One quick thing -- can you update the title of this KIP to reflect the
decision to implement these metrics for all state stores implementations
rather than just RocksDB?
On Tue, May 14, 2024 at 1:36 PM Nick Telford <nick.telf...@gmail.com> wrote:
> Woops! Thanks for the catch Lucas. Given this was just a typo, I don't
> think this affects the voting.
>
> Cheers,
> Nick
>
> On Tue, 14 May 2024 at 18:06, Lucas Brutschy <lbruts...@confluent.io
> .invalid>
> wrote:
>
> > Hi Nick,
> >
> > you are still referring to oldest-open-iterator-age-ms in the
> > `Proposed Changes` section.
> >
> > Cheers,
> > Lucas
> >
> > On Thu, May 2, 2024 at 4:00 PM Lucas Brutschy <lbruts...@confluent.io>
> > wrote:
> > >
> > > Hi Nick!
> > >
> > > I agree, the age variant is a bit nicer since the semantics are very
> > > clear from the name. If you'd rather go for the simple implementation,
> > > how about calling it `oldest-iterator-open-since-ms`? I believe this
> > > could be understood without docs. Either way, I think we should be
> > > able to open the vote for this KIP because nobody raised any major /
> > > blocking concerns.
> > >
> > > Looking forward to getting this voted on soon!
> > >
> > > Cheers
> > > Lucas
> > >
> > > On Sun, Mar 31, 2024 at 5:23 PM Nick Telford <nick.telf...@gmail.com>
> > wrote:
> > > >
> > > > Hi Matthias,
> > > >
> > > > > For the oldest iterator metric, I would propose something simple
> like
> > > > > `iterator-opened-ms` and it would just be the actual timestamp when
> > the
> > > > > iterator was opened. I don't think we need to compute the actual
> age,
> > > > > but user can to this computation themselves?
> > > >
> > > > That works for me; it's easier to implement like that :-D I'm a
> little
> > > > concerned that the name "iterator-opened-ms" may not be obvious
> enough
> > > > without reading the docs.
> > > >
> > > > > If we think reporting the age instead of just the timestamp is
> > better, I
> > > > > would propose `iterator-max-age-ms`. I should be sufficient to call
> > out
> > > > > (as it's kinda "obvious" anyway) that the metric applies to open
> > > > > iterator only.
> > > >
> > > > While I think it's preferable to record the timestamp, rather than
> the
> > age,
> > > > this does have the benefit of a more obvious metric name.
> > > >
> > > > > Nit: the KIP says it's a store-level metric, but I think it would
> be
> > > > > good to say explicitly that it's recorded with DEBUG level only?
> > > >
> > > > Yes, I've already updated the KIP with this information in the table.
> > > >
> > > > Regards,
> > > >
> > > > Nick
> > > >
> > > > On Sun, 31 Mar 2024 at 10:53, Matthias J. Sax <mj...@apache.org>
> > wrote:
> > > >
> > > > > The time window thing was just an idea. Happy to drop it.
> > > > >
> > > > > For the oldest iterator metric, I would propose something simple
> like
> > > > > `iterator-opened-ms` and it would just be the actual timestamp when
> > the
> > > > > iterator was opened. I don't think we need to compute the actual
> age,
> > > > > but user can to this computation themselves?
> > > > >
> > > > > If we think reporting the age instead of just the timestamp is
> > better, I
> > > > > would propose `iterator-max-age-ms`. I should be sufficient to call
> > out
> > > > > (as it's kinda "obvious" anyway) that the metric applies to open
> > > > > iterator only.
> > > > >
> > > > > And yes, I was hoping that the code inside MetereXxxStore might
> > already
> > > > > be setup in a way that custom stores would inherit the iterator
> > metrics
> > > > > automatically -- I am just not sure, and left it as an exercise for
> > > > > somebody to confirm :)
> > > > >
> > > > >
> > > > > Nit: the KIP says it's a store-level metric, but I think it would
> be
> > > > > good to say explicitly that it's recorded with DEBUG level only?
> > > > >
> > > > >
> > > > >
> > > > > -Matthias
> > > > >
> > > > >
> > > > > On 3/28/24 2:52 PM, Nick Telford wrote:
> > > > > > Quick addendum:
> > > > > >
> > > > > > My suggested metric "oldest-open-iterator-age-seconds" should be
> > > > > > "oldest-open-iterator-age-ms". Milliseconds is obviously a better
> > > > > > granularity for such a metric.
> > > > > >
> > > > > > Still accepting suggestions for a better name.
> > > > > >
> > > > > > On Thu, 28 Mar 2024 at 13:41, Nick Telford <
> nick.telf...@gmail.com
> > >
> > > > > wrote:
> > > > > >
> > > > > >> 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
> > > > > >>>>>>>>>>>>>
> > > > > >>>>>>>>>>>>
> > > > > >>>>>>>>>>
> > > > > >>>>>>>>
> > > > > >>>>>>>
> > > > > >>>>>>
> > > > > >>>>>
> > > > > >>>>
> > > > > >>>
> > > > > >>
> > > > > >
> > > > >
> >
>
