On Fri, 2005-08-12 at 10:35 -0700, Bryan Henderson wrote: > >>How much extra work does Linux have to do for each interrupt? > > > >usually 1 pci mmio read; the rest is negligible. > > I was hoping you would cater better to my ignorance of how PCI interrupt > handling works in Linux. > > Is it the case that Linux invokes the registered interrupt handler of each > of the drivers for the devices that share the interrupt, and each does an > mmio read of its device to find out if it had reason to generate an > interrupt? So the waste is that extra call, and you're saying the CPU > instructions involved are negligible compared to the mmio read? yes. A function call is like half a cycle (a function pointer call is maybe 40) an mmio read is a lot more > Are these level-sensitive interrupts, so that if both devices need service > at the same time, they generate just one interrupt and neither device > driver call is wasted? ok this is more complex, but if 2 cards raise it quickly after eachother, before the ISR has run, then you only get the handler called once afaik. - : send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
