HOWTO for multiqueue network device support =========================================== Section 1: Base driver requirements for implementing multiqueue support Section 2: Qdisc support for multiqueue devices Section 3: Brief howto using PRIO or RR for multiqueue devices Intro: Kernel support for multiqueue devices --------------------------------------------------------- Kernel support for multiqueue devices is only an API that is presented to the netdevice layer for base drivers to implement. This feature is part of the core networking stack, and all network devices will be running on the multiqueue-aware stack. If a base driver only has one queue, then these changes are transparent to that driver. Section 1: Base driver requirements for implementing multiqueue support ----------------------------------------------------------------------- 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.). Finally, the base driver should indicate that it is a multiqueue device. The feature flag NETIF_F_MULTI_QUEUE should be added to the netdev->features bitmap on device initialization. Below is an example from e1000: #ifdef CONFIG_E1000_MQ if ( (adapter->hw.mac.type == e1000_82571) || (adapter->hw.mac.type == e1000_82572) || (adapter->hw.mac.type == e1000_80003es2lan)) netdev->features |= NETIF_F_MULTI_QUEUE; #endif Section 2: Qdisc support for multiqueue devices ----------------------------------------------- Currently two qdiscs support multiqueue devices. A new round-robin qdisc, sch_rr, and sch_prio. The qdisc is responsible for classifying the skb's to bands and queues, and will store the queue mapping into skb->queue_mapping. Use this field in the base driver to determine which queue to send the skb to. sch_rr has been added for hardware that doesn't want scheduling policies from software, so it's a straight round-robin qdisc. It uses the same syntax and classification priomap that sch_prio uses, so it should be intuitive to configure for people who've used sch_prio. In order to utilitize the multiqueue features of the qdiscs, the network device layer needs to enable multiple queue support. This can be done by selecting NETDEVICES_MULTIQUEUE under Drivers. The PRIO qdisc naturally plugs into a multiqueue device. If NETDEVICES_MULTIQUEUE is selected, then on qdisc load, the number of bands requested is compared to the number of queues on the hardware. If they are equal, it sets a one-to-one mapping up between the queues and bands. If they're not equal, it will not load the qdisc. This is the same behavior for RR. Once the association is made, any skb that is classified will have skb->queue_mapping set, which will allow the driver to properly queue skb's to multiple queues. Section 3: Brief howto using PRIO and RR for multiqueue devices --------------------------------------------------------------- The userspace command 'tc,' part of the iproute2 package, is used to configure qdiscs. To add the PRIO qdisc to your network device, assuming the device is called eth0, run the following command: # tc qdisc add dev eth0 root handle 1: prio bands 4 multiqueue This will create 4 bands, 0 being highest priority, and associate those bands to the queues on your NIC. Assuming eth0 has 4 Tx queues, the band mapping would look like: band 0 => queue 0 band 1 => queue 1 band 2 => queue 2 band 3 => queue 3 Traffic will begin flowing through each queue if your TOS values are assigning traffic across the various bands. For example, ssh traffic will always try to go out band 0 based on TOS -> Linux priority conversion (realtime traffic), so it will be sent out queue 0. ICMP traffic (pings) fall into the "normal" traffic classification, which is band 1. Therefore pings will be send out queue 1 on the NIC. Note the use of the multiqueue keyword. This is only in versions of iproute2 that support multiqueue networking devices; if this is omitted when loading a qdisc onto a multiqueue device, the qdisc will load and operate the same if it were loaded onto a single-queue device (i.e. - sends all traffic to queue 0). Another alternative to multiqueue band allocation can be done by using the multiqueue option and specify 0 bands. If this is the case, the qdisc will allocate the number of bands to equal the number of queues that the device reports, and bring the qdisc online. The behavior of tc filters remains the same, where it will override TOS priority classification. Author: Peter P. Waskiewicz Jr.