Hi everyone,
Sorry for leaving this for so long. So much for "3 weeks until KIP freeze"!
On Sophie's comments:
1. Would Matthias's suggestion of a separate metric tracking the age of the
oldest open iterator (within the tag set) satisfy this? That way we can
keep iterator-duration-(avg|max) for closed iterators, which can be useful
for performance debugging for iterators that don't leak. I'm not sure what
we'd call this metric, maybe: "oldest-open-iterator-age-seconds"? Seems
like a mouthful.
2. You're right, it makes more sense to provide
iterator-duration-(avg|max). Honestly, I can't remember why I had "total"
before, or why I was computing a rate-of-change over it.
3, 4, 5, 6. Agreed, I'll make all those changes as suggested.
7. Combined with Matthias's point about RocksDB, I'm convinced that this is
the wrong KIP for these. I'll introduce the additional Rocks metrics in
another KIP.
On Matthias's comments:
A. Not sure about the time window. I'm pretty sure all existing avg/max
metrics are since the application was started? Any other suggestions here
would be appreciated.
B. Agreed. See point 1 above.
C. Good point. My focus was very much on Rocks memory leaks when I wrote
the first draft. I can generalise it. My only concern is that it might make
it more difficult to detect Rocks iterator leaks caused *within* our
high-level iterator, e.g. RocksJNI, RocksDB, RocksDBStore, etc. But we
could always provide a RocksDB-specific metric for this, as you suggested.
D. Hmm, we do already have MeteredKeyValueIterator, which automatically
wraps the iterator from inner-stores of MeteredKeyValueStore. If we
implemented these metrics there, then custom stores would automatically
gain the functionality, right? This seems like a pretty logical place to
implement these metrics, since MeteredKeyValueStore is all about adding
metrics to state stores.
> I imagine the best way to implement this would be to do so at the
> high-level iterator rather than implementing it separately for each
> specific iterator implementation for every store type.
Sophie, does MeteredKeyValueIterator fit with your recommendation?
Thanks for your thoughts everyone, I'll update the KIP now.
Nick
On Thu, 14 Mar 2024 at 03:37, Sophie Blee-Goldman <sop...@responsive.dev>
wrote:
> About your last two points: I completely agree that we should try to
> make this independent of RocksDB, and should probably adopt a
> general philosophy of being store-implementation agnostic unless
> there is good reason to focus on a particular store type: eg if it was
> only possible to implement for certain stores, or only made sense in
> the context of a certain store type but not necessarily stores in general.
>
> While leaking memory due to unclosed iterators on RocksDB stores is
> certainly the most common issue, I think Matthias sufficiently
> demonstrated that the problem of leaking iterators is not actually
> unique to RocksDB, and we should consider including in-memory
> stores at the very least. I also think that at this point, we may as well
> just implement the metrics for *all* store types, whether rocksdb or
> in-memory or custom. Not just because it probably applies to all
> store types (leaking iterators are rarely a good thing!) but because
> I imagine the best way to implement this would be to do so at the
> high-level iterator rather than implementing it separately for each
> specific iterator implementation for every store type.
>
> That said, I haven't thought all that carefully about the implementation
> yet -- it just strikes me as easiest to do at the top level of the store
> hierarchy rather than at the bottom. My gut instinct may very well be
> wrong, but that's what it's saying
>
> On Thu, Mar 7, 2024 at 10:43 AM Matthias J. Sax <mj...@apache.org> wrote:
>
> > Seems I am late to this party. Can we pick this up again aiming for 3.8
> > release? I think it would be a great addition. Few comments:
> >
> >
> > - I think it does make sense to report `iterator-duration-avg` and
> > `iterator-duration-max` for all *closed* iterators -- it just seems to
> > be a useful metric (wondering if this would be _overall_ or bounded to
> > some time window?)
> >
> > - About the duration iterators are currently open, I believe the only
> > useful way is to report the "oldest iterator", ie, the smallest iterator
> > open-time, of all currently open-iterator? We all agree that in general,
> > leaking iterator would bump the count metric, and if there is a few ones
> > which are not closed and open for a long time, it seem sufficient to
> > detect the single oldest one for alerting purpose?
> >
> > - What I don't like about the KIP is it focus on RocksDB. I don't think
> > we should build on the internal RocksDB counters as proposed (I guess,
> > we could still expose them, similar to other RocksDB metrics which we
> > expose already). However, for this new metric, we should track it
> > ourselves and thus make it independent of RocksDB -- in the end, an
> > in-memory store could also leak memory (and kill a JVM with an
> > out-of-memory error) and we should be able to track it.
> >
> > - Not sure if we would like to add support for custom stores, to allow
> > them to register their iterators with this metric? Or would this not be
> > necessary, because custom stores could just register a custom metric
> > about it to begin with?
> >
> >
> >
> > -Matthias
> >
> > On 10/25/23 4:41 PM, Sophie Blee-Goldman wrote:
> > >>
> > >> If we used "iterator-duration-max", for
> > >> example, would it not be confusing that it includes Iterators that are
> > >> still open, and therefore the duration is not yet known?
> > >
> > >
> > > 1. Ah, I think I understand your concern better now -- I totally agree
> > that
> > > a
> > > "iterator-duration-max" metric would be confusing/misleading. I was
> > > thinking about it a bit differently, something more akin to the
> > > "last-rebalance-seconds-ago" consumer metric. As the name suggests,
> > > that basically just tracks how long the consumer has gone without
> > > rebalancing -- it doesn't purport to represent the actual duration
> > between
> > > rebalances, just the current time since the last one. The hard part is
> > > really
> > > in choosing a name that reflects this -- maybe you have some better
> ideas
> > > but off the top of my head, perhaps something like
> > "iterator-lifetime-max"?
> > >
> > > 2. I'm not quite sure how to interpret the "iterator-duration-total"
> > metric
> > > -- what exactly does it mean to add up all the iterator durations? For
> > > some context, while this is not a hard-and-fast rule, in general you'll
> > > find that Kafka/Streams metrics tend to come in pairs of avg/max or
> > > rate/total. Something that you might measure the avg for usually is
> > > also useful to measure the max, whereas a total metric is probably
> > > also useful as a rate but not so much as an avg. I actually think this
> > > is part of why it feels like it makes so much sense to include a "max"
> > > version of this metric, as Lucas suggested, even if the name of
> > > "iterator-duration-max" feels misleading. Ultimately the metric names
> > > are up to you, but for this reason, I would personally advocate for
> > > just going with an "iterator-duration-avg" and "iterator-duration-max"
> > >
> > > I did see your example in which you mention one could monitor the
> > > rate of change of the "-total" metric. While this does make sense to
> > > me, if the only way to interpret a metric is by computing another
> > > function over it, then why not just make that computation the metric
> > > and cut out the middle man? And in this case, to me at least, it feels
> > > much easier to understand a metric like "iterator-duration-max" vs
> > > something like "iterator-duration-total-rate"
> > >
> > > 3. By the way, can you add another column to the table with the new
> > metrics
> > > that lists the recording level? My suggestion would be to put the
> > > "number-open-iterators" at INFO and the other two at DEBUG. See
> > > the following for my reasoning behind this recommendation
> > >
> > > 4. I would change the "Type" entry for the "number-open-iterators" from
> > > "Value" to "Gauge". This helps justify the "INFO" level for this
> metric,
> > > since unlike the other metrics which are "Measurables", the current
> > > timestamp won't need to be retrieved on each recording
> > >
> > > 5. Can you list the tags that would be associated with each of these
> > > metrics (either in the table, or separately above/below if they will be
> > > the same for all)
> > >
> > > 6. Do you have a strong preference for the name "number-open-iterators"
> > > or would you be alright in shortening this to "num-open-iterators"? The
> > > latter is more in line with the naming scheme used elsewhere in Kafka
> > > for similar kinds of metrics, and a shorter name is always nice.
> > >
> > > 7. With respect to the rocksdb cache metrics, those sound useful but
> > > if it was me, I would probably save them for a separate KIP mainly just
> > > because the KIP freeze deadline is in a few weeks, and I wouldn't want
> > > to end up blocking all the new metrics just because there was ongoing
> > > debate about a subset of them. That said, you do have 3 full weeks, so
> > > I would hope that you could get both sets of metrics agreed upon in
> > > that timeframe!
> > >
> > >
> > > On Tue, Oct 24, 2023 at 6:35 AM Nick Telford <nick.telf...@gmail.com>
> > wrote:
> > >
> > >> I don't really have a problem with adding such a metric, I'm just not
> > >> entirely sure how it would work. If we used "iterator-duration-max",
> for
> > >> example, would it not be confusing that it includes Iterators that are
> > >> still open, and therefore the duration is not yet known? When graphing
> > that
> > >> over time, I suspect it would be difficult to understand.
> > >>
> > >> 3.
> > >> FWIW, this would still be picked up by "open-iterators", since that
> > metric
> > >> is only decremented when Iterator#close is called (via the
> > >> ManagedKeyValueIterator#onClose hook).
> > >>
> > >> I'm actually considering expanding the scope of this KIP slightly to
> > >> include improved Block Cache metrics, as my own memory leak
> > investigations
> > >> have trended in that direction. Do you think the following metrics
> > should
> > >> be included in this KIP, or should I create a new KIP?
> > >>
> > >> - block-cache-index-usage (number of bytes occupied by index
> blocks)
> > >> - block-cache-filter-usage (number of bytes occupied by filter
> > blocks)
> > >>
> > >> Regards,
> > >> Nick
> > >>
> > >> On Tue, 24 Oct 2023 at 07:09, Sophie Blee-Goldman <
> > sop...@responsive.dev>
> > >> wrote:
> > >>
> > >>> I actually think we could implement Lucas' suggestion pretty easily
> and
> > >>> without too much additional effort. We have full control over the
> > >> iterator
> > >>> that is returned by the various range queries, so it would be easy to
> > >>> register a gauge metric for how long it has been since the iterator
> was
> > >>> created. Then we just deregister the metric when the iterator is
> > closed.
> > >>>
> > >>> With respect to how useful this metric would be, both Nick and Lucas
> > have
> > >>> made good points: I would agree that in general, leaking iterators
> > would
> > >>> mean an ever-increasing iterator count that should be possible to
> spot
> > >>> without this. However, a few things to consider:
> > >>>
> > >>> 1. it's really easy to set up an alert based on some maximum
> threshold
> > of
> > >>> how long an iterator should remain open for. It's relatively more
> > tricky
> > >> to
> > >>> set up alerts based on the current count of open iterators and how it
> > >>> changes over time.
> > >>> 2. As Lucas mentioned, it only takes a few iterators to wreak havoc
> in
> > >>> extreme cases. Sometimes more advanced applications end up with just
> a
> > >> few
> > >>> leaking iterators despite closing the majority of them. I've seen
> this
> > >>> happen just once personally, but it was driving everyone crazy until
> we
> > >>> figured it out.
> > >>> 3. A metric for how long the iterator has been open would help to
> > >> identify
> > >>> hanging iterators due to some logic where the iterator is properly
> > closed
> > >>> but for whatever reason just isn't being advanced to the end, and
> thus
> > >> not
> > >>> reached the iterator#close line of the user code. This case seems
> > >> difficult
> > >>> to spot without the specific metric for iterator lifetime
> > >>> 4. Lastly, I think you could argue that all of the above are fairly
> > >>> advanced use cases, but this seems like a fairly advanced feature
> > >> already,
> > >>> so it doesn't seem unreasonable to try and cover all the bases.
> > >>>
> > >>> All that said, my philosophy is that the KIP author gets the final
> word
> > >> on
> > >>> what to pull into scope as long as the proposal isn't harming anyone
> > >>> without the extra feature/changes. So it's up to you Nick -- just
> > wanted
> > >>> to add some context on how it could work, and why it would be helpful
> > >>>
> > >>> Thanks for the KIP!
> > >>>
> > >>> On Wed, Oct 18, 2023 at 7:04 AM Lucas Brutschy
> > >>> <lbruts...@confluent.io.invalid> wrote:
> > >>>
> > >>>> Hi Nick,
> > >>>>
> > >>>> I did not think in detail about how to implement it, just about what
> > >>>> metrics would be nice to have. You are right, we'd have to
> > >>>> register/deregister the iterators during open/close. This would be
> > >>>> more complicated to implement than the other metrics, but I do not
> see
> > >>>> a fundamental problem with it.
> > >>>>
> > >>>> As far as I understand, even a low number of leaked iterators can
> hurt
> > >>>> RocksDB compaction significantly. So we may even want to detect if
> the
> > >>>> iterators are opened by one-time / rare queries against the state
> > >>>> store.
> > >>>>
> > >>>> But, as I said, that would be an addition and not a change of the
> > >>>> current contents of the KIP, so I'd support the KIP moving forward
> > >>>> even without this extension.
> > >>>>
> > >>>> Cheers, Lucas
> > >>>>
> > >>>>
> > >>>>
> > >>>> On Tue, Oct 17, 2023 at 3:45 PM Nick Telford <
> nick.telf...@gmail.com>
> > >>>> wrote:
> > >>>>>
> > >>>>> Hi Lucas,
> > >>>>>
> > >>>>> Hmm, I'm not sure how we could reliably identify such leaked
> > >> Iterators.
> > >>>> If
> > >>>>> we tried to include open iterators when calculating
> > >> iterator-duration,
> > >>>> we'd
> > >>>>> need some kind of registry of all the open iterator creation
> > >>> timestamps,
> > >>>>> wouldn't we?
> > >>>>>
> > >>>>> In general, if you have a leaky Iterator, it should manifest as a
> > >>>>> persistently climbing "open-iterators" metric, even on a busy node,
> > >>>> because
> > >>>>> each time that Iterator is used, it will leak another one. So even
> in
> > >>> the
> > >>>>> presence of many non-leaky Iterators on a busy instance, the metric
> > >>>> should
> > >>>>> still consistently climb.
> > >>>>>
> > >>>>> Regards,
> > >>>>>
> > >>>>> Nick
> > >>>>>
> > >>>>> On Mon, 16 Oct 2023 at 14:24, Lucas Brutschy <
> lbruts...@confluent.io
> > >>>> .invalid>
> > >>>>> wrote:
> > >>>>>
> > >>>>>> Hi Nick!
> > >>>>>>
> > >>>>>> thanks for the KIP! I think this could be quite useful, given the
> > >>>>>> problems that we had with leaks due to RocksDB resources not being
> > >>>>>> closed.
> > >>>>>>
> > >>>>>> I don't have any pressing issues why we can't accept it like it
> is,
> > >>>>>> just one minor point for discussion: would it also make sense to
> > >> make
> > >>>>>> it possible to identify a few very long-running / leaked
> > >> iterators? I
> > >>>>>> can imagine on a busy node, it would be hard to spot that 1% of
> the
> > >>>>>> iterators never close when looking only at closed iterator or the
> > >>>>>> number of iterators. But it could still be good to identify those
> > >>>>>> leaks early. One option would be to add `iterator-duration-max`
> and
> > >>>>>> take open iterators into account when computing the metric.
> > >>>>>>
> > >>>>>> Cheers,
> > >>>>>> Lucas
> > >>>>>>
> > >>>>>>
> > >>>>>> On Thu, Oct 5, 2023 at 3:50 PM Nick Telford <
> > >> nick.telf...@gmail.com>
> > >>>>>> wrote:
> > >>>>>>>
> > >>>>>>> Hi Colt,
> > >>>>>>>
> > >>>>>>> I kept the details out of the KIP, because KIPs generally
> > >> document
> > >>>>>>> high-level design, but the way I'm doing it is like this:
> > >>>>>>>
> > >>>>>>> final ManagedKeyValueIterator<Bytes, byte[]>
> > >>>>>>> rocksDbPrefixSeekIterator = cf.prefixScan(accessor, prefixBytes);
> > >>>>>>> --> final long startedAt = System.nanoTime();
> > >>>>>>> openIterators.add(rocksDbPrefixSeekIterator);
> > >>>>>>> rocksDbPrefixSeekIterator.onClose(() -> {
> > >>>>>>> -->
> > >>> metricsRecorder.recordIteratorDuration(System.nanoTime()
> > >>>> -
> > >>>>>>> startedAt);
> > >>>>>>> openIterators.remove(rocksDbPrefixSeekIterator);
> > >>>>>>> });
> > >>>>>>>
> > >>>>>>> The lines with the arrow are the new code. This pattern is
> > >> repeated
> > >>>>>>> throughout RocksDBStore, wherever a new RocksDbIterator is
> > >> created.
> > >>>>>>>
> > >>>>>>> Regards,
> > >>>>>>> Nick
> > >>>>>>>
> > >>>>>>> On Thu, 5 Oct 2023 at 12:32, Colt McNealy <c...@littlehorse.io>
> > >>>> wrote:
> > >>>>>>>
> > >>>>>>>> Thank you for the KIP, Nick!
> > >>>>>>>>
> > >>>>>>>> This would be highly useful for many reasons. Much more sane
> > >> than
> > >>>>>> checking
> > >>>>>>>> for leaked iterators by profiling memory usage while running
> > >>> 100's
> > >>>> of
> > >>>>>>>> thousands of range scans via interactive queries (:
> > >>>>>>>>
> > >>>>>>>> One small question:
> > >>>>>>>>
> > >>>>>>>>> The iterator-duration metrics will be updated whenever an
> > >>>> Iterator's
> > >>>>>>>> close() method is called
> > >>>>>>>>
> > >>>>>>>> Does the Iterator have its own "createdAt()" or equivalent
> > >> field,
> > >>>> or
> > >>>>>> do we
> > >>>>>>>> need to keep track of the Iterator's start time upon creation?
> > >>>>>>>>
> > >>>>>>>> Cheers,
> > >>>>>>>> Colt McNealy
> > >>>>>>>>
> > >>>>>>>> *Founder, LittleHorse.dev*
> > >>>>>>>>
> > >>>>>>>>
> > >>>>>>>> On Wed, Oct 4, 2023 at 9:07 AM Nick Telford <
> > >>>> nick.telf...@gmail.com>
> > >>>>>>>> wrote:
> > >>>>>>>>
> > >>>>>>>>> Hi everyone,
> > >>>>>>>>>
> > >>>>>>>>> KIP-989 is a small Kafka Streams KIP to add a few new metrics
> > >>>> around
> > >>>>>> the
> > >>>>>>>>> creation and use of RocksDB Iterators, to aid users in
> > >>>> identifying
> > >>>>>>>>> "Iterator leaks" that could cause applications to leak native
> > >>>> memory.
> > >>>>>>>>>
> > >>>>>>>>> Let me know what you think!
> > >>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>>
> > >>>>>>
> > >>>>
> > >>>
> > >>
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-989%3A+RocksDB+Iterator+Metrics
> > >>>>>>>>>
> > >>>>>>>>> P.S. I'm not too sure about the formatting of the "New
> > >> Metrics"
> > >>>>>> table,
> > >>>>>>>> any
> > >>>>>>>>> advice there would be appreciated.
> > >>>>>>>>>
> > >>>>>>>>> Regards,
> > >>>>>>>>> Nick
> > >>>>>>>>>
> > >>>>>>>>
> > >>>>>>
> > >>>>
> > >>>
> > >>
> > >
> >
>
