Hello, I have spent a fair bit of time trying to nail how exactly block filter digests[1] should be done to optimize bandwidth, space, resource usage.
The report can be found here: http://bc-2.jp/bfd-profile.pdf This graph shows bandwidth use of 200 wallets simulated over 5000 blocks: http://bc-2.jp/bandwidth_bfd.png (black line is "sync once per block" wallet, yellow is "sync once per 144 blocks" wallet, red is average across all wallets). An interesting insight made during the experiments: when allowing digests to contain multiple blocks, the false positive rate of high block count digests can be higher than normal, because the probability of a false positive hit for a given entry in multiple digests, assuming their sizes differ, is almost completely independent. The results look rather promising to me, but I would like to hear comments, in particular on the approach taken, if I made any faulty assumptions, bad math mistakes, etc. I am also curious what people consider to be acceptable costs in terms of bandwidth use and memory (I couldn't find any stats on bandwidth use of bloom filters). In the profiling, I restricted the field sizes to 2^27 = 128 MB. I assumed this was appropriate as these fields are very short lived, and in worst case, a client *could* do the scan and decode simultaneously, without allocating up the space for the field at all. For high block count digests (e.g. 1024 blocks), this is sometimes overfilled. I wonder if 2^28 (256 MB) fields would be at all acceptable or if an over-filled (high false positive rate) field is better. For that matter, I am not entirely sure 1024-block digests are necessary, but they do come with an average 15 kb/block which is pretty good. I also wonder if the serialization approach taken is overkill or not. It does save some space instead of simply storing "BBBAAAAA" but adds some complexity that may not be warranted. [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-May/012636.html _______________________________________________ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
