Hi Brain, I wanted it to be created under https://cwiki.apache.org/confluence/display/CASSANDRA/Discussion but it looks like I do not have grants to add a page there and Confluence automatically selected this space to store the page. I do not have permission to move it too :-( Can I get grants to create pages under https://cwiki.apache.org/confluence/display/CASSANDRA/ ?
Thank you, Dmitry On Fri, 3 Jan 2025 at 14:12, Brian Proffitt <b...@apache.org> wrote: > Dmitry: > > You are using a section of the Confluence wiki that is dedicated to > Community Over Code, the Apache Conference. Please move that page to a more > appropriate part of the Apache wiki as soon as you can. > > Thanks! > BKP > > On 2025/01/03 13:55:49 Dmitry Konstantinov wrote: > > I have summarized information from this mail thread to > > > https://cwiki.apache.org/confluence/display/COC/SSTable%27s+partition+cardinality+implementation > > Probably later it can be transformed to a CEP.. > > Regarding experience of DataSketches library's authors and publications > > here there is a good summary in Background section: > > > https://cwiki.apache.org/confluence/display/INCUBATOR/DataSketchesProposal > > . It looks good.. > > > > On Fri, 3 Jan 2025 at 13:06, Štefan Miklošovič <smikloso...@apache.org> > > wrote: > > > > > Right ... that sounds reasonable. Let's "sleep on it" for a while. It > is > > > not something which is urgent to deal with right now but I find myself > > > quite often to identify the functionality where we go to the disk more > > > often than necessary and this was next on the list to take a look at > > > reading CASSANDRA-13338. So I took a look ... and here we are. > > > > > > If you guys go to bump SSTable version in 5.1 / 6.0, this change might > be > > > just shipped with that too. > > > > > > On Fri, Jan 3, 2025 at 1:47 PM Benedict <bened...@apache.org> wrote: > > > > > >> I’ve had a quick skim of the data sketches library, and it does seem > to > > >> have made some more efficient decisions in its design than > clearspring, > > >> appears to maybe support off-heap representations, and has reasonably > good > > >> documentation about the theoretical properties of the sketches. The > chair > > >> of the project is a published author on the topic, and the library has > > >> newer algorithms for cardinality estimation than HLL. > > >> > > >> So, honestly, it might not be a bad idea to (carefully) consider a > > >> migration, even if the current library isn’t broken for our needs. > > >> > > >> It would not be high up my priority list for the project, but I would > > >> support it if it scratches someone’s itch. > > >> > > >> On 3 Jan 2025, at 12:16, Štefan Miklošovič <smikloso...@apache.org> > > >> wrote: > > >> > > >> > > >> Okay ... first problems. > > >> > > >> These 2000 bytes I have mentioned in my response to Chris were indeed > > >> correct, but that was with Datasketches and the main parameter for > Hall > > >> Sketch (DEFAULT_LG_K) was 12. When I changed that to 13 to match what > we > > >> currently have in Cassandra with Clearspring, that doubled the size to > > >> ~4000 bytes. > > >> > > >> When we do not use Datasketches, what Clearspring generates is about > > >> ~5000 bytes for the array itself but that array is wrapped into an > > >> ICardinality object of Clearspring and we need that object in order to > > >> merge another ICardinality into that. So, we would need to cache this > > >> ICardinality object instead of just an array itself. If we don't cache > > >> whole ICardinality, we would then need to do basically what > > >> CompactionMetadata.CompactionMetadataSerializer.deserialize is doing > which > > >> would allocate a lot / often (ICardinality cardinality = > > >> HyperLogLogPlus.Builder.build(that_cached_array)). > > >> > > >> To avoid the allocations every time we compute, we would just cache > that > > >> whole ICardinality of Clearspring, but that whole object measures like > > >> 11/12 KB. So even 10k tables would occupy like 100MB. 50k tables > 500MB. > > >> That is becoming quite a problem. > > >> > > >> On the other hand, HllSketch of Datasketches, array included, adds > > >> minimal overhead. Like an array has 5000 bytes and the whole object > like > > >> 5500. You got the idea ... > > >> > > >> If we are still OK with these sizes, sure ... I am just being > transparent > > >> about the consequences here. > > >> > > >> A user would just opt-in into this (by default it would be turned > off). > > >> > > >> On the other hand, if we have 10k SSTables, reading that 10+KB from > disk > > >> takes around 2-3ms so we would read the disk 20/30 seconds every time > we > > >> would hit that metric (and we haven't even started to merge the logs). > > >> > > >> If this is still not something which would sell Datasketches as a > viable > > >> alternative then I guess we need to stick to these numbers and cache > it all > > >> with Clearspring, occupying way more memory. > > >> > > >> On Thu, Jan 2, 2025 at 10:15 PM Benedict <bened...@apache.org> wrote: > > >> > > >>> I would like to see somebody who has some experience writing data > > >>> structures, preferably someone we trust as a community to be > competent at > > >>> this (ie having some experience within the project contributing at > this > > >>> level), look at the code like they were at least lightly reviewing > the > > >>> feature as a contribution to this project. > > >>> > > >>> This should be the bar for any new library really, but triply so for > > >>> replacing a library that works fine. > > >>> > > >>> On 2 Jan 2025, at 21:02, Štefan Miklošovič <smikloso...@apache.org> > > >>> wrote: > > >>> > > >>> > > >>> Point 2) is pretty hard to fulfil, I can not imagine what would be > > >>> "enough" for you to be persuaded. What should concretely happen? > Because > > >>> whoever comes and says "yeah this is a good lib, it works" is > probably not > > >>> going to be enough given the vague requirements you put under 2) You > would > > >>> like to see exactly what? > > >>> > > >>> The way it looks to me is to just shut it down because of perceived > > >>> churn caused by that and there will always be some argument against > that. > > >>> > > >>> Based on (1) I don't think what we have is bug free. > > >>> > > >>> Jeff: > > >>> > > >>> Thank you for that answer, I think we are on the same page that > caching > > >>> it is just fine, that's what I got from your last two paragraphs. > > >>> > > >>> So the path from here is > > >>> > > >>> 1) add datasketches and cache > > >>> 2) don't add datasketches and cache it anyway > > >>> > > >>> The introduction of datasketches lib is not the absolute must in > order > > >>> to achieve that, we can cache / compute it parallel with Clearspring > as > > >>> well, it is just a bitter-sweet solution which just doesn't feel > right. > > >>> > > >>> (1) https://github.com/addthis/stream-lib/issues > > >>> > > >>> On Thu, Jan 2, 2025 at 9:26 PM Benedict <bened...@apache.org> wrote: > > >>> > > >>>> Your message seemed to be all about the caching proposal, which I > have > > >>>> proposed we separate, hence my confusion. > > >>>> > > >>>> To restate my answer to your question, I think that unless the new > > >>>> library actually offers us concrete benefits we can point to that we > > >>>> actually care about then yes it’s a bad idea to incur the churn of > > >>>> migration. > > >>>> > > >>>> I’m not inherently opposed to a migration but simply “new is > better” is > > >>>> just plain wrong. Nothing you’ve presented yet convinces me this > library is > > >>>> worth the effort of vetting given our current solution works fine. > > >>>> > > >>>> My position is that for any new library we should: > > >>>> > > >>>> 1) Point to something it solves that we actually want and is worth > the > > >>>> time investment > > >>>> 2) Solicit folk in the community competent in the relevant data > > >>>> structures to vet the library for the proposed functionality > > >>>> > > >>>> The existing solution never went through (2) because it dates from > the > > >>>> dark ages where we just threw dependencies in willynilly. But it > has the > > >>>> benefit of having been used for a very long time without incident. > > >>>> > > >>>> > > >>>> > > >>>> On 2 Jan 2025, at 20:12, Štefan Miklošovič <smikloso...@apache.org> > > >>>> wrote: > > >>>> > > >>>> > > >>>> Hi Benedict, > > >>>> > > >>>> you wrote: > > >>>> > > >>>> I am strongly opposed to updating libraries simply for the sake of > it. > > >>>> Something like HLL does not need much ongoing maintenance if it > works. > > >>>> We’re simply asking for extra work and bugs by switching, and some > risk > > >>>> without understanding the quality control for the new library > project’s > > >>>> releases. > > >>>> > > >>>> I understand this. But really, do you think that it is a bad idea to > > >>>> switch to a well maintained library which is already used quite > widely (the > > >>>> website mentions extensions for sketches in Apache Druid, Hive, > Pig, Pinot > > >>>> and PostgreSQL) and using the library which was abandoned for 6 > years? > > >>>> > > >>>> As I mentioned there is also extensive comparison with Clearspring > (1) > > >>>> where all performance benefits / speedups etc present in detail > with charts > > >>>> attached. > > >>>> > > >>>> I think this is a mature project, under Apache, so when we think > that a > > >>>> 6 years old and abandoned library is better than what Apache > Datasketches > > >>>> provides, then the question is what are we doing here? Are we not > believing > > >>>> what Apache itself offers and we need to rely on a 6 years old and > dead > > >>>> library instead of that? Huh? That lib has 3k commits, releases > often, it's > > >>>> a pretty active project ... > > >>>> > > >>>> I don't say that we should not test more deeply how it behaves, we > > >>>> might even re-consider the parameters of hyperloglog as we do that. > But I > > >>>> don't think that having this library introduced would cause some > kind of a > > >>>> widespread / systemic risk. > > >>>> > > >>>> (1) https://datasketches.apache.org/docs/HLL/Hll_vs_CS_Hllpp.html > > >>>> > > >>>> On Thu, Jan 2, 2025 at 5:03 PM Benedict <bened...@apache.org> > wrote: > > >>>> > > >>>>> I am strongly opposed to updating libraries simply for the sake of > it. > > >>>>> Something like HLL does not need much ongoing maintenance if it > works. > > >>>>> We’re simply asking for extra work and bugs by switching, and some > risk > > >>>>> without understanding the quality control for the new library > project’s > > >>>>> releases. > > >>>>> > > >>>>> That said, I was not very impressed with the clear spring library > when > > >>>>> I looked at it, so I would be open to a stronger argument about > data > > >>>>> sketches being superior otherwise in a way that matters to us. > > >>>>> > > >>>>> If we are to replace the library, we should at the very least do > > >>>>> proper due diligence by reviewing the new library’s > implementation(s) > > >>>>> ourselves. We cannot simply assume the new library behaves well > for our use > > >>>>> cases, or is well maintained. > > >>>>> > > >>>>> We should also not use the fallback intersection method, as this > would > > >>>>> represent a regression to compaction on upgrade. We should really > convert > > >>>>> from one HLL to another. > > >>>>> > > >>>>> The proposal to reduce allocations appears to be orthogonal to this > > >>>>> library, so let’s separate out that discussion? If there’s > evidence this > > >>>>> library alone improves the memory profile let’s discuss that. > > >>>>> > > >>>>> > > >>>>> On 2 Jan 2025, at 15:26, Chris Lohfink <clohfin...@gmail.com> > wrote: > > >>>>> > > >>>>> > > >>>>> I think switching to datasketches is a good idea first off simply > > >>>>> because of the lack of maintenance and improvements from > clearspring. I am > > >>>>> however, am not sold that it will actually improve anything > significantly. > > >>>>> Caches might help on small cases, but those small cases probably > are not > > >>>>> actually impacted. In the large cases the caches cost more in > complexity, > > >>>>> memory, and ultimately wont matter when theres 50k sstables and > the cache > > >>>>> holds 1k so everythings hitting disk anyway. > > >>>>> > > >>>>> The 5% is missing some relevant information like what the > allocation > > >>>>> rate was, how many tables there are etc. On an idle system thats > > >>>>> meaningless, if there were 5gb/s allocations of reads/writes > happening at > > >>>>> the time thats huge. > > >>>>> > > >>>>> On Thu, Jan 2, 2025 at 8:42 AM Štefan Miklošovič < > > >>>>> smikloso...@apache.org> wrote: > > >>>>> > > >>>>>> Interesting, thanks for this. Well ... 5% here, 5% there ... it > > >>>>>> compounds. I think it is worth trying to do something with this. > Would be > > >>>>>> great if you were part of this effort! > > >>>>>> > > >>>>>> On Thu, Jan 2, 2025 at 3:38 PM Dmitry Konstantinov < > > >>>>>> netud...@gmail.com> wrote: > > >>>>>> > > >>>>>>> I have seen this place in async profiler memory allocation > profile > > >>>>>>> on one of production environments some time ago, it was visible > but not > > >>>>>>> dramatic, about 5% of allocations: > > >>>>>>> <image.png> > > >>>>>>> > > >>>>>>> The amount of overhead also depends on a metric collection > frequency > > >>>>>>> (in my case it was once per 60 seconds) > > >>>>>>> > > >>>>>>> Regards, > > >>>>>>> Dmitry > > >>>>>>> > > >>>>>>> On Thu, 2 Jan 2025 at 14:21, Štefan Miklošovič < > > >>>>>>> smikloso...@apache.org> wrote: > > >>>>>>> > > >>>>>>>> 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 > > >>>>>>>>>> > > >>>>>>>>> > > >>>>>>> > > >>>>>>> -- > > >>>>>>> Dmitry Konstantinov > > >>>>>>> > > >>>>>> > > > > -- > > Dmitry Konstantinov > > > -- Dmitry Konstantinov
