[PATCH] vm: vfs shrinkage tuning

Some people want the dentry and inode caches shrink harder, others want them
shrunk more reluctantly.

The patch adds /proc/sys/vm/vfs_cache_pressure, which tunes the vfs cache
versus pagecache scanning pressure.

- at vfs_cache_pressure=0 we don't shrink dcache and icache at all.

- at vfs_cache_pressure=100 there is no change in behaviour.

- at vfs_cache_pressure > 100 we reclaim dentries and inodes harder.


The number of megabytes of slab left after a slocate.cron on my 256MB test
box:

vfs_cache_pressure=100000   33480
vfs_cache_pressure=10000    61996
vfs_cache_pressure=1000     104056
vfs_cache_pressure=200      166340
vfs_cache_pressure=100      190200
vfs_cache_pressure=50       206168

Of course, this just left more directory and inode pagecache behind instead of
vfs cache.  Interestingly, on this machine the entire slocate run fits into
pagecache, but not into VFS caches.


Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 13 insertions, 1 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 995da97a263319..b84b7a7cc7231c 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -1115,6 +1115,18 @@ program to load modules on demand.
 The files  in  this directory can be used to tune the operation of the virtual
 memory (VM)  subsystem  of  the  Linux  kernel.
 
+vfs_cache_pressure
+------------------
+
+Controls the tendency of the kernel to reclaim the memory which is used for
+caching of directory and inode objects.
+
+At the default value of vfs_cache_pressure=100 the kernel will attempt to
+reclaim dentries and inodes at a "fair" rate with respect to pagecache and
+swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
+to retain dentry and inode caches.  Increasing vfs_cache_pressure beyond 100
+causes the kernel to prefer to reclaim dentries and inodes.
+
 dirty_background_ratio
 ----------------------
 
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index fc3e413c37210f..c873ef92fbfab5 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -28,7 +28,7 @@ Currently, these files are in /proc/sys/vm:
 ==============================================================
 
 dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
-dirty_writeback_centisecs:
+dirty_writeback_centisecs, vfs_cache_pressure:
 
 See Documentation/filesystems/proc.txt