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
