Peter Bailis created CASSANDRA-4261:
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Summary: [Patch] Support consistency-latency prediction in nodetool
Key: CASSANDRA-4261
URL: https://issues.apache.org/jira/browse/CASSANDRA-4261
Project: Cassandra
Issue Type: New Feature
Components: Tools
Affects Versions: 1.2
Reporter: Peter Bailis
.h1 Introduction
Cassandra supports a variety of replication configurations: ReplicationFactor
is set per-ColumnFamily and ConsistencyLevel is set per-request. Setting
ConsistencyLevel to QUORUM for reads and writes ensures strong consistency, but
QUORUM is often slower than ONE, TWO, or THREE. What should users choose?
This patch provides a latency-consistency analysis within nodetool. Users can
accurately predict Cassandra behavior in their production environments without
interfering with performance.
What's the probability that we'll read a write t seconds after it completes?
What about reading one of the last k writes? nodetool predictconsistency
provides this:
{{nodetool predictconsistency ReplicationFactor TimeAfterWrite Versions}}
{code:title=Example output|borderStyle=solid}
//N == ReplicationFactor
//R == read ConsistencyLevel
//W == write ConsistencyLevel
user@test:$ nodetool predictconsistency 3 100 1
100ms after a given write, with maximum version staleness of k=1
N=3, R=1, W=1
Probability of consistent reads: 0.811700
Average read latency: 6.896300ms (99.900th %ile 174ms)
Average write latency: 8.788000ms (99.900th %ile 252ms)
N=3, R=1, W=2
Probability of consistent reads: 0.867200
Average read latency: 6.818200ms (99.900th %ile 152ms)
Average write latency: 33.226101ms (99.900th %ile 420ms)
N=3, R=1, W=3
Probability of consistent reads: 1.000000
Average read latency: 6.766800ms (99.900th %ile 111ms)
Average write latency: 153.764999ms (99.900th %ile 969ms)
N=3, R=2, W=1
Probability of consistent reads: 0.951500
Average read latency: 18.065800ms (99.900th %ile 414ms)
Average write latency: 8.322600ms (99.900th %ile 232ms)
N=3, R=2, W=2
Probability of consistent reads: 0.983000
Average read latency: 18.009001ms (99.900th %ile 387ms)
Average write latency: 35.797100ms (99.900th %ile 478ms)
N=3, R=3, W=1
Probability of consistent reads: 0.993900
Average read latency: 101.959702ms (99.900th %ile 1094ms)
Average write latency: 8.518600ms (99.900th %ile 236ms)
{code}
.h1 Demo
Here's an example scenario you can run using
[ccm|https://github.com/pcmanus/ccm]. The prediction is fast:
{code:borderStyle=solid}
cd <cassandra-source-dir with patch applied>
ant
# turn on consistency logging
sed -i .bak 's/log_latencies_for_consistency_prediction:
false/log_latencies_for_consistency_prediction: true/' conf/cassandra.yaml
ccm create consistencytest --cassandra-dir=.
ccm populate -n 5
ccm start
# if start fails, you might need to initialize more loopback interfaces
# e.g., sudo ifconfig lo0 alias 127.0.0.2
# use stress to get some sample latency data
tools/bin/stress -d 127.0.0.1 -l 3 -n 10000 -o insert
tools/bin/stress -d 127.0.0.1 -l 3 -n 10000 -o read
bin/nodetool -h 127.0.0.1 -p 7100 predictconsistency 3 100 1
{code}
.h1 What and Why
We've implemented [Probabilistically Bounded
Staleness|http://pbs.cs.berkeley.edu/#demo], a new technique for predicting
consistency-latency trade-offs within Cassandra. Our
[paper||http://arxiv.org/pdf/1204.6082.pdf] will appear in [VLDB
2012|http://www.vldb2012.org/], and, in it, we've used PBS to profile a range
of Dynamo-style data store deployments at places like LinkedIn and Yammer in
addition to profiling our own Cassandra deployments. In our experience,
prediction is both accurate and much more lightweight than trying out different
configurations (especially in production!).
This analysis is important for the many users we've talked to and heard about
who use "partial quorum" operation (e.g., non-QUORUM ConsistencyLevels). Should
they use CL=ONE? CL=TWO? It likely depends on their runtime environment and,
short of profiling in production, there's no existing way to answer these
questions. (Keep in mind, Cassandra defaults to CL=ONE, meaning users don't
know how stale their data will be.)
This patch allows users to perform this prediction in production using
{{nodetool}}. Users enable tracing of latency data by setting
{{log_latencies_for_consistency_prediction: true}} in {{cassandra.yaml}}.
Cassandra logs {{max_logged_latencies_for_consistency_prediction}} latencies
(each latency is 8 bytes, and there are 4 distributions we require, so the
space overhead is {{32*logged_latencies}} bytes of memory for the predicting
node) and then predicts the latency and consistency for each possible
ConsistencyLevel setting (reads and writes) by running
{{number_trials_for_consistency_prediction}} Monte Carlo trials per
configuration. Users shouldn't have to touch these parameters, and the defaults
work well. The more latencies they log, the better the predictions will be.
We outline limitations of the current approach after describing how it's done.
We believe that this is a useful feature that can provide guidance and fairly
accurate estimation for most users.
.h1 Implementation
This patch is fairly lightweight, requiring minimal changes to existing code.
The high-level overview is that we gather empirical latency distributions then
perform Monte Carlo analysis using the gathered data.
.h2 Latency Data
We log latency data in {{service.PBSPredictor}}, recording four relevant
distributions:
* *W*: time from when the coordinator sends a mutation to the time that a
replica begins to serve the new value(s)
* *A*: time from when a replica accepting a mutation sends an
* *R*: time from when the coordinator sends a read request to the time that
the replica performs the read
* *S*: time from when the replica sends a read response to the time when the
coordinator receives it
We augment {{net.MessageIn}} and {{net.MessageOut}} to store timestamps along
with every message (8 bytes overhead required for millisecond {{long}}). In
{{net.MessagingService}}, we log the start of every mutation and read, and, in
{{net.ResponseVerbHandler}}, we log the end of every mutation and read.
Jonathan Ellis mentioned that
[1123|https://issues.apache.org/jira/browse/CASSANDRA-1123] had similar latency
tracing, but, as far as we can tell, these latencies aren't in that patch.
.h2 Prediction
When prompted by nodetool, we call {{service.PBSPredictor.doPrediction}}, which
performs the actual Monte Carlo analysis based on the provided data. It's
straightforward, and we've commented this analysis pretty well but can
elaborate more here if required.
.h2 Testing
We've modified the unit test for {{SerializationsTest}} and provided a new unit
test for {{PBSPredictor}} ({{PBSPredictorTest}}). You can run the
{{PBSPredictor}} test with {{ant pbs-test}}.
.h2 Overhead
This patch introduces 8 bytes of overhead per message. We could reduce this
overhead and add timestamps on-demand, but this would require changing
{{net.MessageIn}} and {{net.MessageOut}} serialization at runtime, which is
messy.
If enabled, consistency tracing requires {{32*logged_latencies}} bytes of
memory on the node on which tracing is enabled.
.h1 Caveats
The predictions are conservative, or worst-case, meaning we may predict more
staleness than in practice in the following ways:
* We do not account for read repair.
* We do not account for Merkle tree exchange.
* Multi-version staleness is particularly conservative.
* We simulate non-local reads and writes. We assume that the coordinating
Cassandra node is not itself a replica for a given key.
The predictions are optimistic in the following ways:
* We do not predict the impact of node failure.
* We do not model hinted handoff.
We can talk about how to improve these if you're interested. This is an area of
active research.
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