Thanks for the further explanations, Yu!
1. The inflight buffer spilling process is indeed handled asynchronously. While
the buffer is not finished spilling, it would not be recycled to reuse again.
Your understanding is right. I guess I misunderstood your previous concern of
additional memory consumption from the perspective of buffer usage.
My point of no additional memory consumption is from the perspective of total
network memory size which would not be increased as a result.
2. We treat the inflight buffers as input&output states which are equivalent
with existing operator states, and try to make use of all the existing
mechanisms for
state handle and assignment during recovery. So i guess for the local recovery
it should be the similar case. I would think through whether it has some special
work to do around with local recovery, and then clarify it in FLIP after we
reach an agreement internally. BTW, this FLIP has not finalized yet.
3. Yes, the previous proposal is for measuring how many inflight buffers to be
spilled which refers to the data size if really taking this way. I think the
proposed option
in FLIP are the initial thoughts for various of possibilities. Which way we
decide to take for the first version, I guess we need to further finalize
before voting.
4. I think there probably exists the requirements or scenarios from users as
you mentioned. Actually we have not finalized the way of switching to unaligned
checkpoint yet.
Anyway we could provide an option for users to try out this feature at the
beginning, although it might be not the most ideal one. Another input is that
we know the motivation
of unaligned checkpoint is from the scenarios of backpressure, but it might
also performs well in the case of non backpressure, even shorten the checkpoint
duration without
obvious performance regression in our previous POC testing. So the backpressure
might not be the only factor to switch to the unaligned way in practice I
guess. Anyway your
inputs are valuable for us to make the final decision.
Best,
Zhijiang
------------------------------------------------------------------
From:Yu Li <car...@gmail.com>
Send Time:2020 Feb. 26 (Wed.) 15:59
To:dev <dev@flink.apache.org>; Zhijiang <wangzhijiang...@aliyun.com>
Subject:Re: [DISCUSS] FLIP-76: Unaligned checkpoints
Hi Zhijiang,
Thanks for the quick reply!
For the 1st question, please allow me to confirm, that when doing asynchronous
checkpointing, disk spilling should happen in background in parallel with
receiving/sending new data, or else it would become synchronous, right? Based
on such assumption, some copy-on-write like mechanism would be necessary to
make sure the new updates won't modify the to-be-checkpointed data, and this is
where the additional memory consumption comes from.
About point #2, I suggest we write it down in the FLIP document about local
recovery support (if reach a consensus here), to make sure it won't be
neglected in later implementation (I believe there're still some work to do
following existing local recovery mechanism). What do you think?
For the 3rd topic, do you mean UNALIGNED_WITH_MAX_INFLIGHT_DATA would set some
kind of threshold about "how much in-flight data to checkpoint"? If so, could
you further clarify the measurement (data size? record number? others?) since
there seems to be no description in the current FLIP doc? This is somewhat
different from my understanding after reading the FLIP...
Regarding question #4, I have no doubt that the new unaligned checkpoint
mechanism could make fast checkpoint possible, at the cost of more memory,
network bandwidth and disk space consumption. However, (correct me if I'm
wrong) for users who are satisfied with the existing aligned checkpoint
interval, taking the constant cost to prevent delayed checkpoint during back
pressure - a relatively low frequency event - may not be that pragmatic.
Best Regards,
Yu
On Wed, 26 Feb 2020 at 15:07, Zhijiang <wangzhijiang...@aliyun.com.invalid>
wrote:
Hi Yu,
Thanks for concerning of this FLIP and sharing your thoughts! Let me try to
answer some below questions.
1. Yes, the asynchronous checkpointing should be part of whole process and be
supported naturally. As for the network memory concern,
the inflight-buffers would be spilled into persistent storage while triggering
checkpoint, and are recycled to receive/send data after finish spilling.
We still reuse the current network memory setting, so the maximum
inflight-buffers would not exceed that amount, and there would not have
additional memory consumption.
2. Yes, we would try to reuse the existing checkpoint recovery mechanism for
simple implementation.
3. UNALIGNED_WITH_MAX_INFLIGHT_DATA and UNALIGNED_WITH_UNLIMITED_INFLIGHT_DATA
are for the consideration of triggering checkpoint
at proper time, the tradeoff between checkpoint duration and spilling inflight
data, etc. I guess it still makes sense for the single input channel.
Assuming there were already accumulated 100 unconsumed buffers in one remote
input channel when the barrier arrives from the network, then we can
decide whether to trigger checkpoint immediately based on
UNALIGNED_WITH_UNLIMITED_INFLIGHT_DATA or based on
UNALIGNED_WITH_MAX_INFLIGHT_DATA
if 100 is not reaching its max threshold.
4. I remembered that we ever discussed the options internally before. I agree
with that the adaptive way might seem more flexible, but also mean more
complicated
in design and implementation. As the first step of unaligned checkpoint, it
seems more make sense to take an easy way for only concentrating on the
function and
practical effect. After getting some feedbacks to convince us, I guess the
adaptive way might be probably an option to consider if really necessary in
future.
Best,
Zhijiang
------------------------------------------------------------------
From:Yu Li <car...@gmail.com>
Send Time:2020 Feb. 26 (Wed.) 12:59
To:dev <dev@flink.apache.org>
Subject:Re: [DISCUSS] FLIP-76: Unaligned checkpoints
Hi All,
Sorry for being late to the discussion. I've gone through the latest FLIP
document and have below questions/suggestions:
1. Do we support asynchronous checkpointing on the in-flight data?
* From the doc the answer seems to be yes (state-based storage for the
first version), and if so, there would be additional memory consumption on
network buffer during checkpoint and we should take this into account,
especially in container environment.
2. I suggest we also take local recovery into consideration during
implementation, which could speed up the recovery speed especially when the
amount of in-flight data is huge.
3. About checkpointing policy, are the below understanding correct? Maybe
it helps if we map them more explicitly in FLIP doc, IMHO:
* For single input channel, there's no difference between
UNALIGNED_WITH_MAX_INFLIGHT_DATA
and UNALIGNED_WITH_UNLIMITED_INFLIGHT_DATA, both means we start the
checkpoint once observe the barrier in the input channel.
* For multiple input channels, UNALIGNED_WITH_MAX_INFLIGHT_DATA means
starting checkpoint only when barrier appears in all input channels,
while UNALIGNED_WITH_UNLIMITED_INFLIGHT_DATA means starting checkpoint when
barrier appears in any one of the input channels.
4. It seems now we only support pre-defined options, but is it possible to
switch in between dynamically? For example, if we predefine the policy to
ALIGNED, could we supply a command to switch
to UNALIGNED_WITH_MAX_INFLIGHT_DATA when severe back pressure observed? Or
switch to ALIGNED if we see too much data persisted for
UNALIGNED_WITH_MAX_INFLIGHT_DATA? Maybe I'm neglecting something, but
what's preventing us from being more adaptive?
Thanks!
Best Regards,
Yu
On Tue, 22 Oct 2019 at 15:44, Piotr Nowojski <pi...@ververica.com> wrote:
> Hi,
>
> I would like to propose a modification to this FLIP.
>
> Based on the feedback that we were receiving after publishing this
> document and during Flink Forward, I was growing more and more anxious
> about one issue here: having to persist all buffered in-flight data at
> once. As the volume of this data might be large (GBs per TaskManager even
> with small clusters and relatively simple jobs), the time to persist all of
> this data at once might be quite substantial.
>
>
>
> To address this issue, I would like to propose that at first we implement
> a variant of unaligned checkpoints, just as written down in FLIP-76, but
> with continuous spilling - all data will be persisted/spilled continuously,
> all the time as they come - not at once when the checkpoint starts. Think
> about this proposal as incremental way of persisting the data.
>
> Pros of continuous spilling:
> + faster checkpointing, as there will be no need to store GBs of data,
> just flush/close.
> + more predictable behaviour. Instead of jerky/varying/spike IO/CPU loads,
> steady records throughput and spilling.
>
> Cons of continuous spilling:
> - need to persist all of the network traffic instead of persisting just
> the in-flight data
>
> Larger volume of persisted data doesn’t matter that much from the
> perspective of the throughput, as if you are unable to spill the data
> faster than to process them, unaligned checkpoints are worse option
> compared to the aligned checkpoints [1]. If checkpoints are frequent it
> also doesn’t matter [2]. The true downside is if checkpoints are infrequent
> and you have to for example pay $ for the extra storage or extra network
> traffic to the storage.
>
> On the other hand, continuous spilling (persistent communication
> channels?) might have an added benefit of enabling us localised failures -
> failure of one node will not necessarily bring down the whole cluster.
>
>
>
> As I mentioned, I’m proposing to just start with the continuous spilling.
> It might be more costly in some scenarios, but it will offer the most
> stable and predictable performance with the lowest checkpoint latency. It’s
> not perfect, it won’t solve all of the use cases, but frankly all of the
> other options have their own blind spots, and continuous spilling should at
> least fully solve relatively low throughput use cases. We can later build
> on top of that solution, expanding it with the following features:
>
> 1. Do not spill continuously if there is no backpressure. For example
> provide a timeout: start spilling pre-emptively/continuously if some buffer
> was not processed within X seconds.
> 2. Start spilling only once the checkpoint starts (this is the exact
> proposal from the current FLIP-76).
> 3. Initially we want to spill to a Flink’s FileSystem (for example S3),
> but in the future we are considering other options, for example Apache
> Bookeeper.
>
> What do you think?
>
> Piotrek
>
>
>
> [1] I’m assuming that the spilling throughput per node can go up to
> ~30MB/s. If your Flink's job data processing rate is 100MB/s, spilling
> in-flight data will take 3.3 times longer than waiting for the alignment.
> On the other hand if data processing rate is 10MB/s, overhead of continuous
> spilling is relatively low.
> [2] With checkpoints every one minute, with data processing throughput
> 30MB/s per node, we would have to persist 1.8GB of data per node between
> the checkpoints, which is similar order of magnitude as buffered in-flight
> data under the back-pressure. With higher throughput, unaligned checkpoints
> are not helping ([1]). With lower throughput, both the original proposal
> and continuous spilling would have to effectively persist all of the data
> anyway.
>
> > On 10 Oct 2019, at 19:51, Yun Tang <myas...@live.com> wrote:
> >
> > Hi Arvid
> >
> > +1 for this future which has been hoped for a long time. End-to-end
> exactly once job could benefit from quicker checkpoint completion.
> >
> >
> > Best
> > Yun Tang
> > ________________________________
> > From: Yun Gao <yungao...@aliyun.com.INVALID>
> > Sent: Thursday, October 10, 2019 18:39
> > To: dev <dev@flink.apache.org>
> > Subject: Re: [DISCUSS] FLIP-76: Unaligned checkpoints
> >
> > Hi Arvid,
> >
> > Very thanks for bring up the discussion! From our side unable
> to finish the checkpoint is commonly met for online jobs, therefore +1 from
> my side to implement this.
> > A tiny issue of the FLIP is that the Discussion Thread URL
> attached seems to be not right.
> >
> >
> > Best,
> > Yun
> >
> >
> > ------------------------------------------------------------------
> > From:Arvid Heise <ar...@ververica.com>
> > Send Time:2019 Sep. 30 (Mon.) 20:31
> > To:dev <dev@flink.apache.org>
> > Subject:[DISCUSS] FLIP-76: Unaligned checkpoints
> >
> > Hi Devs,
> >
> > I would like to start the formal discussion about FLIP-76 [1], which
> > improves the checkpoint latency in systems under backpressure, where a
> > checkpoint can take hours to complete in the worst case. I recommend the
> > thread "checkpointing under backpressure" [2] to get a good idea why
> users
> > are not satisfied with the current behavior. The key points:
> >
> > - Since the checkpoint barrier flows much slower through the
> > back-pressured channels, the other channels and their upstream
> operators
> > are effectively blocked during checkpointing.
> > - The checkpoint barrier takes a long time to reach the sinks causing
> > long checkpointing times. A longer checkpointing time in turn means
> that
> > the checkpoint will be fairly outdated once done. Since a heavily
> utilized
> > pipeline is inherently more fragile, we may run into a vicious cycle of
> > late checkpoints, crash, recovery to a rather outdated checkpoint, more
> > back pressure, and even later checkpoints, which would result in
> little to
> > no progress in the application.
> >
> > The FLIP proposes "unaligned checkpoints" which improves the current
> state,
> > such that
> >
> > - Upstream processes can continue to produce data, even if some
> operator
> > still waits on a checkpoint barrier on a specific input channel.
> > - Checkpointing times are heavily reduced across the execution graph,
> > even for operators with a single input channel.
> > - End-users will see more progress even in unstable environments as
> more
> > up-to-date checkpoints will avoid too many recomputations.
> > - Facilitate faster rescaling.
> >
> > The key idea is to allow checkpoint barriers to be forwarded to
> downstream
> > tasks before the synchronous part of the checkpointing has been conducted
> > (see Fig. 1). To that end, we need to store in-flight data as part of the
> > checkpoint as described in greater details in this FLIP.
> >
> > Although the basic idea was already sketched in [2], we would like get
> > broader feedback in this dedicated mail thread.
> >
> > Best,
> >
> > Arvid
> >
> > [1]
> >
> https://cwiki.apache.org/confluence/display/FLINK/FLIP-76%3A+Unaligned+Checkpoints
> > [2]
> >
> http://apache-flink-mailing-list-archive.1008284.n3.nabble.com/Checkpointing-under-backpressure-td31616.html
> >
>
>
