December 2005
Intermediate to advanced
1066 pages
33h 38m
English
Content preview from Understanding Linux Network InternalsBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Congestion Management
Congestion management is an important component of the input frame-processing task. An overloaded CPU can become unstable and introduce a big latency into the system. The section "Interrupts" in Chapter 9 explained why the interrupts generated by a high load can cripple the system. For this reason, congestion management mechanisms are needed to make sure the system's stability is not compromised under high network load. Common ways to reduce the CPU load under high traffic loads include:
- Reducing the number of interrupts if possible
This is accomplished by coding drivers either to process several frames with a single interrupt (see the section "Processing Multiple Frames During an Interrupt" in Chapter 9), or to use NAPI.
- Discarding frames as early as possible in the ingress path
If code knows that a frame is going to be dropped by higher layers, it can save CPU time by dropping the frame quickly. For instance, if a device driver knew that the ingress queue was full, it could drop a frame right away instead of relaying it to the kernel and having the latter drop it.
The second point is what we cover in this section.
A similar optimization applies to the egress path: if a device driver does not have resources to accept new frames for transmission (that is, if the device is out of memory), it would be a waste of CPU time to have the kernel pushing new frames down to the driver for transmission. This point is discussed in Chapter 11 in the section "Enabling and ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.