• Christoph Lameter's avatar
    [PATCH] optional ZONE_DMA: deal with cases of ZONE_DMA meaning the first zone · 6267276f
    Christoph Lameter authored
    This patchset follows up on the earlier work in Andrew's tree to reduce the
    number of zones.  The patches allow to go to a minimum of 2 zones.  This one
    allows also to make ZONE_DMA optional and therefore the number of zones can be
    reduced to one.
    ZONE_DMA is usually used for ISA DMA devices.  There are a number of reasons
    why we would not want to have ZONE_DMA
    1. Some arches do not need ZONE_DMA at all.
    2. With the advent of IOMMUs DMA zones are no longer needed.
       The necessity of DMA zones may drastically be reduced
       in the future. This patchset allows a compilation of
       a kernel without that overhead.
    3. Devices that require ISA DMA get rare these days. All
       my systems do not have any need for ISA DMA.
    4. The presence of an additional zone unecessarily complicates
       VM operations because it must be scanned and balancing
       logic must operate on its.
    5. With only ZONE_NORMAL one can reach the situation where
       we have only one zone. This will allow the unrolling of many
       loops in the VM and allows the optimization of varous
       code paths in the VM.
    6. Having only a single zone in a NUMA system results in a
       1-1 correspondence between nodes and zones. Various additional
       optimizations to critical VM paths become possible.
    Many systems today can operate just fine with a single zone.  If you look at
    what is in ZONE_DMA then one usually sees that nothing uses it.  The DMA slabs
    are empty (Some arches use ZONE_DMA instead of ZONE_NORMAL, then ZONE_NORMAL
    will be empty instead).
    On all of my systems (i386, x86_64, ia64) ZONE_DMA is completely empty.  Why
    constantly look at an empty zone in /proc/zoneinfo and empty slab in
    /proc/slabinfo?  Non i386 also frequently have no need for ZONE_DMA and zones
    stay empty.
    The patchset was tested on i386 (UP / SMP), x86_64 (UP, NUMA) and ia64 (NUMA).
    The RFC posted earlier (see
    ) had lots
    of #ifdefs in them.  An effort has been made to minize the number of #ifdefs
    and make this as compact as possible.  The job was made much easier by the
    ongoing efforts of others to extract common arch specific functionality.
    I have been running this for awhile now on my desktop and finally Linux is
    using all my available RAM instead of leaving the 16MB in ZONE_DMA untouched:
    christoph@pentium940:~$ cat /proc/zoneinfo
    Node 0, zone   Normal
      pages free     4435
            min      1448
            low      1810
            high     2172
            active   241786
            inactive 210170
            scanned  0 (a: 0 i: 0)
            spanned  524224
            present  524224
        nr_anon_pages 61680
        nr_mapped    14271
        nr_file_pages 390264
        nr_slab_reclaimable 27564
        nr_slab_unreclaimable 1793
        nr_page_table_pages 449
        nr_dirty     39
        nr_writeback 0
        nr_unstable  0
        nr_bounce    0
        cpu: 0 pcp: 0
                  count: 156
                  high:  186
                  batch: 31
        cpu: 0 pcp: 1
                  count: 9
                  high:  62
                  batch: 15
      vm stats threshold: 20
        cpu: 1 pcp: 0
                  count: 177
                  high:  186
                  batch: 31
        cpu: 1 pcp: 1
                  count: 12
                  high:  62
                  batch: 15
      vm stats threshold: 20
      all_unreclaimable: 0
      prev_priority:     12
      temp_priority:     12
      start_pfn:         0
    This patch:
    In two places in the VM we use ZONE_DMA to refer to the first zone.  If
    ZONE_DMA is optional then other zones may be first.  So simply replace
    ZONE_DMA with zone 0.
    This also fixes ZONETABLE_PGSHIFT.  If we have only a single zone then
    ZONES_PGSHIFT may become 0 because there is no need anymore to encode the zone
    number related to a pgdat.  However, we still need a zonetable to index all
    the zones for each node if this is a NUMA system.  Therefore define
    ZONETABLE_SHIFT unconditionally as the offset of the ZONE field in page flags.
    [apw@shadowen.org: fix mismerge]
    Acked-by: default avatarChristoph Hellwig <hch@infradead.org>
    Signed-off-by: default avatarChristoph Lameter <clameter@sgi.com>
    Cc: Andi Kleen <ak@suse.de>
    Cc: "Luck, Tony" <tony.luck@intel.com>
    Cc: Kyle McMartin <kyle@mcmartin.ca>
    Cc: Matthew Wilcox <willy@debian.org>
    Cc: James Bottomley <James.Bottomley@steeleye.com>
    Cc: Paul Mundt <lethal@linux-sh.org>
    Signed-off-by: default avatarAndy Whitcroft <apw@shadowen.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
page_alloc.c 90.9 KB