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Rik van Riel wrote: > Nick Piggin wrote: > >> OK, sure. I think we need more numbers though. > > > Thinking about the issue some more, I think I know just the > number we might want to know. > > It is pretty obvious that the kernel needs to do less work > with the MADV_FREE code present. However, it is possible > that userspace needs to do more work, by accessing pages > that are not in the CPU cache, or in another CPU's cache. > > In the test cases where you see similar performance on the > workload with and without the MADV_FREE code, are you by any > chance seeing lower system time and higher user time? I didn't actually check system and user times for the mysql benchmark, but that's exactly what I had in mind when I mentioned the poor cache behaviour this patch could cause. I definitely did see user times go up in benchmarks where I measured. We have percpu and cache affine page allocators, so when userspace just frees a page, it is likely to be cache hot, so we want to free it up so it can be reused by this CPU ASAP. Likewise, when we newly allocate a page, we want it to be one that is cache hot on this CPU. > I think that maybe for 2.6.22 we should just alias MADV_FREE > to run with the MADV_DONTNEED functionality, so that the glibc > people can make the change on their side while we figure out > what will be the best thing to do on the kernel side. > > I'll send in a patch that does that once Linus has committed > your most recent flood of patches. What do you think? I'll let you and Ulrich decide on that. Keep in mind that older kernels (without the mmap_sem patch for MADV_DONTNEED) still seem to get a pretty decent improvement from using MADV_DONTNEED, so it is possible glibc will want to start using that anyway. -- SUSE Labs, Novell Inc. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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