From: "Jose R. Santos" It alleviates some issues seen with Linux when accessing millions of files on machines with large amounts of RAM (+32GB). Both algorithms are base on some studies that Dominique Heger was doing on hash table efficiencies in Linux. The dentry hash table has been tested in small systems with one internal IDE hard disk as well as in large SMP with many fiberchanel disks. Dominique claims that in all the testing done, they did not see one case were this has function provided worst performance and that in most test they were seeing better performance. The inode hash function was done by me base on Dominique's original work and has only been stress tested with SpecSFS. It provided a 3% improvement over the default algorithm in the SpecSFS results and speed ups in the response time of almost all filesystem operations the benchmark stress. With the better distribution is as also possible to reduce the number of inode buckets for 32 million to 16 million and still get a slightly better results. Anton was nice enough to provide some graphs that show the distribution before and after the patch at http://samba.org/~anton/linux/sfs/1/ For the dentry hash function, some of my other coorkers had put this hash function through various testing and have concluded that the hash function was equal or better than the default hash function. These runs were done with a (hopefully to be Open Source soon) benchmark called FFSB which can simulate various io patters across many filesystems and variable file sizes. SpecSFS fileset is basically a lot of small file which varies depending on the size of the run. For a not so big SMP system the number of file is in the +20 Million files range. Of those 20 million files only 10% are access randomly by the client. The purpose of this is that the benchmark tries to stress not only the NFS layer but, VM and Filesystems layers as well. The filesets are also hundreds of gigabytes in size in order to promote disk head movement by guaranteeing cache misses in memory. SFS 27% of the workload are lookups __d_lookup has showing high in my profiles. For the inode hash the problem that I see is that when running a benchmark with this huge fileset we end up trying to free a lot of inode entries during the run while trying to put new entries in cache. We end up calling ifind_fast() which calls find_inodes_fast() held under inode_lock. In order to avoid holding the inode_lock we needed to avoid having long chains in that hash function. When I took a look at the original hash function, I found it to be a bit to simple for any workload. My solution (which I took advantage of Dominique's work) was to create a hash that function that could generate completely different hashes depending on the hashval and the superblock in order to have the hash scale as we added more filesystems to the machine. Both of these problems can be somewhat tuned out by increasing the number of buckets of both d and i cache but it got to a point were I had 256MB of inode and 128MB in dentry hash buckets on a not so large SMP. With the hash changes I have been able to reduce the number of buckets to 128MB for inode cache and to 32MB for dentry cache and still get better performance. If it help my case... I haven't been running this benchmark for long, so I haven't been able to find a way to cheat. I need to come up with generic solutions until I can find a cheat for the benchmark. :) --- 25-akpm/fs/dcache.c | 5 +++-- 25-akpm/fs/inode.c | 9 +++++---- 2 files changed, 8 insertions(+), 6 deletions(-) diff -puN fs/dcache.c~dentry-and-inode-cache-hash-algorithm-performance-changes fs/dcache.c --- 25/fs/dcache.c~dentry-and-inode-cache-hash-algorithm-performance-changes 2004-05-04 20:55:54.595321992 -0700 +++ 25-akpm/fs/dcache.c 2004-05-04 20:55:54.601321080 -0700 @@ -21,6 +21,7 @@ #include #include #include +#include #include #include #include @@ -796,8 +797,8 @@ struct dentry * d_alloc_root(struct inod static inline struct hlist_head * d_hash(struct dentry * parent, unsigned long hash) { - hash += (unsigned long) parent / L1_CACHE_BYTES; - hash = hash ^ (hash >> D_HASHBITS); + hash += ((unsigned long) parent ^ GOLDEN_RATIO_PRIME) / L1_CACHE_BYTES; + hash = hash ^ ((hash ^ GOLDEN_RATIO_PRIME) >> D_HASHBITS); return dentry_hashtable + (hash & D_HASHMASK); } diff -puN fs/inode.c~dentry-and-inode-cache-hash-algorithm-performance-changes fs/inode.c --- 25/fs/inode.c~dentry-and-inode-cache-hash-algorithm-performance-changes 2004-05-04 20:55:54.596321840 -0700 +++ 25-akpm/fs/inode.c 2004-05-04 20:55:54.603320776 -0700 @@ -671,13 +671,14 @@ static struct inode * get_new_inode_fast static inline unsigned long hash(struct super_block *sb, unsigned long hashval) { - unsigned long tmp = hashval + ((unsigned long) sb / L1_CACHE_BYTES); - tmp = tmp + (tmp >> I_HASHBITS); + unsigned long tmp; + + tmp = (hashval * (unsigned long)sb) ^ (GOLDEN_RATIO_PRIME + hashval) / + L1_CACHE_BYTES; + tmp = tmp ^ ((tmp ^ GOLDEN_RATIO_PRIME) >> I_HASHBITS); return tmp & I_HASHMASK; } -/* Yeah, I know about quadratic hash. Maybe, later. */ - /** * iunique - get a unique inode number * @sb: superblock _