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 > > > > > Thanks George and Andreas for the comments. Andreas you mentions a good point, e_bdev pointer is not needed when having one mb_cache for each block device. I'll integrate that into my patch, removing the e_bdev pointer, and run some comparison between one large hash table vs multiple hash tables, as suggested by George. 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
