2장. Tuning IRQ balancing

To troubleshoot performance issues, understand how a computer manages tasks and responds to events. This process relies on interrupts, which signal the CPU to handle an urgent task, and interrupt handlers, which are the specific routines that process those requests.

When a network interface controller (NIC) receives incoming data, it copies the data into kernel buffers by using Direct Memory Access (DMA). The NIC then notifies the kernel about this data by triggering a hard interrupt. These interrupts are processed by interrupt handlers which do minimal work, as they have already interrupted another task and the handlers cannot interrupt themselves. Hard interrupts can be costly in terms of CPU usage, especially if they use kernel locks.

The hard interrupt handler then leaves the majority of packet reception to a software interrupt request (SoftIRQ) process. The kernel can schedule these processes more fairly.

# grep -E "CPU|enp1s0" /proc/interrupts
         CPU0     CPU1     CPU2    CPU3    CPU4   CPU5
 105:  141606        0        0       0       0      0  IR-PCI-MSI-edge      enp1s0-rx-0
 106:       0   141091        0       0       0      0  IR-PCI-MSI-edge      enp1s0-rx-1
 107:       2        0   163785       0       0      0  IR-PCI-MSI-edge      enp1s0-rx-2
 108:       3        0        0  194370       0      0  IR-PCI-MSI-edge      enp1s0-rx-3
 109:       0        0        0       0       0      0  IR-PCI-MSI-edge      enp1s0-tx