From: Bruce Richardson <bruce.richardson@intel.com>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: <jerinj@marvell.com>, <dev@dpdk.org>,
Thomas Monjalon <thomas@monjalon.net>,
Ferruh Yigit <ferruh.yigit@amd.com>,
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Subject: Re: [dpdk-dev] [RFC] ethdev: support Tx queue free descriptor query
Date: Tue, 9 Jan 2024 08:47:01 +0000 [thread overview]
Message-ID: <ZZ0IBRNw4-Qc-drs@bricha3-MOBL.ger.corp.intel.com> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F11E@smartserver.smartshare.dk>
On Mon, Jan 08, 2024 at 10:15:40PM +0100, Morten Brørup wrote:
> > 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).
>
Ok, thanks for the info. All good with me so.
/Bruce
next prev parent reply other threads:[~2024-01-09 8:47 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
2024-01-09 8:47 ` Bruce Richardson [this message]
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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