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In this case the signal about the interrupt will come to the CPU first, so the processor will read not yet valid data. If MSI is used, information about the MSI is transmitted in the same way as data messages, and so it can’t come earlier.
Data may still be dropped in software, though, and other tuning is required to reduce or eliminate drops completely. MSI-X daemon-tools-lite.down4you.software interrupts are the preferred method, especially for NICs that support multiple RX queues. This is because each RX queue can have its own hardware interrupt assigned, which can then be handled by a specific CPU (with irqbalance or by modifying /proc/irq/IRQ_NUMBER/smp_affinity). As we’ll see shortly, the CPU that handles the interrupt will be the CPU that processes the packet.
Recall in previous sections that device drivers allocate a region of memory for the device to perform DMA to incoming packets. Just as it is the responsibility of the driver to allocate those regions, it is also the responsibility of the driver to unmap those regions, harvest the data, and send it up the network stack. The softirq system in the Linux kernel is a mechanism for executing code outside of the context of an interrupt handler implemented in a driver. This system is important because hardware interrupts may be disabled during all or part of the execution of an interrupt handler. The longer interrupts are disabled, the greater chance that events may be missed.
Hardware interrupts are used by devices to communicate that they require attention from the operating system. For example, pressing a key on the keyboard or moving the mouse triggers hardware interrupts that cause the processor to read the keystroke or mouse position. The act of initiating a hardware interrupt is referred to as an interrupt request . This means incoming packets can be DMA’d to a separate memory region for each queue, with a separate NAPI structure to manage polling this region, as well. Thus multiple CPUs will handle interrupts from the device and also process packets.
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So, it is important to defer any long running actions outside of the interrupt handler so that it can complete as quickly as possible and re-enable interrupts from the device. Some NICs and their drivers also support adjusting the size of the RX queue. Exactly how this works is hardware specific, but luckily ethtool provides a generic way for users to adjust the size. Increasing the size of the RX queue can help prevent network data drops at the NIC during periods where large numbers of data frames are received.
