• Adam Litke's avatar
    hugetlb: Move update_and_free_page · 6af2acb6
    Adam Litke authored
    Dynamic huge page pool resizing.
    
    In most real-world scenarios, configuring the size of the hugetlb pool
    correctly is a difficult task.  If too few pages are allocated to the pool,
    applications using MAP_SHARED may fail to mmap() a hugepage region and
    applications using MAP_PRIVATE may receive SIGBUS.  Isolating too much memory
    in the hugetlb pool means it is not available for other uses, especially those
    programs not using huge pages.
    
    The obvious answer is to let the hugetlb pool grow and shrink in response to
    the runtime demand for huge pages.  The work Mel Gorman has been doing to
    establish a memory zone for movable memory allocations makes dynamically
    resizing the hugetlb pool reliable within the limits of that zone.  This patch
    series implements dynamic pool resizing for private and shared mappings while
    being careful to maintain existing semantics.  Please reply with your comments
    and feedback; even just to say whether it would be a useful feature to you.
    Thanks.
    
    How it works
    ============
    
    Upon depletion of the hugetlb pool, rather than reporting an error immediately,
    first try and allocate the needed huge pages directly from the buddy allocator.
    Care must be taken to avoid unbounded growth of the hugetlb pool, so the
    hugetlb filesystem quota is used to limit overall pool size.
    
    The real work begins when we decide there is a shortage of huge pages.  What
    happens next depends on whether the pages are for a private or shared mapping.
    Private mappings are straightforward.  At fault time, if alloc_huge_page()
    fails, we allocate a page from the buddy allocator and increment the source
    node's surplus_huge_pages counter.  When free_huge_page() is called for a page
    on a node with a surplus, the page is freed directly to the buddy allocator
    instead of the hugetlb pool.
    
    Because shared mappings require all of the pages to be reserved up front, some
    additional work must be done at mmap() to support them.  We determine the
    reservation shortage and allocate the required number of pages all at once.
    These pages are then added to the hugetlb pool and marked reserved.  Where that
    is not possible the mmap() will fail.  As with private mappings, the
    appropriate surplus counters are updated.  Since reserved huge pages won't
    necessarily be used by the process, we can't be sure that free_huge_page() will
    always be called to return surplus pages to the buddy allocator.  To prevent
    the huge page pool from bloating, we must free unused surplus pages when their
    reservation has ended.
    
    Controlling it
    ==============
    
    With the entire patch series applied, pool resizing is off by default so unless
    specific action is taken, the semantics are unchanged.
    
    To take advantage of the flexibility afforded by this patch series one must
    tolerate a change in semantics.  To control hugetlb pool growth, the following
    techniques can be employed:
    
     * A sysctl tunable to enable/disable the feature entirely
     * The size= mount option for hugetlbfs filesystems to limit pool size
    
    Performance
    ===========
    
    When contiguous memory is readily available, it is expected that the cost of
    dynamicly resizing the pool will be small.  This series has been performance
    tested with 'stream' to measure this cost.
    
    Stream (http://www.cs.virginia.edu/stream/
    
    ) was linked with libhugetlbfs to
    enable remapping of the text and data/bss segments into huge pages.
    
    Stream with small array
    -----------------------
    Baseline: 	nr_hugepages = 0, No libhugetlbfs segment remapping
    Preallocated:	nr_hugepages = 5, Text and data/bss remapping
    Dynamic:	nr_hugepages = 0, Text and data/bss remapping
    
    				Rate (MB/s)
    Function	Baseline	Preallocated	Dynamic
    Copy:		4695.6266	5942.8371	5982.2287
    Scale:		4451.5776	5017.1419	5658.7843
    Add:		5815.8849	7927.7827	8119.3552
    Triad:		5949.4144	8527.6492	8110.6903
    
    Stream with large array
    -----------------------
    Baseline: 	nr_hugepages =  0, No libhugetlbfs segment remapping
    Preallocated:	nr_hugepages = 67, Text and data/bss remapping
    Dynamic:	nr_hugepages =  0, Text and data/bss remapping
    
    				Rate (MB/s)
    Function	Baseline	Preallocated	Dynamic
    Copy:		2227.8281	2544.2732	2546.4947
    Scale:		2136.3208	2430.7294	2421.2074
    Add:		2773.1449	4004.0021	3999.4331
    Triad:		2748.4502	3777.0109	3773.4970
    
    * All numbers are averages taken from 10 consecutive runs with a maximum
      standard deviation of 1.3 percent noted.
    
    This patch:
    
    Simply move update_and_free_page() so that it can be reused later in this
    patch series.  The implementation is not changed.
    Signed-off-by: default avatarAdam Litke <agl@us.ibm.com>
    Acked-by: default avatarAndy Whitcroft <apw@shadowen.org>
    Acked-by: default avatarDave McCracken <dave.mccracken@oracle.com>
    Acked-by: default avatarWilliam Irwin <bill.irwin@oracle.com>
    Cc: David Gibson <david@gibson.dropbear.id.au>
    Cc: Ken Chen <kenchen@google.com>
    Cc: Badari Pulavarty <pbadari@us.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    6af2acb6