• Roman Gushchin's avatar
    mm: use only per-device readahead limit · 600e19af
    Roman Gushchin authored
    Maximal readahead size is limited now by two values:
     1) by global 2Mb constant (MAX_READAHEAD in max_sane_readahead())
     2) by configurable per-device value* (bdi->ra_pages)
    
    There are devices, which require custom readahead limit.
    For instance, for RAIDs it's calculated as number of devices
    multiplied by chunk size times 2.
    
    Readahead size can never be larger than bdi->ra_pages * 2 value
    (POSIX_FADV_SEQUNTIAL doubles readahead size).
    
    If so, why do we need two limits?
    I suggest to completely remove this max_sane_readahead() stuff and
    use per-device readahead limit everywhere.
    
    Also, using right readahead size for RAID disks can significantly
    increase i/o performance:
    
    before:
      dd if=/dev/md2 of=/dev/null bs=100M count=100
      100+0 records in
      100+0 records out
      10485760000 bytes (10 GB) copied, 12.9741 s, 808 MB/s
    
    after:
      $ dd if=/dev/md2 of=/dev/null bs=100M count=100
      100+0 records in
      100+0 records out
      10485760000 bytes (10 GB) copied, 8.91317 s, 1.2 GB/s
    
    (It's an 8-disks RAID5 storage).
    
    This patch doesn't change sys_readahead and madvise(MADV_WILLNEED)
    behavior introduced by 6d2be915 ("mm/readahead.c: fix readahead
    failure for memoryless NUMA nodes and limit readahead pages").
    Signed-off-by: default avatarRoman Gushchin <klamm@yandex-team.ru>
    Cc: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Wu Fengguang <fengguang.wu@intel.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: onstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    600e19af
readahead.c 15.4 KB