From: Ferruh Yigit <ferruh.yigit@amd.com>
To: "Morten Brørup" <mb@smartsharesystems.com>,
"Bruce Richardson" <bruce.richardson@intel.com>,
jerinj@marvell.com
Cc: dev@dpdk.org, Thomas Monjalon <thomas@monjalon.net>,
Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>,
ferruh.yigit@xilinx.com, ajit.khaparde@broadcom.com,
aboyer@pensando.io, beilei.xing@intel.com, chas3@att.com,
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viacheslavo@nvidia.com, xiao.w.wang@intel.com,
cloud.wangxiaoyun@huawei.com, yisen.zhuang@huawei.com,
yongwang@vmware.com, xuanziyang2@huawei.com,
cristian.dumitrescu@intel.com
Subject: Re: [dpdk-dev] [RFC] ethdev: support Tx queue free descriptor query
Date: Fri, 12 Jan 2024 10:56:39 +0000 [thread overview]
Message-ID: <65c85f06-3cdc-44f2-9a17-217dfcbf7fad@amd.com> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F11E@smartserver.smartshare.dk>
On 1/8/2024 9:15 PM, 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.
>
Agree on the risk,
perhaps adding some documentation to the API helps, we can highlight
intended usecase is QoS implementations, like RED etc..
>>
>> 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:[~2024-01-12 10:57 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
2024-01-12 10:56 ` Ferruh Yigit [this message]
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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