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Alluxio can cache objects in memory that are used downstream in other steps or even other jobs. In Spark, the biggest performance gains are realized when all data stays in memory. But, the Spark memory architecture is kind of opaque. It's pretty well documented, and there are some good blogs out there, but there are a lot of factors in play. For example, let's say we start with 1 TB of compressed data and take a guess that it will uncompress to 10 TB. So, we do a naive calculation and guess that we'll need 20 nodes, each with 500 GB of memory. For our example, Spark will be managed by Yarn. Yarn is going to take a portion of each node's memory. I forget how much, but let's say 10%. Then, depending on how many executors we have per node, each will also have memory overhead. Again, I don't know exactly, but let's just say another 10%. Then, some memory will be dedicated to system processes, etc. Then there's the Spark driver. So, after all of that, let's say 25-30% of memory is used outside of the executor. So, that leaves us with 10 executors per node, each with ~35 GB of memory. For maximum performance, we have to ensure that all 10 TB fits across all executors. So, we may actually need 29 nodes, or a lot of data is going to spill to disk. Then, let's say that our processing job doesn't evenly distribute data across partitions. Some partitions get 5x as much data as other partitions. Again, data spills to disk - or the VM runs out of heap. Or, maybe our 10% compression estimate is off by 5% - more spills to disk. |