Can you elaborate what “the bad” is here? Maybe a scenario would help. I’m trying to visualize what kind of workload would be running where you wouldn’t have timeouts or a deep queue yet a node is overloaded. What is “the bad” if requests aren’t timing out? How is a node overloaded if there isn’t work queued up?
The levers we have under our control are concurrent work and the queue of work. Applying a rate limiter to the queue is a roundabout way of limiting the queue depth, so i view it as unnecessary in a world where the queue has a limit. Dynamically sizing the depth of the queue based on timeouts allows us to adjust it based on the heuristic of the amount of work that was able to be completed successfully in the past. If you know that the queue has seen timeouts when it gets deeper than 100k requests, you stop accepting requests after 100k is reached. The rate of rejection will be roughly correlated to the number of requests that would otherwise timeout. Basically we say, we can’t serve you, so don’t even get in line, and we use the length of the line as our throttle. Fwiw, I fully acknowledge that I didn’t invent any of this, it’s how hardware works already. I’m just applying the same concepts to our software queues that cpu, disks, and network cards already successfully use. Jon On Sat, Sep 21, 2024 at 12:05 PM Jordan West <jorda...@gmail.com> wrote: > I agree with Josh. We need to be able to protect from a sudden burst of > traffic. 19534 went a long way in that regard — at least wrt to minimizing > the effects. The challenge with latency and queue depths can be that they > trigger when the bad has already occurred. > > One other thing we are considering internally in that project I mentioned > is having separate thresholds. One that starts to slow traffic and one that > rejects it. > > All this said, I think our intuition can guide us to initial > implementations but we I’m still pretty adamant that our decisions should > be guided by testing and numbers for this CEP. > > Jordan > > On Sat, Sep 21, 2024 at 04:36 Josh McKenzie <jmcken...@apache.org> wrote: > >> Are those three sufficient to protect against a client that unexpectedly >> comes up with 100x a previous provisioned-for workload? Or 100 clients at >> 100x concurrently? Given that can be 100x in terms of quantity (helped by >> queueing and shedding), but also 100x in terms of *computational and >> disk implications*? We don't have control over what users do, and while >> in an ideal world client applications have fairly predictable workloads >> over time, in practice things spike around various events for different >> applications. >> >> i.e. 1 thing in a queue could produce orders of magnitude more work than >> 1 other thing in a queue. Especially as we move into a world with SAI and >> Accord. >> >> we need to solve load balancing (summarized by the above three points) >> before we start working on the rate limiter >> >> I see these 2 things as complementary, not as interdependent. Is there >> something I'm missing? >> >> At least for me, mentally I frame this as "How can nodes protect >> themselves from variable and changing user behavior in a way that's >> minimally disruptive to the user and requires as little configuration as >> possible for operators?". Basically, how do we keep the limits of node >> performance from leaking into the scope of user awareness and >> responsibility outside simply pushing various exceptions to the client to >> indicate what's going on (OverloadedException to the client, etc). >> >> It seems to me both rate limiting and resource balancing are integral >> parts of this, but also parts that could be worked on independently. Were >> we to wave a magic wand tomorrow and have robust rate limiting, as we >> improved load balancing over time it would raise the ceiling at which rate >> limiting kicked in. >> >> So concretely to the thread, I think I agree with Jon: >> >> * use the rate of timeouts to limit the depth of the queues for each of >> the thread pools >> * reject requests when the queue is full with an OverloadedException. >> >> followed by: >> >> If you want to follow this up with the ability to dynamically resize >> thread pools that could be interesting. >> >> >> Simple is very much a feature here. >> >> On Sat, Sep 21, 2024, at 5:20 AM, Alex Petrov wrote: >> >> > Personally, I’m a bit skeptical that we will come up with a metric >> based heuristic that works well in most scenarios and doesn’t require >> significant knowledge and tuning. I think past implementations of the >> dynamic snitch are good evidence of that. >> >> I am more optimistic on that font. I think we can achieve a lot. However, >> in my opinion, we need to focus on balancing the load rather than rate >> limiting. Rate limiting is going to be important if/when we decide to >> implement workload isolation. Until then, I think we should focus on three >> things: >> >> * Node health (Nodes should produce useful work and should be stable >> and not overloaded) >> * Latency (we always need to find an optimal way to process request and >> minimize overall queueing time) >> * Fairness (avoid workload and utilization imbalances) >> >> All three points are achievable with very straightforward approaches that >> will not require much operator involvement. >> >> I guess my main point is we need to solve load balancing (summarized by >> the above three points) before we start working on the rate limiter, but >> there's a good chance we may not need one apart from use cases that require >> workload isolation. >> >> >> On Fri, Sep 20, 2024, at 8:14 PM, Jordan West wrote: >> >> +1 to Benedict’s (and others) comments on plugability and low overhead >> when disabled. The latter I think needs little justification. The reason I >> am big on the former is, in my opinion: decisions on approach need to be >> settled with numbers not anecdotes or past experience (including my own). >> So I would like to see us compare different approaches (what metrics to >> use, etc). >> >> Personally, I’m a bit skeptical that we will come up with a metric based >> heuristic that works well in most scenarios and doesn’t require significant >> knowledge and tuning. I think past implementations of the dynamic snitch >> are good evidence of that. However, I expressed the same concerns >> internally for a client level project where we exposed metrics to induce >> back pressure and early experiments are encouraging / contrary to my >> expectations. At different layers different approaches can work better or >> worse. Same with different workloads. I don’t think we should dismiss >> approaches out right in this thread without hard numbers. >> >> In short, I think the testing and evaluation of this CEP is as important >> as its design and implementation. We will need to test a wide variety of >> workloads and potentially implementations and that’s where pluggability >> will be a huge benefit. I would go as far as saying the CEP should focus >> more on a framework for pluggable implementations that has low to zero cost >> when disabled than a specific set of metrics to use or specific approach. >> >> Jordan >> >> On Thu, Sep 19, 2024 at 14:38 Benedict Elliott Smith <bened...@apache.org> >> wrote: >> >> I just want to flag here that this is a topic I have strong opinions on, >> but the CEP is not really specific or detailed enough to understand >> precisely how it will be implemented. So, if a patch is already being >> produced, most of my feedback is likely to be provided some time after a >> patch appears, through the normal review process. I want to flag this now >> to avoid any surprise. >> >> I will say that upfront that, ideally, this system should be designed to >> have ~zero overhead when disabled, and with minimal coupling (between its >> own components and C* itself), so that entirely orthogonal approaches can >> be integrated in future without polluting the codebase. >> >> >> On 19 Sep 2024, at 19:14, Patrick McFadin <pmcfa...@gmail.com> wrote: >> >> The work has begun but we don't have a VOTE thread for this CEP. Can one >> get started? >> >> On Mon, May 6, 2024 at 9:24 PM Jaydeep Chovatia < >> chovatia.jayd...@gmail.com> wrote: >> >> Sure, Caleb. I will include the work as part of CASSANDRA-19534 >> <https://issues.apache.org/jira/browse/CASSANDRA-19534> in the CEP-41. >> >> Jaydeep >> >> On Fri, May 3, 2024 at 7:48 AM Caleb Rackliffe <calebrackli...@gmail.com> >> wrote: >> >> FYI, there is some ongoing sort-of-related work going on in >> CASSANDRA-19534 <https://issues.apache.org/jira/browse/CASSANDRA-19534> >> >> On Wed, Apr 10, 2024 at 6:35 PM Jaydeep Chovatia < >> chovatia.jayd...@gmail.com> wrote: >> >> Just created an official CEP-41 >> <https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-41+%28DRAFT%29+Apache+Cassandra+Unified+Rate+Limiter> >> incorporating the feedback from this discussion. Feel free to let me know >> if I may have missed some important feedback in this thread that is not >> captured in the CEP-41. >> >> Jaydeep >> >> On Thu, Feb 22, 2024 at 11:36 AM Jaydeep Chovatia < >> chovatia.jayd...@gmail.com> wrote: >> >> Thanks, Josh. I will file an official CEP with all the details in a few >> days and update this thread with that CEP number. >> Thanks a lot everyone for providing valuable insights! >> >> Jaydeep >> >> On Thu, Feb 22, 2024 at 9:24 AM Josh McKenzie <jmcken...@apache.org> >> wrote: >> >> >> Do folks think we should file an official CEP and take it there? >> >> +1 here. >> >> Synthesizing your gdoc, Caleb's work, and the feedback from this thread >> into a draft seems like a solid next step. >> >> On Wed, Feb 7, 2024, at 12:31 PM, Jaydeep Chovatia wrote: >> >> I see a lot of great ideas being discussed or proposed in the past to >> cover the most common rate limiter candidate use cases. Do folks think we >> should file an official CEP and take it there? >> >> Jaydeep >> >> On Fri, Feb 2, 2024 at 8:30 AM Caleb Rackliffe <calebrackli...@gmail.com> >> wrote: >> >> I just remembered the other day that I had done a quick writeup on the >> state of compaction stress-related throttling in the project: >> >> >> https://docs.google.com/document/d/1dfTEcKVidRKC1EWu3SO1kE1iVLMdaJ9uY1WMpS3P_hs/edit?usp=sharing >> >> I'm sure most of it is old news to the people on this thread, but I >> figured I'd post it just in case :) >> >> On Tue, Jan 30, 2024 at 11:58 AM Josh McKenzie <jmcken...@apache.org> >> wrote: >> >> >> 2.) We should make sure the links between the "known" root causes of >> cascading failures and the mechanisms we introduce to avoid them remain >> very strong. >> >> Seems to me that our historical strategy was to address individual known >> cases one-by-one rather than looking for a more holistic load-balancing and >> load-shedding solution. While the engineer in me likes the elegance of a >> broad, more-inclusive *actual SEDA-like* approach, the pragmatist in me >> wonders how far we think we are today from a stable set-point. >> >> i.e. are we facing a handful of cases where nodes can still get pushed >> over and then cascade that we can surgically address, or are we facing a >> broader lack of back-pressure that rears its head in different domains >> (client -> coordinator, coordinator -> replica, internode with other >> operations, etc) at surprising times and should be considered more >> holistically? >> >> On Tue, Jan 30, 2024, at 12:31 AM, Caleb Rackliffe wrote: >> >> I almost forgot CASSANDRA-15817, which introduced >> reject_repair_compaction_threshold, which provides a mechanism to stop >> repairs while compaction is underwater. >> >> On Jan 26, 2024, at 6:22 PM, Caleb Rackliffe <calebrackli...@gmail.com> >> wrote: >> >> >> Hey all, >> >> I'm a bit late to the discussion. I see that we've already discussed >> CASSANDRA-15013 <https://issues.apache.org/jira/browse/CASSANDRA-15013> >> and CASSANDRA-16663 >> <https://issues.apache.org/jira/browse/CASSANDRA-16663> at least in >> passing. Having written the latter, I'd be the first to admit it's a crude >> tool, although it's been useful here and there, and provides a couple >> primitives that may be useful for future work. As Scott mentions, while it >> is configurable at runtime, it is not adaptive, although we did >> make configuration easier in CASSANDRA-17423 >> <https://issues.apache.org/jira/browse/CASSANDRA-17423>. It also is >> global to the node, although we've lightly discussed some ideas around >> making it more granular. (For example, keyspace-based limiting, or limiting >> "domains" tagged by the client in requests, could be interesting.) It also >> does not deal with inter-node traffic, of course. >> >> Something we've not yet mentioned (that does address internode traffic) >> is CASSANDRA-17324 >> <https://issues.apache.org/jira/browse/CASSANDRA-17324>, which I >> proposed shortly after working on the native request limiter (and have just >> not had much time to return to). The basic idea is this: >> >> When a node is struggling under the weight of a compaction backlog and >> becomes a cause of increased read latency for clients, we have two safety >> valves: >> >> >> 1.) Disabling the native protocol server, which stops the node from >> coordinating reads and writes. >> 2.) Jacking up the severity on the node, which tells the dynamic snitch >> to avoid the node for reads from other coordinators. >> >> >> These are useful, but we don’t appear to have any mechanism that would >> allow us to temporarily reject internode hint, batch, and mutation messages >> that could further delay resolution of the compaction backlog. >> >> >> Whether it's done as part of a larger framework or on its own, it still >> feels like a good idea. >> >> Thinking in terms of opportunity costs here (i.e. where we spend our >> finite engineering time to holistically improve the experience of operating >> this database) is healthy, but we probably haven't reached the point of >> diminishing returns on nodes being able to protect themselves from clients >> and from other nodes. I would just keep in mind two things: >> >> 1.) The effectiveness of rate-limiting in the system (which includes the >> database and all clients) as a whole necessarily decreases as we move from >> the application to the lowest-level database internals. Limiting correctly >> at the client will save more resources than limiting at the native protocol >> server, and limiting correctly at the native protocol server will save more >> resources than limiting after we've dispatched requests to some thread pool >> for processing. >> 2.) We should make sure the links between the "known" root causes of >> cascading failures and the mechanisms we introduce to avoid them remain >> very strong. >> >> In any case, I'd be happy to help out in any way I can as this moves >> forward (especially as it relates to our past/current attempts to address >> this problem space). >> >> >> >> >> >>
