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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 >
