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From: Bruce Richardson <bruce.richardson@intel.com>
To: Olivier MATZ <olivier.matz@6wind.com>
Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH v2 3/4] examples: example showing use of callbacks.
Date: Mon, 16 Feb 2015 15:16:23 +0000	[thread overview]
Message-ID: <20150216151622.GA1888@bricha3-MOBL3> (raw)
In-Reply-To: <54E1FFC4.1060605@6wind.com>

On Mon, Feb 16, 2015 at 03:33:40PM +0100, Olivier MATZ wrote:
> Hi John,
> 
> On 02/13/2015 04:39 PM, John McNamara wrote:
> > From: Richardson, Bruce <bruce.richardson@intel.com>
> > 
> > Example showing how callbacks can be used to insert a timestamp
> > into each packet on RX. On TX the timestamp is used to calculate
> > the packet latency through the app, in cycles.
> > 
> > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> 
> 
> I'm looking at the example and I don't understand what is the advantage
> of having callbacks in ethdev layer, knowing that the application can
> do the same job by a standard function call.
> 
> What is the advantage of having callbacks compared to:
> 
> 
> for (port = 0; port < nb_ports; port++) {
> 	struct rte_mbuf *bufs[BURST_SIZE];
> 	const uint16_t nb_rx = rte_eth_rx_burst(port, 0,
> 			bufs, BURST_SIZE);
> 	if (unlikely(nb_rx == 0))
> 		continue;
> 	add_timestamp(bufs, nb_rx);
> 
> 	const uint16_t nb_tx = rte_eth_tx_burst(port ^ 1, 0,
> 			bufs, nb_rx);
> 	calc_latency(bufs, nb_tx);
> 
> 	if (unlikely(nb_tx < nb_rx)) {
> 		uint16_t buf;
> 		for (buf = nb_tx; buf < nb_rx; buf++)
> 			rte_pktmbuf_free(bufs[buf]);
> 	}
> }
> 
> 
> To me, doing like the code above has several advantages:
> 
> - code is more readable: the callback is explicitly invoked, so there is
>   no risk to forget it
> - code is faster: the functions calls can be inlined by the compiler
> - easier to handle error cases in the callback function as the error
>   code is accessible to the application
> - there is no need to add code in ethdev api to do this
> - if the application does not want to use callbacks (I suppose most
>   applications), it won't have any performance impact
> 
> Regards,
> Olivier

In this specific instance, given that the application does little else, there
is no real advantage to using the callbacks - it's just to have a simple example
of how they can be used.

Where callbacks are really designed to be useful, is for extending or augmenting
hardware capabilities. Taking the example of sequence numbers - to use the most
trivial example - an application could be written to take advantage of sequence
numbers written to packets by the hardware which received them. However, if such
an application was to be used with a NIC which does not provide sequence numbering
capability, for example, anything using ixgbe driver, the application writer has
two choices - either modify his application code to check each packet for
a sequence number in the data path, and add it there post-rx, or alternatively,
to check the NIC capabilities at initialization time, and add a callback there
at initialization, if the hardware does not support it. In the latter case,
the main packet processing body of the application can be written as though
hardware always has sequence numbering capability, safe in the knowledge that
any hardware not supporting it will be back-filled by a software fallback at 
initialization-time.

By the same token, we could also look to extend hardware capabilities. For
different filtering or hashing capabilities, there can be limits in hardware
which are far less than what we need to use in software. Again, callbacks will
allow the data path to be written in a way that is oblivious to the underlying
hardware limits, because software will transparently fill in the gaps.

Hope this makes the use case clear.

Regards,
/Bruce

  reply	other threads:[~2015-02-16 15:16 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-12-22 16:47 [dpdk-dev] [PATCH RFC 0/3] DPDK ethdev callback support Bruce Richardson
2014-12-22 16:47 ` [dpdk-dev] [PATCH RFC 1/3] ethdev: rename callbacks field to intr_cbs Bruce Richardson
2014-12-22 16:47 ` [dpdk-dev] [PATCH RFC 2/3] ethdev: Add in data rxtx callback support Bruce Richardson
2014-12-22 16:47 ` [dpdk-dev] [PATCH RFC 3/3] examples: example showing use of callbacks Bruce Richardson
2014-12-22 17:02 ` [dpdk-dev] [PATCH RFC 0/3] DPDK ethdev callback support Thomas Monjalon
2014-12-22 17:33   ` Bruce Richardson
2014-12-22 17:47     ` Neil Horman
2014-12-23  9:28       ` Bruce Richardson
2014-12-23 13:09         ` Neil Horman
2014-12-23 14:09           ` Bruce Richardson
2015-01-05 16:17     ` Bruce Richardson
2014-12-22 18:31 ` Stephen Hemminger
2014-12-23  9:29   ` Bruce Richardson
2014-12-23  4:23 ` Vithal S Mohare
2014-12-23  9:30   ` Bruce Richardson
2014-12-23  9:37     ` Vithal S Mohare
2014-12-24  1:43       ` Zhang, Helin
2014-12-24  5:06 ` Qiu, Michael
2015-02-12 19:57 ` [dpdk-dev] [PATCH " John McNamara
2015-02-12 19:57   ` [dpdk-dev] [PATCH 1/3] ethdev: rename callbacks field to intr_cbs John McNamara
2015-02-12 19:57   ` [dpdk-dev] [PATCH 2/3] ethdev: Add in data rxtx callback support John McNamara
2015-02-12 21:12     ` Neil Horman
2015-02-12 19:57   ` [dpdk-dev] [PATCH 3/3] examples: example showing use of callbacks John McNamara
2015-02-13 14:54   ` [dpdk-dev] [PATCH 0/3] DPDK ethdev callback support Declan Doherty
2015-02-13 15:39 ` [dpdk-dev] [PATCH v2 0/4] " John McNamara
2015-02-13 15:39   ` [dpdk-dev] [PATCH v2 1/4] ethdev: rename callbacks field to intr_cbs John McNamara
2015-02-13 16:06     ` Thomas Monjalon
2015-02-13 16:52       ` Thomas Monjalon
2015-02-13 15:39   ` [dpdk-dev] [PATCH v2 2/4] ethdev: Add in data rxtx callback support John McNamara
2015-02-13 16:33     ` Thomas Monjalon
2015-02-13 17:49       ` Bruce Richardson
2015-02-13 15:39   ` [dpdk-dev] [PATCH v2 3/4] examples: example showing use of callbacks John McNamara
2015-02-13 16:02     ` Thomas Monjalon
2015-02-16 14:33     ` Olivier MATZ
2015-02-16 15:16       ` Bruce Richardson [this message]
2015-02-16 17:34         ` Thomas Monjalon
2015-02-17 12:17           ` Declan Doherty
2015-02-17 12:25           ` Bruce Richardson
2015-02-17 13:28             ` Olivier MATZ
2015-02-17 13:50               ` Bruce Richardson
2015-02-17 15:49                 ` Neil Horman
2015-02-17 16:00                   ` Bruce Richardson
2015-02-17 16:08                     ` Neil Horman
2015-02-17 16:15                       ` Bruce Richardson
2015-02-17 19:27                         ` Neil Horman
2015-02-17 15:32             ` Thomas Monjalon
2015-02-17 15:58               ` Bruce Richardson
2015-02-13 15:39   ` [dpdk-dev] [PATCH v2 4/4] abi: Added rxtx callback functions to ABI versioning John McNamara
2015-02-13 15:59     ` Thomas Monjalon
2015-02-13 15:48   ` [dpdk-dev] [PATCH v2 0/4] DPDK ethdev callback support Declan Doherty
2015-02-18 17:42 ` [dpdk-dev] [PATCH v3 0/3] " John McNamara
2015-02-18 17:42   ` [dpdk-dev] [PATCH v3 1/3] ethdev: Rename callbacks field to link_intr_cbs John McNamara
2015-02-18 17:42   ` [dpdk-dev] [PATCH v3 2/3] ethdev: Add rxtx callback support John McNamara
2015-02-18 18:19     ` Thomas Monjalon
2015-02-19  9:33       ` Mcnamara, John
2015-02-18 17:42   ` [dpdk-dev] [PATCH v3 3/3] examples: example showing use of callbacks John McNamara
2015-02-19 17:56   ` [dpdk-dev] [PATCH v4 0/3] DPDK ethdev callback support John McNamara
2015-02-19 17:56     ` [dpdk-dev] [PATCH v4 1/3] ethdev: rename callbacks field to link_intr_cbs John McNamara
2015-02-19 17:56     ` [dpdk-dev] [PATCH v4 2/3] ethdev: add optional rxtx callback support John McNamara
2015-02-20 10:06       ` Bruce Richardson
2015-02-20 10:31         ` Thomas Monjalon
2015-02-19 17:56     ` [dpdk-dev] [PATCH v4 3/3] examples: example showing use of callbacks John McNamara
2015-02-20 17:03   ` [dpdk-dev] [PATCH v5 0/3] DPDK ethdev callback support John McNamara
2015-02-20 17:03     ` [dpdk-dev] [PATCH v5 1/3] ethdev: rename callbacks field to link_intr_cbs John McNamara
2015-02-20 17:03     ` [dpdk-dev] [PATCH v5 2/3] ethdev: add optional rxtx callback support John McNamara
2015-02-23 15:11       ` Thomas Monjalon
2015-02-23 17:27         ` Mcnamara, John
2015-02-20 17:03     ` [dpdk-dev] [PATCH v5 3/3] examples: example showing use of callbacks John McNamara
2015-02-23 18:30   ` [dpdk-dev] [PATCH v6 0/3] DPDK ethdev callback support John McNamara
2015-02-23 18:30     ` [dpdk-dev] [PATCH v6 1/3] ethdev: rename callbacks field to link_intr_cbs John McNamara
2015-02-23 18:30     ` [dpdk-dev] [PATCH v6 2/3] ethdev: add optional rxtx callback support John McNamara
2015-02-23 18:30     ` [dpdk-dev] [PATCH v6 3/3] examples: example showing use of callbacks John McNamara
2015-02-23 23:39     ` [dpdk-dev] [PATCH v6 0/3] DPDK ethdev callback support Thomas Monjalon

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