On Fri, 13 Dec 2013 15:59:03 -0800 Dave Hansen <dave@xxxxxxxx> wrote: > SLUB depends on a 16-byte cmpxchg for an optimization. For the > purposes of this series, I'm assuming that it is a very important > optimization that we desperately need to keep around. What if we don't do that. > In order to get guaranteed 16-byte alignment (required by the > hardware on x86), 'struct page' is padded out from 56 to 64 > bytes. > > Those 8-bytes matter. We've gone to great lengths to keep > 'struct page' small in the past. It's a shame that we bloat it > now just for alignment reasons when we have extra space. Plus, > bloating such a commonly-touched structure *HAS* to have cache > footprint implications. > > These patches attempt _internal_ alignment instead of external > alignment for slub. > > I also got a bug report from some folks running a large database > benchmark. Their old kernel uses slab and their new one uses > slub. They were swapping and couldn't figure out why. It turned > out to be the 2GB of RAM that the slub padding wastes on their > system. > > On my box, that 2GB cost about $200 to populate back when we > bought it. I want my $200 back. > > This set takes me from 16909584K of reserved memory at boot > down to 14814472K, so almost *exactly* 2GB of savings! It also > helps performance, presumably because it touches 14% fewer > struct page cachelines. A 30GB dd to a ramfs file: > > dd if=/dev/zero of=bigfile bs=$((1<<30)) count=30 > > is sped up by about 4.4% in my testing. This is a gruesome and horrible tale of inefficiency and regression. >From 5-10 minutes of gitting I couldn't see any performance testing results for slub's cmpxchg_double stuff. I am thinking we should just tip it all overboard unless someone can demonstrate sufficiently serious losses from so doing. --- a/arch/x86/Kconfig~a +++ a/arch/x86/Kconfig @@ -78,7 +78,6 @@ config X86 select ANON_INODES select HAVE_ALIGNED_STRUCT_PAGE if SLUB select HAVE_CMPXCHG_LOCAL - select HAVE_CMPXCHG_DOUBLE select HAVE_ARCH_KMEMCHECK select HAVE_USER_RETURN_NOTIFIER select ARCH_BINFMT_ELF_RANDOMIZE_PIE _ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>