[RFC] [PATCH] Improve hash function used for full_name_hash()

[Guenter Roeck <guenter.roeck@ericsson.com>]

Please comment on this proposed patch. It is similar but more generic than 
a previously proposed change to dev_name_hash() which tried to address 
the same problem.

The hash function currently used for full_name_hash() produces a large number
of collisions if hashed names are similar. This can cause performance problems
if a large number of similar names exist in the kernel (e.g., if there is
a large number of virtual interfaces).

For example, when hashing "eth0" .. "eth9999" with a hash table size of 256,
the resulting minimum hash bucket depth is 0, the maximum depth is 563,
and the standard deviation is ~136.

With this patch applied, the same test results in a minimum bucket depth
of 37, a maximum bucket depth of 42, and a standard deviation of ~1.02.

The hash factor of 41 was chosen for the following reasons:
- The resulting standard deviation is significantly better than the standard
  deviation of the original hash function for all tested hash table sizes
  (2^x, x=4..16).
- The hash function is simple.
- The resulting code does not require a multiply instruction
  (tested: x86, mips, powerpc).
- The resulting code is more efficient than the code generated for the
  original hash (x86, gcc -O2: 3 instead of 7 instructions).
- The resulting code also works well with more random strings
  (tested with all file names in a given Linux system).

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
---
 include/linux/dcache.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 30b93b2..772755d 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -53,7 +53,7 @@ extern struct dentry_stat_t dentry_stat;
 static inline unsigned long
 partial_name_hash(unsigned long c, unsigned long prevhash)
 {
-	return (prevhash + (c << 4) + (c >> 4)) * 11;
+	return (prevhash + c) * 41;
 }
 
 /*