTF-8?Q?Morten_Br=c3=b8rup?= , Thomas Monjalon , "Richardson, Bruce" Cc: "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> <98CBD80474FA8B44BF855DF32C47DC35C61868@smartserver.smartshare.dk> From: Ferruh Yigit X-User: ferruhy Message-ID: Date: Fri, 18 Jun 2021 11:28:09 +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 6:05 PM, Ananyev, Konstantin wrote: > > >> On 6/17/2021 4:17 PM, Morten Brørup wrote: >>>> From: Ananyev, Konstantin [mailto:konstantin.ananyev@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-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). >>>> 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)? > > I think it would be better to change rx_pkt_burst() to accept port_id and queue_id, > instead of void *. > I.E: > typedef uint16_t (*eth_rx_burst_t)(uint16_t port_id, uint16_t queue_id, struct rte_mbuf **rx_pkts, uint16_t nb_pkts); > May not need to add 'port_id', since in the callback you are already in the driver scope and all required device specific variables already accessible via help of queue struct. > And we can do actual de-referencing of private rxq data inside the actual rx function. > Yes we can replace queue struct with 'queue_id', and do the referencing in the Rx instead of burst API, but what is the benefit of it? >> 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;