On Mon, 12 Aug 2024 17:46:42 -0400 Martin Karsten wrote: > >> Here's how it is intended to work: > >> - An administrator sets the existing sysfs parameters for > >> defer_hard_irqs and gro_flush_timeout to enable IRQ deferral. > >> > >> - An administrator sets the new sysfs parameter irq_suspend_timeout > >> to a larger value than gro-timeout to enable IRQ suspension. > > > > Can you expand more on what's the problem with the existing gro_flush_timeout? > > Is it defer_hard_irqs_count? Or you want a separate timeout only for the > > perfer_busy_poll case(why?)? Because looking at the first two patches, > > you essentially replace all usages of gro_flush_timeout with a new variable > > and I don't see how it helps. > > gro-flush-timeout (in combination with defer-hard-irqs) is the default > irq deferral mechanism and as such, always active when configured. Its > static periodic softirq processing leads to a situation where: > > - A long gro-flush-timeout causes high latencies when load is > sufficiently below capacity, or > > - a short gro-flush-timeout causes overhead when softirq execution > asynchronously competes with application processing at high load. > > The shortcomings of this are documented (to some extent) by our > experiments. See defer20 working well at low load, but having problems > at high load, while defer200 having higher latency at low load. > > irq-suspend-timeout is only active when an application uses > prefer-busy-polling and in that case, produces a nice alternating > pattern of application processing and networking processing (similar to > what we describe in the paper). This then works well with both low and > high load. What about NIC interrupt coalescing. defer_hard_irqs_count was supposed to be used with NICs which either don't have IRQ coalescing or have a broken implementation. The timeout of 200usec should be perfectly within range of what NICs can support. If the NIC IRQ coalescing works, instead of adding a new timeout value we could add a new deferral control (replacing defer_hard_irqs_count) which would always kick in after seeing prefer_busy_poll() but also not kick in if the busy poll harvested 0 packets. > > Maybe expand more on what code paths are we trying to improve? Existing > > busy polling code is not super readable, so would be nice to simplify > > it a bit in the process (if possible) instead of adding one more tunable. > > There are essentially three possible loops for network processing: > > 1) hardirq -> softirq -> napi poll; this is the baseline functionality > > 2) timer -> softirq -> napi poll; this is deferred irq processing scheme > with the shortcomings described above > > 3) epoll -> busy-poll -> napi poll > > If a system is configured for 1), not much can be done, as it is > difficult to interject anything into this loop without adding state and > side effects. This is what we tried for the paper, but it ended up being > a hack. > > If however the system is configured for irq deferral, Loops 2) and 3) > "wrestle" with each other for control. Injecting the larger > irq-suspend-timeout for 'timer' in Loop 2) essentially tilts this in > favour of Loop 3) and creates the nice pattern describe above.
