From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Bruce Richardson" <bruce.richardson@intel.com>, <jerinj@marvell.com>
Cc: <dev@dpdk.org>, "Thomas Monjalon" <thomas@monjalon.net>,
"Ferruh Yigit" <ferruh.yigit@amd.com>,
"Andrew Rybchenko" <andrew.rybchenko@oktetlabs.ru>,
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<cristian.dumitrescu@intel.com>
Subject: RE: [dpdk-dev] [RFC] ethdev: support Tx queue free descriptor query
Date: Mon, 8 Jan 2024 22:15:40 +0100 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9F11E@smartserver.smartshare.dk> (raw)
In-Reply-To: <ZZvUXA3Jh0DqemLB@bricha3-MOBL.ger.corp.intel.com>
> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Monday, 8 January 2024 11.54
>
> On Tue, Dec 19, 2023 at 10:59:48PM +0530, jerinj@marvell.com wrote:
> > From: Jerin Jacob <jerinj@marvell.com>
> >
> > Introduce a new API to retrieve the number of available free
> descriptors
> > in a Tx queue. Applications can leverage this API in the fast path to
> > inspect the Tx queue occupancy and take appropriate actions based on
> the
> > available free descriptors.
> >
> > A notable use case could be implementing Random Early Discard (RED)
> > in software based on Tx queue occupancy.
> >
> > Signed-off-by: Jerin Jacob <jerinj@marvell.com>
> > ---
>
> Hi Jerin,
>
> while I don't strongly object to this patch, I wonder if it encourages
> sub-optimal code implementations. To determine the number of free
> descriptors in a ring, the driver in many cases will need to do a scan
> of
> the descriptor ring to see how many are free/used. However, I suspect
> that
> in most cases we will see something like the following being done:
>
> count = rte_eth_rx_free_count();
Typo: rte_eth_rx_free_count() -> rte_eth_tx_free_count()
> if (count > X) {
> /* Do something */
> }
>
> For instances like this, scanning the entire ring is wasteful of
> resources.
> Instead it would be better to just check the descriptor at position X
> explicitly. Going to the trouble of checking the exact descriptor count
> is
> unnecessary in this case.
Yes, it introduces such a risk.
All DPDK examples simply call tx_burst() without checking free space first, so I think the probability (of the simple case) is low.
And the probability for the case comparing to X could be mitigated by referring to rte_eth_tx_descriptor_status() in the function description.
>
> Out of interest, are you aware of a case where an app would need to
> know
> exactly the number of free descriptors, and where the result would not
> be
> just compared to one or more threshold values? Do we see cases where it
> would be used in a computation, perhaps?
Yes: RED. When exceeding the minimum threshold, the probability of marking/dropping a packet increases linearly with the queue's fill level.
E.g.:
0-900 packets in queue: don't drop,
901-1000 packets in queue: probability of dropping the packet is 1-100 % (e.g. 980 packets in queue = 80 % drop probability).
next prev parent reply other threads:[~2024-01-08 21:15 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-19 17:29 jerinj
2024-01-04 13:16 ` Dumitrescu, Cristian
2024-01-04 13:35 ` Jerin Jacob
2024-01-04 14:21 ` Konstantin Ananyev
2024-01-04 18:29 ` Thomas Monjalon
2024-01-05 9:57 ` Jerin Jacob
2024-01-05 10:03 ` Thomas Monjalon
2024-01-05 11:12 ` Konstantin Ananyev
2024-01-08 20:54 ` Morten Brørup
2024-01-09 14:45 ` Jerin Jacob
2024-01-04 21:17 ` Thomas Monjalon
2024-01-05 9:54 ` Jerin Jacob
2024-01-05 10:02 ` Thomas Monjalon
2024-01-08 10:54 ` Bruce Richardson
2024-01-08 21:15 ` Morten Brørup [this message]
2024-01-09 8:47 ` Bruce Richardson
2024-01-12 10:56 ` Ferruh Yigit
2024-01-11 15:17 ` [dpdk-dev] [v1] ethdev: support Tx queue used count jerinj
2024-01-11 16:17 ` Andrew Rybchenko
2024-01-12 6:56 ` Jerin Jacob
2024-01-11 16:20 ` Morten Brørup
2024-01-12 6:59 ` Jerin Jacob
2024-01-11 17:00 ` Stephen Hemminger
2024-01-12 7:01 ` Jerin Jacob
2024-01-12 16:30 ` Stephen Hemminger
2024-01-12 8:02 ` David Marchand
2024-01-12 9:29 ` Jerin Jacob
2024-01-12 11:34 ` Ferruh Yigit
2024-01-12 12:11 ` David Marchand
2024-01-12 14:25 ` Ferruh Yigit
2024-01-12 12:29 ` Morten Brørup
2024-01-12 14:29 ` Ferruh Yigit
2024-01-18 9:06 ` Jerin Jacob
2024-01-12 12:33 ` Konstantin Ananyev
2024-01-16 6:37 ` Jerin Jacob
2024-01-18 10:17 ` Konstantin Ananyev
2024-01-18 11:21 ` Jerin Jacob
2024-01-18 13:36 ` Morten Brørup
2024-01-19 9:52 ` Konstantin Ananyev
2024-01-19 10:32 ` Morten Brørup
2024-01-12 16:52 ` Stephen Hemminger
2024-01-18 9:47 ` [dpdk-dev] [v2] " jerinj
2024-01-22 13:00 ` Konstantin Ananyev
2024-01-23 11:46 ` Ferruh Yigit
2024-02-07 20:30 ` Ferruh Yigit
2024-01-29 15:03 ` Ferruh Yigit
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