On Fri, 2005-08-12 at 19:51 +0200, Arjan van de Ven wrote: > 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 a bit off topic, but where u get these information? thx > > > 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. so it is possible that both card raise intr quicker than isr run, then both isr will do mmio and go ahead, but then what is device 1 raise intr device 2 isr run and not belong to it, so quit, but before it clear intr and quit device 2 raise intr device 1 isr run and handle device 1, then clear intr and quit. then device 2 intr lost? or i understand this wrong. :) ming > > > - : 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
