Manu Abraham wrote: > On Wed, Dec 9, 2009 at 3:43 AM, Mauro Carvalho Chehab > <mchehab@xxxxxxxxxx> wrote: >> Even with STB, let's assume a very slow cpu that runs at 100 Megabytes/second. So, the clock >> speed is 10 nanoseconds. Assuming that this CPU doesn't have a good pipeline, being >> capable of handling only one instruction per second, you'll have one instruction at executed >> at each 10 nanoseconds (as a reference, a Pentium 1, running at 133 Mbps is faster than this). > > Incorrect. > A CPU doesn't execute instruction per clock cycle. Clock cycles > required to execute an instruction do vary from 2 cycles 12 cycles > varying from CPU to CPU. See the description of an old Pentium MMX processor (the sucessor of i586, running up to 200 MHz): http://www.intel.com/design/archives/processors/mmx/docs/243185.htm Thanks to superscalar architecture, it runs 2 instructions per clock cycle (IPC). Newer processors can run more instructions per clock cycle. For example, any Pentium-4 processor, can do 3 IPC: http://www.intel.com/support/processors/pentium4/sb/CS-017371.htm >> So, even on such bad hardware that is at least 20x slower than a netbook running at 1Gbps, >> what determines the delay is the amount of I/O you're doing, and not the number of extra >> code. > > > The I/O overhead required to read 4 registers from hardware is the > same whether you use the ioctl approach or s2api. It seems you got my point. What will determinate the delay is the number of I/O's, and not the amount of instructions. > Eventually, as you have pointed out yourself, The data struct will be > in the cache all the time for the ioctl approach. The only new > addition to the existing API in the ioctl case is a CALL instruction > as compared to the numerous instructions in comparison to that you > have pointed out as with the s2api approach. True, but, as shown, the additional delay introduced by the code is less than 0.01%, even on a processor that has half of the speed of a 12-year old very slow CPU (a Pentium MMX @ 100 MHz is capable of 2 IPC. My calculus assumed 1 IPC). So, what will affect the delay is the number of I/O you need to do. To get all data that the ioctl approach struct has, the delay for S2API will be equal. To get less data, S2API will have a small delay. Cheers, Mauro. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
