• Josef Bacik's avatar
    fs: don't set *REFERENCED on single use objects · 563f4001
    Josef Bacik authored
    By default we set DCACHE_REFERENCED and I_REFERENCED on any dentry or
    inode we create.  This is problematic as this means that it takes two
    trips through the LRU for any of these objects to be reclaimed,
    regardless of their actual lifetime.  With enough pressure from these
    caches we can easily evict our working set from page cache with single
    use objects.  So instead only set *REFERENCED if we've already been
    added to the LRU list.  This means that we've been touched since the
    first time we were accessed, and so more likely to need to hang out in
    To illustrate this issue I wrote the following scripts
    on my test box.  It is a single socket 4 core CPU with 16gib of RAM and
    I tested on an Intel 2tib NVME drive.  The cache-pressure.sh script
    creates a new file system and creates 2 6.5gib files in order to take up
    13gib of the 16gib of ram with pagecache.  Then it runs a test program
    that reads these 2 files in a loop, and keeps track of how often it has
    to read bytes for each loop.  On an ideal system with no pressure we
    should have to read 0 bytes indefinitely.  The second thing this script
    does is start a fs_mark job that creates a ton of 0 length files,
    putting pressure on the system with slab only allocations.  On exit the
    script prints out how many bytes were read by the read-file program.
    The results are as follows
    Without patch:
    /mnt/btrfs-test/reads/file1: total read during loops 27262988288
    /mnt/btrfs-test/reads/file2: total read during loops 27262976000
    With patch:
    /mnt/btrfs-test/reads/file2: total read during loops 18640457728
    /mnt/btrfs-test/reads/file1: total read during loops 9565376512
    This patch results in a 50% reduction of the amount of pages evicted
    from our working set.
    Signed-off-by: default avatarJosef Bacik <jbacik@fb.com>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
dcache.c 93.8 KB