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https://issues.apache.org/jira/browse/CASSANDRA-14459?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16649549#comment-16649549
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Joseph Lynch commented on CASSANDRA-14459:
------------------------------------------
[~aweisberg] Alright I think that I've accomplished all of the bullet points
that you listed . Along the way I made DES 100% pluggable by class name as well
as significantly more testable because I know that various folks want to
experiment with further improvements and having it be as easy as implementing a
new subclass and dropping a jar will make experimentation much easier going
forwards... I've also gone ahead and factored out the "measurement" parts of
the DES into subclasses (default Histogram and new EMA option) from the
"ranking" parts of the DES in the abstract base class (ranking, probing, etc).
||trunk||
|[Pull Request for Review|https://github.com/apache/cassandra/pull/283]|
|[!https://circleci.com/gh/jolynch/cassandra/tree/CASSANDRA-14459.png?circle-token=
1102a59698d04899ec971dd36e925928f7b521f5!|https://circleci.com/gh/jolynch/cassandra/tree/CASSANDRA-14459]|
There are a couple of points that I'm unsure about:
* I was able to send small + large messages, but trying to pick the maximum
result is hard. I think that it should be ok because if there is a serious
problem with a stage it'll basically average the "good" stage and "bad" stage
and still get a pretty bad number most likely.
* Right now I've removed the reset interval but I actually think it's a good
idea to keep it in and just re-define what it does. Specifically that would be
the interval that we get to determine the probe list, vs the probe interval
which actually sends a probe. I also think it's nice because we could have the
old Histogram implementation as the default, and then users could _opt in_ to
the new Histogram or the EMA approach as they want to. This would also remove
the somewhat awkward (imo) use of the ping timeout to rate limit how often we
get to generate probe lists.
* The current algorithm only sends probes to nodes that were requested
recently, but it's still potentially a lot of messages at the default interval
of 1s which I know you think is ok but e.g. Jason above didn't like it (e.g.
for a 300 * 3 dc 256 vnode cluster operating at LOCAL_ONE CL we'd be
essentially sending out a probe per second always since there are so many
ranked replicas including remote dcs). We could mitigate this somewhat by
changing the local filtering step in {{ReplicaPlans::forRead}} to happen before
we ask the DES to rank rather than after (this seems like a good idea
regardless).
While I was making it way more testable I ran a quick microbenchmark to show
the CPU and memory benefits of the EMA approach. The tradoff of course is that
the EMA reacts less quickly when the sample size is small and way faster when
there is a significant outlier (e.g. if there is a single 10s timing the EMA
will deprefer that host almost immediately compared to the histogram which ...
won't).
{noformat}
Benchmark
Mode Cnt Score Error Units
SnitchMetricsBench.emaSnitchThreadedUpdate
sample 190609 1.573 ± 0.016 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p0.00
sample 0.283 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p0.50
sample 1.288 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p0.90
sample 1.456 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p0.95
sample 1.597 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p0.99
sample 15.286 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p0.999
sample 21.594 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p0.9999
sample 31.319 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:emaSnitchThreadedUpdate·p1.00
sample 49.086 ms/op
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.alloc.rate
sample 3 4.158 ± 10.217 MB/sec
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.alloc.rate.norm
sample 3 722.283 ± 55.616 B/op
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.churn.PS_Eden_Space
sample 3 3.949 ± 62.392 MB/sec
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.churn.PS_Eden_Space.norm
sample 3 745.116 ± 11782.387 B/op
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.churn.PS_Survivor_Space
sample 3 0.108 ± 3.405 MB/sec
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.churn.PS_Survivor_Space.norm
sample 3 19.861 ± 627.604 B/op
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.count
sample 3 2.000 counts
SnitchMetricsBench.emaSnitchThreadedUpdate:·gc.time
sample 3 6.000 ms
SnitchMetricsBench.emaSnitchUpdate
sample 101392 0.296 ± 0.001 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p0.00
sample 0.263 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p0.50
sample 0.297 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p0.90
sample 0.326 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p0.95
sample 0.351 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p0.99
sample 0.417 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p0.999
sample 0.523 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p0.9999
sample 0.635 ms/op
SnitchMetricsBench.emaSnitchUpdate:emaSnitchUpdate·p1.00
sample 5.300 ms/op
SnitchMetricsBench.emaSnitchUpdate:·gc.alloc.rate
sample 3 2.120 ± 2.248 MB/sec
SnitchMetricsBench.emaSnitchUpdate:·gc.alloc.rate.norm
sample 3 690.546 ± 5.915 B/op
SnitchMetricsBench.emaSnitchUpdate:·gc.churn.PS_Eden_Space
sample 3 1.984 ± 62.688 MB/sec
SnitchMetricsBench.emaSnitchUpdate:·gc.churn.PS_Eden_Space.norm
sample 3 606.680 ± 19170.514 B/op
SnitchMetricsBench.emaSnitchUpdate:·gc.count
sample 3 1.000 counts
SnitchMetricsBench.emaSnitchUpdate:·gc.time
sample 3 4.000 ms
SnitchMetricsBench.histogramSnitchThreaded
sample 31134 9.627 ± 0.097 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p0.00
sample 4.030 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p0.50
sample 7.545 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p0.90
sample 17.662 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p0.95
sample 23.003 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p0.99
sample 29.119 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p0.999
sample 39.697 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p0.9999
sample 53.506 ms/op
SnitchMetricsBench.histogramSnitchThreaded:histogramSnitchThreaded·p1.00
sample 55.575 ms/op
SnitchMetricsBench.histogramSnitchThreaded:·gc.alloc.rate
sample 3 668.096 ± 61.306 MB/sec
SnitchMetricsBench.histogramSnitchThreaded:·gc.alloc.rate.norm
sample 3 708597.630 ± 32119.534 B/op
SnitchMetricsBench.histogramSnitchThreaded:·gc.churn.PS_Eden_Space
sample 3 673.780 ± 134.265 MB/sec
SnitchMetricsBench.histogramSnitchThreaded:·gc.churn.PS_Eden_Space.norm
sample 3 714612.589 ± 88018.271 B/op
SnitchMetricsBench.histogramSnitchThreaded:·gc.churn.PS_Survivor_Space
sample 3 0.154 ± 0.009 MB/sec
SnitchMetricsBench.histogramSnitchThreaded:·gc.churn.PS_Survivor_Space.norm
sample 3 163.122 ± 19.745 B/op
SnitchMetricsBench.histogramSnitchThreaded:·gc.count
sample 3 273.000 counts
SnitchMetricsBench.histogramSnitchThreaded:·gc.time
sample 3 303.000 ms
SnitchMetricsBench.histogramSnitchUpdate
sample 23007 1.303 ± 0.003 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p0.00
sample 1.204 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p0.50
sample 1.268 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p0.90
sample 1.368 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p0.95
sample 1.466 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p0.99
sample 2.121 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p0.999
sample 2.830 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p0.9999
sample 3.926 ms/op
SnitchMetricsBench.histogramSnitchUpdate:histogramSnitchUpdate·p1.00
sample 5.218 ms/op
SnitchMetricsBench.histogramSnitchUpdate:·gc.alloc.rate
sample 3 236.317 ± 3.365 MB/sec
SnitchMetricsBench.histogramSnitchUpdate:·gc.alloc.rate.norm
sample 3 339325.737 ± 47.087 B/op
SnitchMetricsBench.histogramSnitchUpdate:·gc.churn.PS_Eden_Space
sample 3 236.374 ± 15.878 MB/sec
SnitchMetricsBench.histogramSnitchUpdate:·gc.churn.PS_Eden_Space.norm
sample 3 339407.281 ± 21325.135 B/op
SnitchMetricsBench.histogramSnitchUpdate:·gc.churn.PS_Survivor_Space
sample 3 0.142 ± 0.462 MB/sec
SnitchMetricsBench.histogramSnitchUpdate:·gc.churn.PS_Survivor_Space.norm
sample 3 203.675 ± 663.495 B/op
SnitchMetricsBench.histogramSnitchUpdate:·gc.count
sample 3 223.000 counts
SnitchMetricsBench.histogramSnitchUpdate:·gc.time
sample 3 232.000 ms
{noformat}
As you can see the traditional histogram snitch generates 700MBps of garbage vs
the EMA which generates 4MBps, and the Histogram snitch was slower at 9.6ms avg
vs EMA at 1.5 avg. So while the speed improvement of 6x is nice, the reduced
garbage is going to be the really nice part.
> DynamicEndpointSnitch should never prefer latent nodes
> ------------------------------------------------------
>
> Key: CASSANDRA-14459
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14459
> Project: Cassandra
> Issue Type: Improvement
> Components: Coordination
> Reporter: Joseph Lynch
> Assignee: Joseph Lynch
> Priority: Minor
> Labels: 4.0-feature-freeze-review-requested,
> pull-request-available
> Fix For: 4.x
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> The DynamicEndpointSnitch has two unfortunate behaviors that allow it to
> provide latent hosts as replicas:
> # Loses all latency information when Cassandra restarts
> # Clears latency information entirely every ten minutes (by default),
> allowing global queries to be routed to _other datacenters_ (and local
> queries cross racks/azs)
> This means that the first few queries after restart/reset could be quite slow
> compared to average latencies. I propose we solve this by resetting to the
> minimum observed latency instead of completely clearing the samples and
> extending the {{isLatencyForSnitch}} idea to a three state variable instead
> of two, in particular {{YES}}, {{NO}}, {{MAYBE}}. This extension allows
> {{EchoMessages}} and {{PingMessages}} to send {{MAYBE}} indicating that the
> DS should use those measurements if it only has one or fewer samples for a
> host. This fixes both problems because on process restart we send out
> {{PingMessages}} / {{EchoMessages}} as part of startup, and we would reset to
> effectively the RTT of the hosts (also at that point normal gossip
> {{EchoMessages}} have an opportunity to add an additional latency
> measurement).
> This strategy also nicely deals with the "a host got slow but now it's fine"
> problem that the DS resets were (afaik) designed to stop because the
> {{EchoMessage}} ping latency will count only after the reset for that host.
> Ping latency is a more reasonable lower bound on host latency (as opposed to
> status quo of zero).
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