On Wed, Mar 28, 2007 at 01:12:56PM -0700, Christoph Lameter wrote: > The benefit of preconstructed pgds and pmds in the i386 arch code seem to > be debatable. The performance measurements indicate that there may be a slight > benefit but it seems to almost vanish in the noise ratio. > Method used (i386 1G memory): > 1. Boot kernel > 2. make clean > 3. time make all
Getting the relevant results without tremendous amounts of noise from other kernel activity needs something like lmbench's fault and fork() microbenchmarks. Also, /proc/profile and/or oprofile results would be useful here to get useful notions of what's happening performancewise, in particular oprofile with L2 cache miss performance counters. The magnitude of the degradation in the specific operations can't really be established without these sorts of affairs. We need to know how large a fraction of the workload is attributable to the operations in question. Profiling would already give something of a breakdown but it's difficult to get the numbers to add up properly or to separate fork() from faults, so the microbenchmarks focused on the particular operations are needed. The profiles are still needed to understand how the time shuffled around and to either confirm or invalidate notions regarding cache behavior. In any event, I already know the answers because I already did all this a number of years ago: it's all dominated by PTE caching, which was never merged, but I carried out-of-tree for several years, so of course it's in the noise. PMD and PTE caching are only pertinent to fork() anyway, so the vast majority of your workload is unaffected, and it's even more "in the noise" due to that. I can't be arsed to care about it anymore since i386 became a target-system-only architecture for me and am sick of the flak coming at me for my pagetable caching code anyway. Hence this patch. -- wli - 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/
