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    Subject[RFC PATCH 00/20] Cleanup and optimise the page allocator
    Date
    The complexity of the page allocator has been increasing for some time
    and it has now reached the point where the SLUB allocator is doing strange
    tricks to avoid the page allocator. This is obviously bad as it may encourage
    other subsystems to try avoiding the page allocator as well.

    This series of patches is intended to reduce the cost of the page
    allocator by doing the following.

    Patches 1-3 iron out the entry paths slightly and remove stupid sanity
    checks from the fast path.

    Patch 4 uses a lookup table instead of a number of branches to decide what
    zones are usable given the GFP flags.

    Patch 5 avoids repeated checks of the zonelist

    Patch 6 breaks the allocator up into a fast and slow path where the fast
    path later becomes one long inlined function.

    Patches 7-10 avoids calculating the same things repeatedly and instead
    calculates them once.

    Patches 11-13 inline the whole allocator fast path

    Patch 14 avoids calling get_pageblock_migratetype() potentially twice on
    every page free

    Patch 15 reduces the number of times interrupts are disabled by reworking
    what free_page_mlock() does. However, I notice that the cost of calling
    TestClearPageMlocked() is still quite high and I'm guessing it's because
    it's a locked bit operation. It's be nice if it could be established if
    it's safe to use an unlocked version here. Rik, can you comment?

    Patch 16 avoids using the zonelist cache on non-NUMA machines

    Patch 17 removes an expensive and excessively paranoid check in the
    allocator fast path

    Patch 18 avoids a list search in the allocator fast path.

    Patch 19 avoids repeated checking of an empty list.

    Patch 20 gets rid of hot/cold freeing of pages because it incurs cost for
    what I believe to be very dubious gain. I'm not sure we currently gain
    anything by it but it's further discussed in the patch itself.

    Running all of these through a profiler shows me the cost of page allocation
    and freeing is reduced by a nice amount without drastically altering how the
    allocator actually works. Excluding the cost of zeroing pages, the cost of
    allocation is reduced by 25% and the cost of freeing by 12%. Again excluding
    zeroing a page, much of the remaining cost is due to counters, debugging
    checks and interrupt disabling. Of course when a page has to be zeroed,
    the dominant cost of a page allocation is zeroing it.

    Counters are surprising expensive, we spent a good chuck of our time in
    functions like __dec_zone_page_state and __dec_zone_state. In a profiled
    run of kernbench, the time spent in __dec_zone_state was roughly equal to
    the combined cost of the rest of the page free path. A quick check showed
    that almost half of the time in that function is spent on line 233 alone
    which for me is;

    (*p)--;

    That's worth a separate investigation but it might be a case that
    manipulating int8_t on the machine I was using for profiling is unusually
    expensive. Converting this to an int might be faster but the increased
    memory consumption and cache footprint might be a problem. Opinions?

    The downside is that the patches do increase text size because of the
    splitting of the fast path into one inlined blob and the slow path into a
    number of other functions. On my test machine, text increased by 1.2K so
    I might revisit that again and see how much of a difference it really made.

    That all said, I'm seeing good results on actual benchmarks with these
    patches.

    o On many machines, I'm seeing a 0-2% improvement on kernbench. The dominant
    cost in kernbench is the compiler and zeroing allocated pages for
    pagetables.

    o For tbench, I have seen an 8-12% improvement on two x86-64 machines (elm3b6
    on test.kernel.org gained 8%) but generally it was less dramatic on
    x86-64 in the range of 0-4%. On one PPC64, the different was also in the
    range of 0-4%. Generally there were gains, but one specific ppc64 showed a
    regression of 7% for one client but a negligible difference for 8 clients.
    It's not clear why this machine regressed and others didn't.

    o hackbench is harder to conclude anything from. Most machines showed
    performance gains in the 5-11% range but one machine in particular showed
    a mix of gains and losses depending on the number of clients. Might be
    a caching thing.

    o One machine in particular was a major surprise for sysbench with gains
    of 4-8% there which was drastically higher than I was expecting. However,
    on other machines, it was in the more reasonable 0-4% range, still pretty
    respectable. It's not guaranteed though. While most machines showed some
    sort of gain, one ppc64 showed no difference at all.

    So, by and large it's an improvement of some sort.

    I haven't run a page-allocator micro-benchmark to see what sort of figures
    that gives. Christoph, I recall you had some sort of page allocator
    micro-benchmark. Do you want to give it a shot or remind me how to use
    it please?

    All other reviews, comments, alternative benchmark reports are welcome.

    arch/ia64/hp/common/sba_iommu.c | 2 +-
    arch/ia64/kernel/mca.c | 3 +-
    arch/ia64/kernel/uncached.c | 3 +-
    arch/ia64/sn/pci/pci_dma.c | 3 +-
    arch/powerpc/platforms/cell/ras.c | 2 +-
    arch/x86/kvm/vmx.c | 2 +-
    drivers/misc/sgi-gru/grufile.c | 2 +-
    drivers/misc/sgi-xp/xpc_uv.c | 2 +-
    fs/afs/write.c | 4 +-
    fs/btrfs/compression.c | 2 +-
    fs/btrfs/extent_io.c | 4 +-
    fs/btrfs/ordered-data.c | 2 +-
    fs/cifs/file.c | 4 +-
    fs/gfs2/ops_address.c | 2 +-
    fs/hugetlbfs/inode.c | 2 +-
    fs/nfs/dir.c | 2 +-
    fs/ntfs/file.c | 2 +-
    fs/ramfs/file-nommu.c | 2 +-
    fs/xfs/linux-2.6/xfs_aops.c | 4 +-
    include/linux/gfp.h | 58 ++--
    include/linux/mm.h | 1 -
    include/linux/mmzone.h | 8 +-
    include/linux/pagemap.h | 2 +-
    include/linux/pagevec.h | 4 +-
    include/linux/swap.h | 2 +-
    init/main.c | 1 +
    kernel/profile.c | 8 +-
    mm/filemap.c | 4 +-
    mm/hugetlb.c | 4 +-
    mm/internal.h | 10 +-
    mm/mempolicy.c | 2 +-
    mm/migrate.c | 2 +-
    mm/page-writeback.c | 2 +-
    mm/page_alloc.c | 646 ++++++++++++++++++++++-----------
    mm/slab.c | 4 +-
    mm/slob.c | 4 +-
    mm/slub.c | 5 +-
    mm/swap.c | 12 +-
    mm/swap_state.c | 2 +-
    mm/truncate.c | 6 +-
    mm/vmalloc.c | 6 +-
    mm/vmscan.c | 8 +-
    42 files changed, 517 insertions(+), 333 deletions(-)



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