* Alexey Dobriyan ([EMAIL PROTECTED]) wrote: > On Tue, Jul 03, 2007 at 12:40:56PM -0400, Mathieu Desnoyers wrote: > > Use immediate values with lower d-cache hit in optimized version as a > > condition for scheduler profiling call. > > How much difference in performance do you see? >
Hi Alexey, Please have a look at Documentation/immediate.txt for that information. Also note that the main advantage of the load immediate is to free a cache line. Therefore, I guess the best way to quantify the improvement it brings at one single site is not in terms of cycles, but in terms of number of cache lines used by the scheduler code. Since memory bandwidth seems to be an increasing bottleneck (CPU frequency increases faster than the available memory bandwidth), it makes sense to free as much cache lines as we can. Measuring the overall impact on the system of this single modification results in the difference brought by one site within the standard deviation of the normal samples. It will become significant when the number of immediate values used instead of global variables at hot kernel paths (need to ponder with the frequency at which the data is accessed) will start to be significant compared to the L1 data cache size. We could characterize this in memory to L1 cache transfers per seconds. On 3GHz P4: memory read: ~48 cycles So we can definitely say that 48*HZ (approximation of the frequency at which the scheduler is called) won't make much difference, but as it grows, it will. On a 1000HZ system, it results in: 48000 cycles/second, or 16µs/second, or 0.000016% speedup. However, if we place this in code called much more often, such as do_page_fault, we get, with an hypotetical scenario of approximation of 100000 page faults per second: 4800000 cycles/s, 1.6ms/second or 0.0016% speedup. So as the number of immediate values used increase, the overall memory bandwidth required by the kernel will go down. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/