• Mel Gorman's avatar
    mm: add tracepoints for LRU activation and insertions · c6286c98
    Mel Gorman authored
    Andrew Perepechko reported a problem whereby pages are being prematurely
    evicted as the mark_page_accessed() hint is ignored for pages that are
    currently on a pagevec --
    http://www.spinics.net/lists/linux-ext4/msg37340.html .
    
    Alexey Lyahkov and Robin Dong have also reported problems recently that
    could be due to hot pages reaching the end of the inactive list too
    quickly and be reclaimed.
    
    Rather than addressing this on a per-filesystem basis, this series aims
    to fix the mark_page_accessed() interface by deferring what LRU a page
    is added to pagevec drain time and allowing mark_page_accessed() to call
    SetPageActive on a pagevec page.
    
    Patch 1 adds two tracepoints for LRU page activation and insertion. Using
    	these processes it's possible to build a model of pages in the
    	LRU that can be processed offline.
    
    Patch 2 defers making the decision on what LRU to add a page to until when
    	the pagevec is drained.
    
    Patch 3 searches the local pagevec for pages to mark PageActive on
    	mark_page_accessed. The changelog explains why only the local
    	pagevec is examined.
    
    Patches 4 and 5 tidy up the API.
    
    postmark, a dd-based test and fs-mark both single and threaded mode were
    run but none of them showed any performance degradation or gain as a
    result of the patch.
    
    Using patch 1, I built a *very* basic model of the LRU to examine
    offline what the average age of different page types on the LRU were in
    milliseconds.  Of course, capturing the trace distorts the test as it's
    written to local disk but it does not matter for the purposes of this
    test.  The average age of pages in milliseconds were
    
    				    vanilla deferdrain
    Average age mapped anon:               1454       1250
    Average age mapped file:             127841     155552
    Average age unmapped anon:               85        235
    Average age unmapped file:            73633      38884
    Average age unmapped buffers:         74054     116155
    
    The LRU activity was mostly files which you'd expect for a dd-based
    workload.  Note that the average age of buffer pages is increased by the
    series and it is expected this is due to the fact that the buffer pages
    are now getting added to the active list when drained from the pagevecs.
    Note that the average age of the unmapped file data is decreased as they
    are still added to the inactive list and are reclaimed before the
    buffers.
    
    There is no guarantee this is a universal win for all workloads and it
    would be nice if the filesystem people gave some thought as to whether
    this decision is generally a win or a loss.
    
    This patch:
    
    Using these tracepoints it is possible to model LRU activity and the
    average residency of pages of different types.  This can be used to
    debug problems related to premature reclaim of pages of particular
    types.
    Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
    Reviewed-by: default avatarRik van Riel <riel@redhat.com>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Alexey Lyahkov <alexey.lyashkov@gmail.com>
    Cc: Andrew Perepechko <anserper@ya.ru>
    Cc: Robin Dong <sanbai@taobao.com>
    Cc: Theodore Tso <tytso@mit.edu>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Bernd Schubert <bernd.schubert@fastmail.fm>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    c6286c98
swap.c 23.7 KB