Re: [PATCH] fs: don't set *REFERENCED on single use objects

[Josef Bacik <josef@toxicpanda.com>]

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@fb.com>

Hello Al,

Can we get this included in the upcoming merge window?  Thanks,

Josef