Using G1 for Big Data Streams to Cassandra

Background; Adello does mobile ads using a medium size Hadoop cluster (http://www.technavio.com/blog/top-16-hadoop-technology-companies), taking in, among others, the Smaato, Flurry, OpenX, and OMAX feeds. Combined, this is a fair amount of data to handle, about a terabyte per day. Cassandra's turned out to be great for this task, even if we had to fix a race condition and develop new read/write paths from Pig as the existing ones were not able to handle the analysis load (see https://issues.apache.org/jira/browse/CASSANDRA-6788 and https://issues.apache.org/jira/browse/CASSANDRA-6665).

The standard garbage collection config for Cassandra is not bad, but it couldn't quite cope with the 20GB+ heaps we had before we really optimized our Cassandra config. In particular, too much data was pushed into the old space. This creates 30-50 second long stop-the-world GC cycles every 2-4 hours. This was a serious issue when using the token aware writing of Astyanax (https://github.com/Netflix/astyanax), a problem partly solved by optimizing fail-overs, retries, and using Hystrix (https://github.com/Netflix/hystrix) to drop the outliers. Overall, I found Netflix' Github account to be a open-pit gold mine :)


Even though we're no longer using G1, I found the experiences worth sharing. The G1 collector was running in production for a few weeks, and here are our observations:

Initially, it seemed to be a fire-and-forget change; once we switched from CMS, we've immediately saw a major reduction in the time and frequency of long GC cycles.

However, too much data was still pushed into old space. This is caused to the memory-wasteful read/write paths of Cassandra over Thrift.

We soon realized that the time spent on the full cycles increased linearly with the Cassandra's uptime. Over the course of a week, the full GC cycle went from taking 10 seconds to 60 seconds, reclaiming the same amount of memory. This is mostly caused by fragmentation, which G1 was supposed handle better than CMS. Internally, G1 arranges the space into blocks that can be reclaimed/defragmented separately. Clearly, the simplest solution was to restart Cassandra nodes on a regular basis, like Spotify does (http://www.slideshare.net/planetcassandra/8-axel-liljencrantz-23204252). However, with over 4TB per node, Cassandra takes roughly 5 minutes to start, making a serial restart a lengthy process.

To combat the fragmentation issue, we turned to the experimental options for G1. The "-XX:G1NewSizePercent" defaults to 5 and "-XX:G1MaxNewSizePercent" defaults to 60 (they're described here: http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html). Given that Cassandra is wasteful with memory, most of the connection-related data is ready to be reclaimed almost instantly. By using "-XX:G1NewSizePercent=60" and "-XX:G1MaxNewSizePercent=95", we keep almost all the data off old space. When using the defaults, the old space would slowly grow to about 12GB, with the new settings, it stayed at about 1GB. Given that the young collection is more parallel, this was huge benefit both for responsiveness and max heap use.

Some notes: setting the "-XX:InitiatingHeapOccupancyPercent" doesn't seem to have much impact. It defaults to 45, but even setting it to 5 or 0 doesn't mean the GC will step in earlier. Reducing the max heap size, however, makes the GC kick in. This even made it possible to run Cassandra with less than 8GB of RAM, but the risk/benefit wasn't there for us. Also, we used "-XX:MaxGCPauseMillis=100", compared to the default of 200. However, this has little impact as G1 typically collected 7GB of new space in 50ms, which is pretty fantastic performance.

Cassandra's been great for the most part. The main problem is that it has an extremely ungraceful degradation. If you leave Cassandra's intended design space, or run with a non-optimized config, it will quickly turn into a constant source of pain. However, once you've figured out the best config for your application, it just keeps running, with minimal management and almost zero stability issues.


Cheers,
Christian

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    1. In the context of integrating Apache Cassandra with Big Data streams, "G1" could refer to various concepts or components. Here's a plausible interpretation and approach:

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      Apache Cassandra is a distributed NoSQL database designed for handling large amounts of data across multiple nodes in a scalable and fault-tolerant manner. When dealing with Big Data streams and Cassandra, optimizing performance and resource management is crucial. The "G1" aspect likely refers to the Garbage First (G1) garbage collector used in Java Virtual Machines (JVMs). Here’s how it could be relevant:

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  18. The author found that the G1 collector was able to significantly reduce the time and frequency of long garbage collection cycles. However, the author also found that the G1 collector was not able to handle fragmentation as well as the standard garbage collector. Ultimately, the author recommends using a combination of the G1 collector and restarting Cassandra nodes regularly to optimize performance. Thank you for the insights always helpful to you
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  22. Using G1 (Garbage-First) garbage collector for managing big data streams in a Cassandra environment offers improved efficiency and performance, particularly with large heap sizes. G1 is designed to prioritize low-pause garbage collection, making it ideal for Cassandra, which often demands high throughput and low-latency responses. G1's ability to handle concurrent tasks like compaction and writes reduces latency spikes, enhancing real-time data processing in Cassandra nodes. Moreover, its region-based memory management enables more predictable garbage collection pauses, minimizing disruptions in streaming data workloads. This suitability for high-throughput, real-time applications makes G1 a strategic choice for optimizing Cassandra’s performance with big data.
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