Hi Mitch, On 10/23/2012 11:55 AM, Mitch Bradley wrote: > On 10/23/2012 4:49 AM, Jon Hunter wrote: > >> Therefore, I believe it will improve search time and hence, boot time if >> we have interrupt-parent defined in each node. > > I strongly suspect (based on many years of performance tuning, with > special focus on boot time) that the time difference will be completely > insignificant. The total extra time for walking up the interrupt tree > for every interrupt in a large system is comparable to the time it takes > to send a few characters out a UART. So you can get more improvement > from eliminating a single printk() than from globally adding per-node > interrupt-parent. > > Furthermore, the cost of processing all of the interrupt-parent > properties is probably similar to the cost of the avoided tree walks. > > CPU cycles are very fast compared to I/O register accesses, say a factor > of 100. Now consider that many modern devices contain embedded > microcontrollers (SD cards, network interface modules, USB hubs and > devices, ...), and those devices usually require various delays measured > in milliseconds, to ensure that the microcontroller is ready for the > next initialization step. Those delays are extremely long compared to > CPU cycles. Obviously, some of that can be overlapped by careful > multithreading, but that isn't free either. > > The bottom line is that I'm pretty sure that adding per-node > interrupt-parent would not be worthwhile from the standpoint of speeding > up boot time. Absolutely, I don't expect this to miraculously improve the boot time or suggest that this is a major contributor to boot time, but what is the best approach in general in terms of efficiency (memory and time). In other words, is there a best practice? And from your feedback, I understand that adding a global interrupt-parent is a good practice. For a bit of fun, I took an omap4430 board and benchmarked the time taken by the of_irq_find_parent() when interrupt-parent was defined for each node using interrupts and without. There were a total of 47 device nodes using interrupts. Adding the interrupt-parent to all 47 nodes increased the dtb from 13211 bytes to 13963 bytes. On boot-up I saw 117 calls to of_irq_find_parent() for this platform (there appears to be multiple calls for a given device). Without interrupt-parent defined for each node total time spent in of_irq_find_parent() was 1.028 ms where as with interrupt-parent defined for each node the total time was 0.4032 ms. This was done using a 38.4MHz timer and the overhead of reading the timer 117 times was about 36 us. I understand that this does not provide the full picture, but I wanted to get a better handle on the times here. So yes the overall overhead here is not significant for us to worry about. Cheers Jon -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
