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From Benjamin Mahler <bmah...@apache.org>
Subject Re: Metrics collection affected when libprocess queue builds up
Date Thu, 05 Jan 2017 02:28:13 GMT
A patch to update the documentation with a NOTE about this would be great.
It excludes all metrics that were not available within the timeout, there
is no indication within a particular result whether any timed out and were
excluded.

My feeling is that taking the difference between enqueued and dequeued is
> not as obvious as a `Gauge`. If we take this path, we should document it
> clearly how to use the metric.


We could also explore avoiding the dispatch per (3) for this case, since
getting the size is a read only operation and should be thread-safe without
acquiring the lock.

I'm also interested in how other `Gauge` type systems are implemented in
> other well know OSS projects. Maybe we can do some more research on their
> approach?


We did research when the metrics library was originally added. Coda Hale's
library (now called "dropwizard metrics" FWICT) was one of the libraries we
looked at, and its Gauge is equivalent to suggestion (3), as it was
designed for Java-style programs with locking as opposed to actors:

http://metrics.dropwizard.io/3.1.0/getting-started/#gauges

On Fri, Dec 30, 2016 at 10:34 AM, Zhitao Li <zhitaoli.cs@gmail.com> wrote:

> Hi Benjamin,
>
> Thanks for the response. First time heard of the `timeout` parameter. I'll
> fix our monitoring scripts to always specify this.
>
> One question on timeout: does it simply drop any metric callback which is
> not collected within the timeout? Does caller know which metrics are
> dropped due to timeout?
>
> Also, it is not documented in
> http://mesos.apache.org/documentation/latest/monitoring/. Should we send a
> patch to update it?
>
> On Tue, Dec 27, 2016 at 6:12 PM, Benjamin Mahler <bmahler@apache.org>
> wrote:
>
> > The /metrics endpoint exposes a timeout parameter if you want to receive
> a
> > response with all of the metrics that were available within the timeout,
> > e.g. /metrics/snapshot.json?timeout=10secs
> >
> > I'd recommend using this when collecting metrics so that you can maintain
> > visibility when a particular component is backlogged.
> >
> > Should we explore a more reliable way to track metrics independently from
> > > libprocess's queue?
> >
> >
> > Note that this problem applies only to our defer-based "Gauge" metrics
> that
> > execute on the actor. Counters and Timers are immune to this. I would say
> > there are a couple of improvements we can make in increasing order of
> > difficulty:
> >
> > (1) There are instances of Gauges that might be better represented as
> > Counters. For example, we expose the actor queue sizes using a gauge
> (known
> > to be unfortunate!), when instead we could expose two counters for
> > "enqueued" and "dequeued" messages and infer size from these. We can also
> > add the ability for callers to manually increment and decrement their
> > Gauges rather than go through a dispatch.
>
>
> My feeling is that taking the difference between enqueued and dequeued is
> not as obvious as a `Gauge`. If we take this path, we should document it
> clearly how to use the metric.
>
>
> >
> > (2) Allow Gauge dispatches to be sent to the front of the actor's queue,
> > rather than the back. I would hope that we don't wind up with a notion of
> > integer priority for messages. Note that this doesn't solve the problem
> for
> > when the "backlog" is occurring inside a single expensive function. It
> also
> > has the issue of preventing "progress" if metrics are hit frequently
> enough
> > and are expensive enough.
>
>
> > (3) There are instances of Gauges that might be better represented as
> > thread-safe logic. For example, if we need an actor's std::map member's
> > .size(), we could call .size() safely so long as the map is not
> destructed.
> > In other cases, explicit locking may be needed and is more complicated.
> >
> > (4) There are instances of Gauges that might be better represented as a
> > "wrapping" around a data-structure. For example, the std::map could be
> > wrapped as a 'map_wrapper' that injects metric updates into each
> non-const
> > operation that affects the size of the map.
> >
> > So far I've felt that the timeout and (1) will be sufficient for the
> > foreseeable future, while (3) and (4) seem to require a significant
> impact
> > to non-metrics related code complexity, let me know what you think.
> >
>
> I agree that we should not adopt (2) only to address this problem: it seems
> like something larger and also affects how libprocess was generally
> designed, so we should think more carefully about that.
>
> I like the idea of (3) since it can be implemented gradually, and it can
> completely avoid paying the cost of enque/deque message (which is another
> interesting question: how expensive it could be?)
>
> (4) seems like a bigger
>
> I'm also interested in how other `Gauge` type systems are implemented in
> other well know OSS projects. Maybe we can do some more research on their
> approach?
>
>
> > Ben
> >
> > On Mon, Dec 19, 2016 at 6:32 PM, Zameer Manji <zmanji@apache.org> wrote:
> >
> > > I believe Zhitao is referring to `/metrics/snapshot` returning a result
> > > after 10-30 seconds.
> > >
> > > I think in a typical environment, this will cause most metrics
> collection
> > > tooling to timeout. This causes the operator to not have any visibility
> > > into the system, making debugging/fighting the problem very hard.
> > >
> > > On Mon, Dec 19, 2016 at 9:23 PM, haosdent <haosdent@gmail.com> wrote:
> > >
> > > > Hi, @zhitao
> > > >
> > > > > the `/metrics/snapshot` could take 10-30 seconds to respond.
> > > >
> > > > Do you mean it `/metrics/snapshot` return result after 10~30 seconds?
> > > > Or `/metrics/snapshot` takes 10~30 seconds to reflect the change of `
> > > > allocator/mesos/event_queue_dispatches gauge`?
> > > >
> > > > On Mon, Dec 19, 2016 at 1:11 PM, Zhitao Li <zhitaoli.cs@gmail.com>
> > > wrote:
> > > >
> > > > > Hi all,
> > > > >
> > > > > While I was debugging an allocator message queue build up issue on
> > > master
> > > > > (which I plan to share another thread), I noticed that
> > > > `/metrics/snapshot`
> > > > > is also badly affected.
> > > > >
> > > > > For example, when the allocator queue has ~3k dispatches in it
> > > (revealed
> > > > by
> > > > > the allocator/mesos/event_queue_dispatches gauge), the
> > > > `/metrics/snapshot`
> > > > > could take 10-30 seconds to respond.
> > > > >
> > > > > During an active debugging or outage fighting, this is pretty
> > > undesired.
> > > > >
> > > > > My guess is that many stats collection code relies on *deferring*
> to
> > > > > another libprocess and collect the result.
> > > > >
> > > > > Should we explore a more reliable way to track metrics
> independently
> > > from
> > > > > libprocess's queue?
> > > > >
> > > > > --
> > > > > Cheers,
> > > > >
> > > > > Zhitao Li
> > > > >
> > > >
> > > >
> > > >
> > > > --
> > > > Best Regards,
> > > > Haosdent Huang
> > > >
> > > > --
> > > > Zameer Manji
> > > >
> > >
> >
>
>
>
> --
> Cheers,
>
> Zhitao Li
>

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