On Tue, Apr 18, 2017 at 04:04:17PM -0400, Josef Bacik wrote: > 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 > cache. > > To illustrate this issue I wrote the following scripts > > https://github.com/josefbacik/debug-scripts/tree/master/cache-pressure > > 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: Josef Bacik <jbacik@xxxxxx> Hello Al, Can we get this included in the upcoming merge window? Thanks, Josef
