From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bruce Richardson Subject: Re: [RFC 0/9] get Rx and Tx used descriptors Date: Tue, 17 Jan 2017 13:56:30 +0000 Message-ID: <20170117135630.GA8640@bricha3-MOBL3.ger.corp.intel.com> References: <1479981261-19512-1-git-send-email-olivier.matz@6wind.com> <20170113174409.2f1be0b5@platinum> <59AF69C657FD0841A61C55336867B5B035B88D5D@IRSMSX103.ger.corp.intel.com> <20170117092410.5f5950d7@platinum> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "dev@dpdk.org" , "thomas.monjalon@6wind.com" , "Ananyev, Konstantin" , "Lu, Wenzhuo" , "Zhang, Helin" To: Olivier Matz Return-path: Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) by dpdk.org (Postfix) with ESMTP id D1BB7108F for ; Tue, 17 Jan 2017 14:56:41 +0100 (CET) Content-Disposition: inline In-Reply-To: <20170117092410.5f5950d7@platinum> 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 Tue, Jan 17, 2017 at 09:24:10AM +0100, Olivier Matz wrote: > Hi, > > Thanks Bruce for the comments. > > On Fri, 13 Jan 2017 17:32:38 +0000, "Richardson, Bruce" > wrote: > > > -----Original Message----- > > > From: Olivier Matz [mailto:olivier.matz@6wind.com] > > > Sent: Friday, January 13, 2017 4:44 PM > > > To: dev@dpdk.org > > > Cc: thomas.monjalon@6wind.com; Ananyev, Konstantin > > > ; Lu, Wenzhuo ; > > > Zhang, Helin ; Richardson, Bruce > > > > > > Subject: Re: [dpdk-dev] [RFC 0/9] get Rx and Tx used descriptors > > > > > > Hi, > > > > > > On Thu, 24 Nov 2016 10:54:12 +0100, Olivier Matz > > > wrote: > > > > This RFC patchset introduces a new ethdev API function > > > > rte_eth_tx_queue_count() which is the tx counterpart of > > > > rte_eth_rx_queue_count(). It implements this API on some Intel > > > > drivers for reference, and it also optimizes the implementation of > > > > rte_eth_rx_queue_count(). > > > > > > > > > > I'm planning to send a new version of this patchset, fixing the > > > issues seen by Ferruh, plus a bug fix in the e1000 implementation. > > > > > > Does anyone have any comment about the new API or about questions > > > raised in the cover letter? Especially about the real meaning of > > > "used descriptor": should it include the descriptors hold by the > > > driver? > > For TX, I think we just need used/unused, since for TX any driver > > will reuse a slot that has been completed by the NIC, and doesn't > > hold the mbufs back for buffering at all. > > Agree > > > For RX, strictly speaking, we should have three categories, rather > > than trying to work it into 2. I don't see why we can't report a slot > > as used/unused/unavailable. > > With the rte_eth_rx_queue_count() API, we don't have this opportunity > since it just returns an int. > > Something I found a bit strange when doing this patchset is that the > user does not have the full control of the number of hold buffers. With > default parameters, the effective size of a ring of 128 is 64. > > So it is, we could introduce an API to retrieve the status: > used/unused/unavailable. > > > > Any comment about the method (binary search to find the used > > > descriptors)? > > > > I think binary search should work ok, though linear search may work > > better for smaller ranges as we can prefetch ahead since we know what > > we will check next. Linear can also go backward only if we want > > accuracy (going forward risks having race conditions between read and > > NIC write). Overall, though I think binary search should work well > > enough. > > > > > > > > I'm also wondering about adding rte_eth_tx_descriptor_done() in the > > > API at the same time. > > > > > > > Let me switch the question around - do we need the queue_count APIs at > > all, and is it not more efficient to just supply the > > descriptor_done() APIs? If an app wants to know the use of the ring, > > and take some action based on it, that app is going to have one or > > more thresholds for taking the action, right? In that case, rather > > than scanning descriptors to find the absolute number of free/used > > descriptors, it would be more efficient for the app to just check the > > descriptor on the threshold - and take action based just on that > > value. > > Yes, I reached the same conclusion (...after posting the RFC patchset > unfortunatly). > > > Any app that really does need the absolute value of the ring > > capacity can presumably do its own binary search or linear search to > > determine the value itself. However, I think just doing a done > > function should encourage people to use the more efficient solution > > of just checking the minimum number of descriptors needed. > > > The question is: now that the work is done, is there any application > that would require this absolute values? For instance, monitoring. > > If not, I have no problem to the patchset, I just need to validate my > application with a descriptor_done() API. In this case we can also > deprecate rx_queue_count() and tx_queue_count(). I wouldn't have a problem with deprecating these functions. > > The rte_eth_rx_descriptor_done() function could be updated into: > > /** > * Check the status of a RX descriptor in the queue. > * > * @param port_id > * The port identifier of the Ethernet device. > * @param queue_id > * The queue id on the specific port. > * @param offset > * The offset of the descriptor ID from tail (0 is the next packet to > * be received by the driver). > * - (2) Descriptor is unavailable (hold by driver, not yet returned to hw) > * - (1) Descriptor is done (filled by hw, but not processed by the driver, > * i.e. in the receive queue) > * - (0) Descriptor is available for the hardware to receive a packet. > * - (-ENODEV) if *port_id* invalid. > * - (-ENOTSUP) if the device does not support this function > */ > static inline int rte_eth_rx_descriptor_done(uint8_t port_id, > uint16_t queue_id, uint16_t offset) > > > A similar rte_eth_tx_descriptor_done() would be introduced: > > /** > * Check the status of a TX descriptor in the queue. > * > * @param port_id > * The port identifier of the Ethernet device. > * @param queue_id > * The queue id on the specific port. > * @param offset > * The offset of the descriptor ID from tail (0 is the place where the next > * packet will be send). > * - (1) Descriptor is beeing processed by the hw, i.e. in the transmit queue > * - (0) Descriptor is available for the driver to send a packet. > * - (-ENODEV) if *port_id* invalid. > * - (-ENOTSUP) if the device does not support this function > */ > static inline int rte_eth_tx_descriptor_done(uint8_t port_id, > uint16_t queue_id, uint16_t offset) > > > > An alternative would be to rename these functions in descriptor_status() > instead of descriptor_done(). Seems good naming to me. /Bruce