Indeed, I plan to measure it and compare, maybe some bench test would be cool to add ..
I strongly suspect that the primary reason for the slowness (if it is verified to be true) is us going to the disk every time and reading stats for every SSTable all over again. While datasketches say that it is way faster to update (1), we are living in a realm of nanoseconds here and I don't think that itself would make any meaningful difference when merging one hyperloglog with another as part of partition rows estimation computation. The only place we are updating is SortableTableWriter#endParition which calls metadatacollector.addKey(key.getKey()) which eventually updates the estimator via cardinality#offeredHashed. In other words, I think that going to the disk and reading it repeatedly is disproportionally more IO / time intensive than switching the hyperloglog implementation. However, I consider the replacement of the library still important. I feel uneasy about staying with an abandoned library where there is clearly a well-maintained replacement. What we could do is to cache all cardinality estimators and just merge it all when asked upon metric resolution. That is different from going to disk to deserialize StatsComponent's all over again. Then on SSTable removal, we would remove that from cache too. I think there is some kind of an observer when SSTable is removed ... However, I am not sure I can just hold it all in the memory, it works for laptop testing but if we have thousands of SSTables with non-trivial number of rows things start to get interesting. (1) https://datasketches.apache.org/docs/HLL/Hll_vs_CS_Hllpp.html - section HllSketch vs. HyperLogLogPlus Update Speed Behavior On Thu, Jan 2, 2025 at 2:46 PM Jon Haddad <j...@rustyrazorblade.com> wrote: > Sounds interesting. I took a look at the issue but I'm not seeing any > data to back up "expensive". Can this be quantified a bit more? > > Anytime we have a performance related issue, there should be some data to > back it up, even if it seems obvious. > > Jon > > On Thu, Jan 2, 2025 at 8:20 AM Štefan Miklošovič <smikloso...@apache.org> > wrote: > >> Hello, >> >> I just stumbled upon this library we are using for getting estimations of >> the number of partitions in a SSTable which are used e.g. in >> EstimatedPartitionCount metric. (1) >> >> A user reported in (1) that it is an expensive operation. When one looks >> into what it is doing, it calls SSTableReader.getApproximateKeyCount() (6) >> which basically goes to disk every single time, it loads all Stats >> components and it looks into CompactionMetadata where the cardinality >> estimator is located. >> >> We are serializing the hyperloglog to disk as part of a SSTable and we >> deserialize it back in runtime for every SSTable in a CF and we merge them >> all to one cardinality again. >> >> I do not think there is a way around this because of the nature of how a >> cardinality estimator works (hyperloglog). We can not "cache it", it would >> work only in case we are adding SSTables only - hence we would just merge >> again - but if we remove an SSTable as part of the compaction, we can not >> "unmerge" it. >> >> That being said, we are currently using this library for hyperloglog (1) >> which was archived in summer 2020 and nothing was contributed to that for 6 >> years. That lib is dead. >> >> There is very nice replacement of that (2) directly from Apache (!!!) and >> they are even giving the detailed and in-depth comparison of hyperloglog >> implementation found in stream-lib we happen to use (3) (stream-lib = >> Clearspring) where they are saying that updating is way faster and it is >> also giving better estimations in general. >> >> I have implemented the usage of both cardinality estimators (4), (5). The >> reason we need to keep the old one around is that we may have old SSTables >> around and we need to work with them too. That translates to a new SSTable >> version (ob) which uses new implementation and for versions < ob, it uses >> the old one. When SSTables are upgraded from oa to ob, the old estimator >> will not be used anymore. >> >> There is also a case of a user not upgrading his oa SSTables, turning a >> node on and creating new SSTables with ob version. When this happens and we >> ask what is the cardinality (e.g via nodetool tablestats), I am checking if >> all SSTables are on the same version or not. If they are, they will use >> either an old or new estimator. (we can not merge estimations from two >> different hyperloglog implementations). If they are not, it will compute >> that from index summaries. (The computation for index summaries was already >> in place (6) as a fail-over in case the estimation computation failed / was >> not present). >> >> Does this all make sense to drive further to the completion and >> eventually merge this work to trunk? >> >> Worth to add that Apache Datasketches are just two dependencies for us, >> it has zero external dependencies. >> >> (1) https://github.com/addthis/stream-lib >> (2) https://datasketches.apache.org/ >> (3) https://datasketches.apache.org/docs/HLL/Hll_vs_CS_Hllpp.html >> (4) https://issues.apache.org/jira/browse/CASSANDRA-13338 >> (5) https://github.com/apache/cassandra/pull/3767 >> (6) >> https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/io/sstable/format/SSTableReader.java#L284-L338 >> >> Regards >> >
