On 01/28/2014 02:09 PM, Andreas Dilger wrote: > On Jan 28, 2014, at 5:26 AM, George Spelvin <linux@xxxxxxxxxxx> wrote: >>> The third part of the patch further increases the scalablity of an ext4 >>> filesystem by having each ext4 fielsystem allocate and use its own private >>> mbcache structure, instead of sharing a single mcache structures across all >>> ext4 filesystems, and increases the size of its mbcache hash tables. >> >> Are you sure this helps? The idea behind having one large mbcache is >> that one large hash table will always be at least as well balanced as >> multiple separate tables, if the total size is the same. >> >> If you have two size 2^n hash tables, the chance of collision is equal to >> one size 2^(n+1) table if they're equally busy, and if they're unequally >> busy. the latter is better. The busier file system will take less time >> per search, and since it's searched more often than the less-busy one, >> net win. >> >> How does it compare with just increasing the hash table size but leaving >> them combined? > > Except that having one mbcache per block device would avoid the need > to store the e_bdev pointer in thousands/millions of entries. Since > the blocks are never shared between different block devices, there > is no caching benefit even if the same block is on two block devices. > > Cheers, Andreas > On all 3 systems, with 80, 60 and 20 cores, that I ran aim7 on, spreading test files across 4 ext4 filesystems, there seems to be no different in performance either with a single large hash table or a smaller one per filesystem. Having said that, I still believe that having a separate hash table for each filesystem should scale better, as the size of a larger single hash table would be very arbitrary. As Andres mentioned above, with an mbcache per filesystem we would be able to remove the e_bdev member from the mb_cache_entry. It would also work well and also result in less mb_cache_entry lock contention, if we are to use the blockgroup locks, which are also on a per filesystem base, to implement the mb_cache_entry lock as suggested by Andreas. Please let me know if you have any further comment or concerns. Thanks, Mak. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
