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| HOWTO for multiqueue network device support |
| =========================================== |
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| Section 1: Base driver requirements for implementing multiqueue support |
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| Intro: Kernel support for multiqueue devices |
| --------------------------------------------------------- |
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| Kernel support for multiqueue devices is always present. |
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| Section 1: Base driver requirements for implementing multiqueue support |
| ----------------------------------------------------------------------- |
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| Base drivers are required to use the new alloc_etherdev_mq() or |
| alloc_netdev_mq() functions to allocate the subqueues for the device. The |
| underlying kernel API will take care of the allocation and deallocation of |
| the subqueue memory, as well as netdev configuration of where the queues |
| exist in memory. |
| |
| The base driver will also need to manage the queues as it does the global |
| netdev->queue_lock today. Therefore base drivers should use the |
| netif_{start|stop|wake}_subqueue() functions to manage each queue while the |
| device is still operational. netdev->queue_lock is still used when the device |
| comes online or when it's completely shut down (unregister_netdev(), etc.). |
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| Section 2: Qdisc support for multiqueue devices |
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| ----------------------------------------------- |
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| Currently two qdiscs are optimized for multiqueue devices. The first is the |
| default pfifo_fast qdisc. This qdisc supports one qdisc per hardware queue. |
| A new round-robin qdisc, sch_multiq also supports multiple hardware queues. The |
| qdisc is responsible for classifying the skb's and then directing the skb's to |
| bands and queues based on the value in skb->queue_mapping. Use this field in |
| the base driver to determine which queue to send the skb to. |
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| sch_multiq has been added for hardware that wishes to avoid head-of-line |
| blocking. It will cycle though the bands and verify that the hardware queue |
| associated with the band is not stopped prior to dequeuing a packet. |
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| On qdisc load, the number of bands is based on the number of queues on the |
| hardware. Once the association is made, any skb with skb->queue_mapping set, |
| will be queued to the band associated with the hardware queue. |
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| Section 3: Brief howto using MULTIQ for multiqueue devices |
| --------------------------------------------------------------- |
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| The userspace command 'tc,' part of the iproute2 package, is used to configure |
| qdiscs. To add the MULTIQ qdisc to your network device, assuming the device |
| is called eth0, run the following command: |
| |
| # tc qdisc add dev eth0 root handle 1: multiq |
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| The qdisc will allocate the number of bands to equal the number of queues that |
| the device reports, and bring the qdisc online. Assuming eth0 has 4 Tx |
| queues, the band mapping would look like: |
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| band 0 => queue 0 |
| band 1 => queue 1 |
| band 2 => queue 2 |
| band 3 => queue 3 |
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| Traffic will begin flowing through each queue based on either the simple_tx_hash |
| function or based on netdev->select_queue() if you have it defined. |
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| The behavior of tc filters remains the same. However a new tc action, |
| skbedit, has been added. Assuming you wanted to route all traffic to a |
| specific host, for example 192.168.0.3, through a specific queue you could use |
| this action and establish a filter such as: |
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| tc filter add dev eth0 parent 1: protocol ip prio 1 u32 \ |
| match ip dst 192.168.0.3 \ |
| action skbedit queue_mapping 3 |
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| Author: Alexander Duyck <[email protected]> |
| Original Author: Peter P. Waskiewicz Jr. <[email protected]> |
