The task code was optimized from 32-bit jvm (yes, people use 64-bit jvm for servers and 32-bit jvm for tasks in production), because it's more memory efficient with the same Xmx, which by default is smaller than 2GiB. Feel free to open a jira for improvement. I'd preserve the optimization for 32-bit jvm by checking the os.arch property.
On Sat, Oct 27, 2012 at 7:35 AM, Lijie Xu <[email protected]> wrote: > Hi, all. > I'm debugging Hadoop's source code and find an incomprehensible setting. > In hadoop-0.20.2 and hadoop-1.0.3, reducer's shuffle buffer size cannot > exceed 2048MB (i.e., Integer.MAX_VALUE). In this way, although*//* > reducer's JVM size can be set more than 2048MB (e.g., > mapred.child.java.opts=-Xmx4000m), the heap size used for shuffle buffer is > at most "2048MB * maxInMemCopyUse (default 0.7)" not "4000MB * > maxInMemCopyUse". > I think it's not a reasonable setting for large memory machines. > > The following code taken from "org.apache.hadoop.mapred.ReduceTask" shows > the concrete algorithm. > I'm wondering why maxSize is declared as "long" but set as "(int)". I point > out the corresponding code with "-->" > > Thanks for any suggestion. > --------------------------------------------------------------------------------------------------------------------------- > private final long maxSize; > private final long maxSingleShuffleLimit; > > private long size = 0; > > private Object dataAvailable = new Object(); > private long fullSize = 0; > private int numPendingRequests = 0; > private int numRequiredMapOutputs = 0; > private int numClosed = 0; > private boolean closed = false; > > public ShuffleRamManager(Configuration conf) throws IOException { > final float maxInMemCopyUse = > conf.getFloat("mapred.job.shuffle.input.buffer.percent", 0.70f); > if (maxInMemCopyUse > 1.0 || maxInMemCopyUse < 0.0) { > throw new IOException("mapred.job.shuffle.input.buffer.percent" + > maxInMemCopyUse); > } > // Allow unit tests to fix Runtime memory > --> maxSize = (int)(conf.getInt("mapred.job.reduce.total.mem.bytes", > --> (int)Math.min(Runtime.getRuntime().maxMemory(), > Integer.MAX_VALUE)) > --> * maxInMemCopyUse); > maxSingleShuffleLimit = (long)(maxSize * > MAX_SINGLE_SHUFFLE_SEGMENT_FRACTION); > LOG.info("ShuffleRamManager: MemoryLimit=" + maxSize + > ", MaxSingleShuffleLimit=" + maxSingleShuffleLimit); > }
