• Alexander Duyck's avatar
    mm: adjust shuffle code to allow for future coalescing · a2129f24
    Alexander Duyck authored
    Patch series "mm / virtio: Provide support for free page reporting", v17.
    
    This series provides an asynchronous means of reporting free guest pages
    to a hypervisor so that the memory associated with those pages can be
    dropped and reused by other processes and/or guests on the host.  Using
    this it is possible to avoid unnecessary I/O to disk and greatly improve
    performance in the case of memory overcommit on the host.
    
    When enabled we will be performing a scan of free memory every 2 seconds
    while pages of sufficiently high order are being freed.  In each pass at
    least one sixteenth of each free list will be reported.  By doing this we
    avoid racing against other threads that may be causing a high amount of
    memory churn.
    
    The lowest page order currently scanned when reporting pages is
    pageblock_order so that this feature will not interfere with the use of
    Transparent Huge Pages in the case of virtualization.
    
    Currently this is only in use by virtio-balloon however there is the hope
    that at some point in the future other hypervisors might be able to make
    use of it.  In the virtio-balloon/QEMU implementation the hypervisor is
    currently using MADV_DONTNEED to indicate to the host kernel that the page
    is currently free.  It will be zeroed and faulted back into the guest the
    next time the page is accessed.
    
    To track if a page is reported or not the Uptodate flag was repurposed and
    used as a Reported flag for Buddy pages.  We walk though the free list
    isolating pages and adding them to the scatterlist until we either
    encounter the end of the list or have processed at least one sixteenth of
    the pages that were listed in nr_free prior to us starting.  If we fill
    the scatterlist before we reach the end of the list we rotate the list so
    that the first unreported page we encounter is moved to the head of the
    list as that is where we will resume after we have freed the reported
    pages back into the tail of the list.
    
    Below are the results from various benchmarks.  I primarily focused on two
    tests.  The first is the will-it-scale/page_fault2 test, and the other is
    a modified version of will-it-scale/page_fault1 that was enabled to use
    THP.  I did this as it allows for better visibility into different parts
    of the memory subsystem.  The guest is running with 32G for RAM on one
    node of a E5-2630 v3.  The host has had some features such as CPU turbo
    disabled in the BIOS.
    
    Test                   page_fault1 (THP)    page_fault2
    Name            tasks  Process Iter  STDEV  Process Iter  STDEV
    Baseline            1    1012402.50  0.14%     361855.25  0.81%
                       16    8827457.25  0.09%    3282347.00  0.34%
    
    Patches Applied     1    1007897.00  0.23%     361887.00  0.26%
                       16    8784741.75  0.39%    3240669.25  0.48%
    
    Patches Enabled     1    1010227.50  0.39%     359749.25  0.56%
                       16    8756219.00  0.24%    3226608.75  0.97%
    
    Patches Enabled     1    1050982.00  4.26%     357966.25  0.14%
     page shuffle      16    8672601.25  0.49%    3223177.75  0.40%
    
    Patches enabled     1    1003238.00  0.22%     360211.00  0.22%
     shuffle w/ RFC    16    8767010.50  0.32%    3199874.00  0.71%
    
    The results above are for a baseline with a linux-next-20191219 kernel,
    that kernel with this patch set applied but page reporting disabled in
    virtio-balloon, the patches applied and page reporting fully enabled, the
    patches enabled with page shuffling enabled, and the patches applied with
    page shuffling enabled and an RFC patch that makes used of MADV_FREE in
    QEMU.  These results include the deviation seen between the average value
    reported here versus the high and/or low value.  I observed that during
    the test memory usage for the first three tests never dropped whereas with
    the patches fully enabled the VM would drop to using only a few GB of the
    host's memory when switching from memhog to page fault tests.
    
    Any of the overhead visible with this patch set enabled seems due to page
    faults caused by accessing the reported pages and the host zeroing the
    page before giving it back to the guest.  This overhead is much more
    visible when using THP than with standard 4K pages.  In addition page
    shuffling seemed to increase the amount of faults generated due to an
    increase in memory churn.  The overehad is reduced when using MADV_FREE as
    we can avoid the extra zeroing of the pages when they are reintroduced to
    the host, as can be seen when the RFC is applied with shuffling enabled.
    
    The overall guest size is kept fairly small to only a few GB while the
    test is running.  If the host memory were oversubscribed this patch set
    should result in a performance improvement as swapping memory in the host
    can be avoided.
    
    A brief history on the background of free page reporting can be found at:
    https://lore.kernel.org/lkml/29f43d5796feed0dec8e8bb98b187d9dac03b900.camel@linux.intel.com/
    
    This patch (of 9):
    
    Move the head/tail adding logic out of the shuffle code and into the
    __free_one_page function since ultimately that is where it is really
    needed anyway.  By doing this we should be able to reduce the overhead and
    can consolidate all of the list addition bits in one spot.
    Signed-off-by: default avatarAlexander Duyck <alexander.h.duyck@linux.intel.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Reviewed-by: default avatarDan Williams <dan.j.williams@intel.com>
    Acked-by: default avatarMel Gorman <mgorman@techsingularity.net>
    Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
    Cc: Yang Zhang <yang.zhang.wz@gmail.com>
    Cc: Pankaj Gupta <pagupta@redhat.com>
    Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Cc: Nitesh Narayan Lal <nitesh@redhat.com>
    Cc: Rik van Riel <riel@surriel.com>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: Luiz Capitulino <lcapitulino@redhat.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Wei Wang <wei.w.wang@intel.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Oscar Salvador <osalvador@suse.de>
    Cc: Michael S. Tsirkin <mst@redhat.com>
    Cc: wei qi <weiqi4@huawei.com>
    Link: http://lkml.kernel.org/r/20200211224602.29318.84523.stgit@localhost.localdomainSigned-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    a2129f24
shuffle.h 1.61 KB