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From: Morten Brørup <mb@smartsharesystems.com>
To: "Bruce Richardson" <bruce.richardson@intel.com>
Cc: "Jeff Guo" <jia.guo@intel.com>,
	"Thomas Monjalon" <thomas@monjalon.net>,
	"Ferruh Yigit" <ferruh.yigit@intel.com>,
	"Andrew Rybchenko" <arybchenko@solarflare.com>,
	<qiming.yang@intel.com>, <beilei.xing@intel.com>,
	<wei.zhao1@intel.com>, <qi.z.zhang@intel.com>,
	<jingjing.wu@intel.com>, <dev@dpdk.org>, <helin.zhang@intel.com>,
	<barbette@kth.se>
Subject: Re: [dpdk-dev] [RFC] ethdev: rte_eth_rx_burst() requirements for nb_pkts
Date: Thu, 27 Aug 2020 11:31:15 +0200
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35C6125A@smartserver.smartshare.dk> (raw)
In-Reply-To: <20200827090944.GA569@bricha3-MOBL.ger.corp.intel.com>

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Thursday, August 27, 2020 11:10 AM
> 
> On Thu, Aug 27, 2020 at 10:40:11AM +0200, Morten Brørup wrote:
> > Jeff and Ethernet API maintainers Thomas, Ferruh and Andrew,
> >
> > I'm hijacking this patch thread to propose a small API modification
> that prevents unnecessarily performance degradations.
> >
> > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Jeff Guo
> > > Sent: Thursday, August 27, 2020 9:55 AM
> > >
> > > The limitation of burst size in vector rx was removed, since it
> should
> > > retrieve as much received packets as possible. And also the
> scattered
> > > receive path should use a wrapper function to achieve the goal of
> > > burst maximizing.
> > >
> > > This patch set aims to maximize vector rx burst for for
> > > ixgbe/i40e/ice/iavf PMDs.
> > >
> >
> > Now I'm going to be pedantic and say that it still doesn't conform to
> the rte_eth_rx_burst() API, because the API does not specify any
> minimum requirement for nb_pkts.
> >
> > In theory, that could also be fixed in the driver by calling the non-
> vector function from the vector functions if nb_pkts is too small for
> the vector implementation.
> >
> > However, I think that calling rte_eth_rx_burst() with a small nb_pkts
> is silly and not in the spirit of DPDK, and introducing an additional
> comparison for a small nb_pkts in the driver vector functions would
> degrade their performance (only slightly, but anyway).
> >
> 
> Actually, I'd like to see a confirmed measurement showing a slowdown
> before
> we discard such an option. :-)

Good point!

> While I agree that using small bursts is
> not
> keeping with the design approach of DPDK of using large bursts to
> amortize
> costs and allow prefetching, there are cases where a user/app may want
> a
> small burst size, e.g. 4, for latency reasons, and we need a way to
> support
> that.
> 
I assume that calling rte_eth_rx_burst() with nb_pkts=32 returns 4 packets if only 4 packets are available, so you would need to be extremely latency sensitive to call it with a smaller nb_pkts. I guess that high frequency trading is the only real life scenario here.

> Since the path selection is dynamic, we need to either:
> a) provide a way for the user to specify that they will use smaller
> bursts
> and so that vector functions should not be used
> b) have the vector functions transparently fallback to the scalar ones
> if
> used with smaller bursts
> 
> Of these, option b) is simpler, and should be low cost since any check
> is
> just once per burst, and - assuming an app is written using the same
> request-size each time - should be entirely predictable after the first
> call.
> 
Why does everyone assume that DPDK applications are so simple that the branch predictor will cover the entire data path? I hear this argument over and over again, and by principle I disagree with it!

How about c): add rte_eth_rx() and rte_eth_tx() functions for receiving/transmitting a single packet. The ring library has such functions.

Optimized single-packet functions might even perform better than calling the burst functions with nb_pkts=1. Great for latency focused applications. :-)

> /Bruce
> 


  reply	other threads:[~2020-08-27  9:31 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-27  7:54 [dpdk-dev] [PATCH v1 0/4] maximize vector rx burst for PMDs Jeff Guo
2020-08-27  7:54 ` [dpdk-dev] [PATCH v1 1/4] net/ixgbe: maximize vector rx burst for ixgbe Jeff Guo
2020-08-27  7:54 ` [dpdk-dev] [PATCH v1 2/4] net/i40e: maximize vector rx burst for i40e Jeff Guo
2020-08-27  7:54 ` [dpdk-dev] [PATCH v1 3/4] net/ice: maximize vector rx burst for ice Jeff Guo
2020-08-27  7:54 ` [dpdk-dev] [PATCH v1 4/4] net/iavf: maximize vector rx burst for iavf Jeff Guo
2020-08-27  8:40 ` [dpdk-dev] [RFC] ethdev: rte_eth_rx_burst() requirements for nb_pkts Morten Brørup
2020-08-27  9:09   ` Bruce Richardson
2020-08-27  9:31     ` Morten Brørup [this message]
2020-08-27  9:43       ` Bruce Richardson
2020-08-27 10:13         ` [dpdk-dev] [RFC] ethdev: rte_eth_rx_burst() requirements fornb_pkts Morten Brørup
2020-08-27 11:41           ` Bruce Richardson
2020-08-28  9:03             ` Morten Brørup
2020-08-28 10:07               ` Bruce Richardson
2020-08-28 10:50                 ` Morten Brørup
2020-08-29 10:15                 ` Morten Brørup
2020-09-09  6:36 ` [dpdk-dev] [PATCH v3 0/5] fix vector rx burst for PMDs Jeff Guo
2020-09-09  6:36   ` [dpdk-dev] [PATCH v3 1/5] net/iavf: fix vector rx burst for iavf Jeff Guo
2020-09-09  6:36   ` [dpdk-dev] [PATCH v3 2/5] net/ixgbe: fix vector rx burst for ixgbe Jeff Guo
     [not found]     ` <VI1PR0802MB23518C6B517B6EAD8E018CD49E260@VI1PR0802MB2351.eurprd08.prod.outlook.com>
2020-09-09  9:54       ` [dpdk-dev] 回复: " Feifei Wang
2020-09-09  6:36   ` [dpdk-dev] [PATCH v3 3/5] net/i40e: fix vector rx burst for i40e Jeff Guo
2020-09-09  6:36   ` [dpdk-dev] [PATCH v3 4/5] net/ice: fix vector rx burst for ice Jeff Guo
2020-09-15  7:10     ` Han, YingyaX
2020-09-09  6:36   ` [dpdk-dev] [PATCH v3 5/5] net/fm10k: fix vector rx burst for fm10k Jeff Guo
2020-09-09  6:45   ` [dpdk-dev] [PATCH v3 0/5] fix vector rx burst for PMDs Wang, Haiyue
2020-09-09  7:03     ` Guo, Jia
2020-09-09  7:05       ` Wang, Haiyue
2020-09-09  7:43         ` Morten Brørup
2020-09-09  7:55           ` Wang, Haiyue
2020-09-09  8:01             ` Guo, Jia
2020-09-17  7:58 ` [dpdk-dev] [PATCH v4 " Jeff Guo
2020-09-17  7:58   ` [dpdk-dev] [PATCH v4 1/5] net/iavf: fix vector rx burst for iavf Jeff Guo
2020-09-17  7:58   ` [dpdk-dev] [PATCH v4 2/5] net/ixgbe: fix vector rx burst for ixgbe Jeff Guo
2020-09-17  7:58   ` [dpdk-dev] [PATCH v4 3/5] net/i40e: fix vector rx burst for i40e Jeff Guo
2020-09-17  7:58   ` [dpdk-dev] [PATCH v4 4/5] net/ice: fix vector rx burst for ice Jeff Guo
2020-09-17 11:03     ` Zhang, Qi Z
2020-09-18  3:20       ` Guo, Jia
2020-09-18  3:41         ` Zhang, Qi Z
2020-09-18  4:41           ` Guo, Jia
2020-09-18  5:39             ` Zhang, Qi Z
2020-09-17  7:58   ` [dpdk-dev] [PATCH v4 5/5] net/fm10k: fix vector rx burst for fm10k Jeff Guo
2020-10-16  9:44 ` [dpdk-dev] [PATCH v5 0/5] fix vector rx burst for PMDs Jeff Guo
2020-10-16  9:44   ` [dpdk-dev] [PATCH v5 1/5] net/ixgbe: fix vector rx burst for ixgbe Jeff Guo
2020-10-16  9:44   ` [dpdk-dev] [PATCH v5 2/5] net/i40e: fix vector rx burst for i40e Jeff Guo
2020-10-16  9:44   ` [dpdk-dev] [PATCH v5 3/5] net/ice: fix vector rx burst for ice Jeff Guo
2020-10-16  9:44   ` [dpdk-dev] [PATCH v5 4/5] net/fm10k: fix vector rx burst for fm10k Jeff Guo
2020-10-16  9:44   ` [dpdk-dev] [PATCH v5 5/5] net/iavf: fix vector rx burst for iavf Jeff Guo
2020-10-23  5:09     ` Ling, WeiX
2020-10-23 10:11   ` [dpdk-dev] [PATCH v5 0/5] fix vector rx burst for PMDs Zhang, Qi Z

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