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Eliminating tasklets

Eliminating tasklets

Posted Jun 28, 2007 14:50 UTC (Thu) by arjan (subscriber, #36785)
In reply to: Eliminating tasklets by rwmj
Parent article: Eliminating tasklets

if you do all work in the irq handler, latency will suck... remember that irq handlers often run with irq's disabled (and at minimum, it's own irq will not happen even if others might).

Offloading the "hard work" out of the hard irq handler means that you can service the hardware short and sweet, with the lowest latency possible. And that the longer taking work gets batched and processed effectively...


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Eliminating tasklets

Posted Jun 29, 2007 21:15 UTC (Fri) by giraffedata (subscriber, #1954) [Link]

But note that the latency that gets improved is the latency of processing interrupts, not the latency of anything a process does. When you consider that a tasklet can't sleep and runs before the CPU returns to regular process stuff, and limit your view to single CPU systems, it isn't as clear that rescheduling interrupt handling for a different time helps any latency. A program that gets interrupted still is not going to get control back until all that interrupt processing is done.

Here's the latency that gets improved: Consider 10 interrupts of the same class that happen one after another. The first 9 take 1ms to service and nobody's urgently waiting for the result. #10 only takes a microsecond, and if you don't respond within 1ms, expensive hardware will go idle. Without tasklets, those interrupts get serviced in order of arrival, so expensive hardware will be idle for 8 ms. With tasklets, you make the code for 1-9 reschedule their work to tasklets (only takes a microsecond to reschedule) and #10 completes in 10 microseconds, soon enough to keep the expensive hardware busy.

Eliminating tasklets

Posted Jun 30, 2007 6:47 UTC (Sat) by dlang (subscriber, #313) [Link]

with workqueues it's not the case that all the interrupt related processing must be completed before userspace gets a chance to run again. with tasklets that is the case. so the switch means that a userspace program that's waiting for some data doesn't need to keep getting delayed while the spu is handling other incomeing data.


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