On Thu, 29 Mar 2012, wrote: > Hi everyone, i'm proceeding reading source code of crush > > i'm reading src/crush/mapper.c which is describing the core > implementation of crush alg. > in reading function bucket_straw_choose, i compare the implementation > with the idea written in the thesis and have some problem > understanding it. > > in the thesis, it said that the length of straw should multiply the > weight of bucket item so that heavier items are more likely to be > selected. > in bucket_straw_choose, i see bucket->straws multiplying the hash value only. > > next, i referred to crush.h to take a look at the data structure of > crush_bucket_straw, and found two members named 'straws' and > 'item_weights'. > > i just don't know whether 'weight' and 'straw' in the implementation > mean the same thing as in thesis. if so, why item_weights is not used > in bucket_straw_choose. The length of the straw is not directly proportional to the weight, and is non-trivial. When the bucket is built/adjusted, the straw lengths are calculated so that the actual choose/placement calculation is fast and simple. See crush_calc_straw() in src/crush/builder.c for the straw calculation. I needed to draw a picture for it to make any sense. sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
