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new c82f01c Post a tech blog about Cisco's practice of using Kylin
c82f01c is described below
commit c82f01cd06f97870658f40e591f0d1b5c775f2d4
Author: Rachelzour <rachelz...@github.com>
AuthorDate: Thu Jan 17 21:58:48 2019 +0800
Post a tech blog about Cisco's practice of using Kylin
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+---
+layout: post-blog
+title: How Cisco's Big Data Team Improved the High Concurrent Throughput of
Apache Kylin by 5x
+date: 2019-01-17 17:30:00
+author: Zongwei Li
+categories: blog
+---
+
+## Background
+
+As part of the development group of Cisco’s Big Data team, one of our
responsibilities is to provide BI reports to our stakeholders. Stakeholders
rely on the reporting system to check the usage of Cisco’s business offerings.
These reports are also used as a reference for billing, so they are critical to
our stakeholders and the business overall.
+
+The raw data for these reports is sourced across multiple tables in our Oracle
database. The monthly data volume for one table is in the billions, and if a
customer wants to run a report for one year, at least one billion to two
billion rows of data need to be aggregated or processed through other
operations. Additionally, all results need to be provided in a short amount of
time. In the course of our research, we discovered Apache Kylin, a distributed
preprocessing engine for massive da [...]
+
+With the simulation test using our production data, we found that Kylin was
ideal for our needs and was indeed capable of providing aggregated results on
one billion rows of data in one second. However, we still needed to undergo
additional tests for another use case. For one stakeholder, we provide 15
charts displayed on a single page. The BI system will send REST API requests to
Kylin to query the data for each chart asynchronously. Based on the production
data volume, if there are 20 [...]
+
+## The Testing Stage
+
+**Precondition**: To reduce the impact from network cost, we deployed the
testing tools in the same network environment with Kylin. Meanwhile, we turned
off the query cache for Kylin to make sure that each request was executed on
the bottom layer.
+
+
+
+**Testing Tools**: Aside from our traditional testing tool, Apache Jmeter, we
also used another open source tool: Gatlin (<https://gatling.io/>) to test the
same case. We excluded the impact from the tools.
+
+
+
+**Testing Strategy**: We simulated user requests of different sizes by
increasing the number of concurrent threads, tracking the average response in
60 seconds, finding the bottleneck for Kylin query responses, and observing the
maximum response time and success rate.
+
+
+
+**Testing Results**:
+
+| Thread | Handled Queries (in 60 seconds) | Handled Queries (per second) |
Mean Response Time (ms) |
+| :----: | :-----------------------------: | :--------------------------: |
:---------------------: |
+| 1 | 773 | 13 |
77 |
+| 15 | 3245 | 54 |
279 |
+| 25 | 3844 | 64 |
390 |
+| 50 | 4912 | 82 |
612 |
+| 75 | 5405 | 90 |
841 |
+| 100 | 5436 | 91 |
1108 |
+| 150 | 5434 | 91 |
1688 |
+
+
+
+Resulting in the line chart as follows:
+
+{:width="500px"
height="300px"}
+
+
+
+**Finding**: When the number of concurrent threads reach 75, executed queries
per second reach a peak of 90. The number does not become better even as we
continue to increase the threads. 90 concurrent query responses in one second
only allows 90/15 = 6 users to view a report at the same time. Even when we
extend the Kylin query nodes to 3, query capability with 18 users per second is
far behind our business demands.
+
+
+
+
+## Root Cause Analysis
+
+
+After reading and analyzing the query engine code of Kylin, we learned that
Kylin's query performs parallel filtering and calculation in HBase's region
server by launching HBase Coprocessor. Based on this information, we checked
the resource usage of the HBase cluster. The count of RPC Tasks processed on
the region server did not increase linearly with the number of Kylin query
requests when high concurrent queries occured. We concluded that there was a
thread block on the Kylin side.
+
+
+
+We used Flame graph and JProfile to collect and analyze data from the Kylin
query node and could not find the root cause. Then we tried to catch a thread
snapshot of Kylin with Jstack. Analyzing the Jstack log, we discovered the root
cause of the bottleneck causing this concurrent query issue. The example is as
follows (Kylin version 2.5.0):
+
+
+
+One thread is locked at sun.misc.URLClassPath.getNextLoader. TID is
0x000000048007a180：
+
+```
+"Query e9c44a2d-6226-ff3b-f984-ce8489107d79-3425" #3425 daemon prio=5
os_prio=0 tid=0x000000000472b000 nid=0x1433 waiting {{for monitor entry
[}}{{0x00007f272e40d000}}]
+ java.lang.Thread.State: BLOCKED (on object monitor)
+ at sun.misc.URLClassPath.getNextLoader(URLClassPath.java:469)
+ - locked <0x000000048007a180> (a sun.misc.URLClassPath)
+ at sun.misc.URLClassPath.findResource(URLClassPath.java:214)
+ at java.net.URLClassLoader$2.run(URLClassLoader.java:569)
+ at java.net.URLClassLoader$2.run(URLClassLoader.java:567)
+ at java.security.AccessController.doPrivileged(Native Method)
+ at java.net.URLClassLoader.findResource(URLClassLoader.java:566)
+ at java.lang.ClassLoader.getResource(ClassLoader.java:1096)
+ at java.lang.ClassLoader.getResource(ClassLoader.java:1091)
+ at
org.apache.catalina.loader.WebappClassLoaderBase.getResource(WebappClassLoaderBase.java:1666)
+ at
org.apache.kylin.common.KylinConfig.buildSiteOrderedProps(KylinConfig.java:338)
+```
+
+
+
+43 threads were waiting to lock <0x000000048007a180> at the same time:
+
+```
+"Query f1f0bbec-a3f7-04b2-1ac6-fd3e03a0232d-4002" #4002 daemon prio=5
os_prio=0 tid=0x00007f27e71e7800 nid=0x1676 waiting {{for monitor entry
[}}{{0x00007f279f503000}}]
+ java.lang.Thread.State: BLOCKED (on object monitor)
+ at sun.misc.URLClassPath.getNextLoader(URLClassPath.java:469)
+ - waiting to lock <0x000000048007a180> (a sun.misc.URLClassPath)
+ at sun.misc.URLClassPath.findResource(URLClassPath.java:214)
+ at java.net.URLClassLoader$2.run(URLClassLoader.java:569)
+ at java.net.URLClassLoader$2.run(URLClassLoader.java:567)
+ at java.security.AccessController.doPrivileged(Native Method)
+ at java.net.URLClassLoader.findResource(URLClassLoader.java:566)
+ at java.lang.ClassLoader.getResource(ClassLoader.java:1096)
+ at java.lang.ClassLoader.getResource(ClassLoader.java:1091)
+ at
org.apache.catalina.loader.WebappClassLoaderBase.getResource(WebappClassLoaderBase.java:1666)
+ at
org.apache.kylin.common.KylinConfig.buildSiteOrderedProps(KylinConfig.java:338)
+```
+
+
+
+We found that the closest code logic to Kylin was:
+
+```
+org.apache.kylin.common.KylinConfig.buildSiteOrderedProps(KylinConfig.java:338)
+```
+
+Further analyzing the Kylin source code showed we were getting close to the
resolution.
+
+
+
+
+
+## Code Analysis
+
+
+
+When Kylin query engine builds a request to HBase Coprocessor, it will export
Kylin properties (various properties used in Kylin) as Strings. This issue is
caused by the relative code logic.
+
+```
+function private static OrderedProperties buildSiteOrderedProps()
+```
+
+
+
+- Each thread will getResouce to load "kylin-defaults.properties" (the default
properties file that users cannot modify).
+
+```
+// 1. load default configurations from classpath.
+// we have a kylin-defaults.properties in kylin/core-common/src/main/resources
+URL resource =
Thread.currentThread().getContextClassLoader().getResource("kylin-defaults.properties");
+Preconditions.checkNotNull(resource);
+logger.info("Loading kylin-defaults.properties from {}", resource.getPath());
+OrderedProperties orderedProperties = new OrderedProperties();
+loadPropertiesFromInputStream(resource.openStream(), orderedProperties);
+```
+
+
+
+- Loop 10 times to getResouce for "kylin-defaults" + (i) + ".properties".
Thread LOCKED occurs here.
+
+```
+for (int i = 0; i < 10; i++) {
+String fileName = "kylin-defaults" + + ".properties";
+ URL additionalResource =
Thread.currentThread().getContextClassLoader().getResource(fileName);
+ if (additionalResource != null) {
+ logger.info("Loading {} from {} ", fileName,
additionalResource.getPath());
+ loadPropertiesFromInputStream(additionalResource.openStream(),
orderedProperties);
+ }
+```
+
+
+
+Those logics were introduced in 2017/6/7, with JIRA ID KYLIN-2659 *Refactor
KylinConfig so that all the default configurations are hidden in
kylin-defaults.properties* reported by Hongbin Ma.
+
+
+
+## Issue Fixing
+
+For the first part of the logic, because kylin-defaults.properties is built in
kylin-core-common-xxxx.jar, there’s no need to getResource for it every time.
We moved this logic to getInstanceFromEnv(). This logic gets called only once
when service starts.
+
+
+
+We found one regression issue when fixing this bug. One class,
CubeVisitService, is a Coprocessor. It will use KylinConfig as util class to
generate KylinConfig object. It’s dangerous to induce any logic to load
properties. Due to this, there is no Kylin.properties file in Coprocessor.
+
+```
+buildDefaultOrderedProperties();
+```
+
+
+
+For the second part, this design should be future-proof and allow users to
define 10 default properties (and override with each other), but after a year
and a half, this logic seemed to never be used. However, to reduce risk, we
kept this logic because it only gets called once during service start up which
resulted in an insignificant waste of additional time.
+
+
+
+
+
+## Performance Testing After Bug Fixes
+
+
+
+Based on the same data volume and testing environment, results were as follows:
+
+| Thread | Handled Queries (in 60 seconds) | Handled Queries (per second) |
Mean Response Time (ms) |
+| :----: | :-----------------------------: | :--------------------------: |
:---------------------: |
+| 1 | 2451 | 41 |
12 |
+| 15 | 12422 | 207 |
37 |
+| 25 | 15600 | 260 |
56 |
+| 50 | 18481 | 308 |
129 |
+| 75 | 21055 | 351 |
136 |
+| 100 | 24036 | 400 |
251 |
+| 150 | 28014 | 467 |
277 |
+
+
+
+And the resulting line chart:
+
+{:width="500px"
height="300px"}
+
+
+
+When the concurrent threads reached 150, Kylin processed 467 requests per
second. The concurrent query capability increased by five times with linear
growth . It could be concluded then that the bottleneck was eliminated . We
didn't increase the concurrent threads due to the Kylin query engine’s settings
for cluster load balancing which meant that increasing the concurrent
connections on a single node increased the workload on the Tomcat server (Max
connection is 150 in Kylin by default) [...]
+
+
+
+After the fix, each Kylin node could now handle requests for 467/15 = 31
users, which meets our business requirement. Additionally, Kylin’s concurrent
query capability can be further improved by several times once we enable query
cache, so it is more than sufficient to fulfill our needs.
+
+
+
+## Summary
+
+Apache Kylin lets you query massive datasets at sub-second latency, thanks to
the pre-calculation design of cubes, the optimization of Apache Calcite
operator in queries, and also the introduction of "Prepared Statement Cache" to
reduce the cost of Calcite SQL parses. Query performance optimization is not
easy. We need to pay more attention to impacts on the Kylin query engine when
new features are introduced or bugs are fixed, since even a minor code change
could spell disaster. Issues [...]
+
+
+
+Lastly, query performance testing should not be limited to a single or small
set of queries. High concurrecy performance testing should take place
considering actual business requirements. For enterprise reporting systems, 3
seconds is the user tolerance limit for new page loading, which includes page
rendering and network consumption. Ultimately, the backend data service should
provide a response within 1 second. This is indeed a big challenge in a
business scenario with big data sets. [...]
+
+This issue has already been submitted on JIRA as
[KYLIN-3672](https://issues.apache.org/jira/browse/KYLIN-3672), and released in
Kylin v2.5.2. Thanks to Shaofeng Shi of Kyligence Inc. for help.
+
+
+
+【1】<https://issues.apache.org/jira/browse/KYLIN-3672>
+
+
+
+
+
+*Author Zongwie Li as a Cisco engineer and a team member in the company’s Big
Data architecture team, currently responsible for OLAP platform construction
and customer business reporting systems.*
+
+
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