Hi, I am trying to compare performance between using parallelism by using more mappers (the way you suggested with reducing the max input split size) and using possible parallelism within the Mapper, there can be advantage to using fewer number of mappers,
Does anyone have any idea on where to start to make the CIMapper thread safe ? Do I have to make changes to every application or I could just change some implementation in the general classes used by all applications? It would be really helpful if someone could point me to the right direction, Thanks Yunming On Dec 20, 2012, at 10:54 PM, Marty Kube <[email protected]> wrote: > Writing thread safe code is hard. Don't do it unless you have too. > > On Dec 20, 2012, at 4:28 AM, Sean Owen <[email protected]> wrote: > >> ... but making the implementation thread-safe won't make it be used by >> multiple threads. If you want more parallelism, suggest to Hadoop to >> use more mappers by reducing the max input split size. But this is >> still not going to require your mappers to be thread-safe. >> >> if you mean you are making your own parallelism in miniature by >> writing a multi-threaded mapper, I wouldn't bother. Just use more >> parallelism via Hadoop. >> >> On Thu, Dec 20, 2012 at 3:31 AM, Yunming Zhang >> <[email protected]> wrote: >>> Thanks Marty, Sean, >>> >>> yeah, I took a look at the source code yesterday and realized that it is >>> not thread safe as well, >>> >>> I am working on a high performance mapper that require making the mapper >>> thread safe so I could exploit the data parallelism that comes with >>> processing multiple input <key, val> pairs to a single mapper, >>> >>> I am currently researching into if there is any easy way that I could make >>> the CIMapper implementation thread safe by may be making a few key data >>> structures that are thread safe, like the OpenIntDoubleHashMap, and >>> hopefully this won't screw up the correctness of the algorithm itself, >>> >>> Yunming >>> >>> On Dec 20, 2012, at 9:07 AM, Marty Kube >>> <[email protected]> wrote: >>> >>>> Sean is right, most MR code is not and does not need to be thread safe. >>>> >>>> Why are you writing a multi-threaded mapper? >>>> >>>> On 12/19/2012 07:50 PM, Sean Owen wrote: >>>>> Hadoop will only use one thread with one Mapper or Reducer instance. >>>>> Unless >>>>> you are somehow spawning threads on your own concurrency should not be an >>>>> issue. I don't known if this behavior is guaranteed but seems to be how it >>>>> always works. >>>>> On Dec 19, 2012 4:03 PM, "Yunming Zhang" <[email protected]> >>>>> wrote: >>>>> >>>>>> Hi , >>>>>> >>>>>> I am developing a custom mapper that is somewhat similar to the >>>>>> multithreaded mapper that came with Hadoop, and I am getting weird errors >>>>>> when running using multiple threads processing multiple input key, value >>>>>> pairs simultaneously, here is the stack trace, I looked into >>>>>> OpenIntDoubleHashMap, and it seems to be stemmed from null values stored >>>>>> in >>>>>> the tables, >>>>>> >>>>>> attempt_201212190955_0004_m_000000_0: >>>>>> java.lang.ArrayIndexOutOfBoundsException: 24 >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.math.map.OpenIntDoubleHashMap.indexOfKey(OpenIntDoubleHashMap.java:278) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.math.map.OpenIntDoubleHashMap.get(OpenIntDoubleHashMap.java:198) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.math.RandomAccessSparseVector.getQuick(RandomAccessSparseVector.java:130) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.math.AbstractVector.assign(AbstractVector.java:738) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.clustering.AbstractCluster.observe(AbstractCluster.java:263) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.clustering.AbstractCluster.observe(AbstractCluster.java:234) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.clustering.AbstractCluster.observe(AbstractCluster.java:229) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.clustering.AbstractCluster.observe(AbstractCluster.java:37) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.clustering.classify.ClusterClassifier.train(ClusterClassifier.java:158) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.clustering.iterator.CIMapper.map(CIMapper.java:46) >>>>>> attempt_201212190955_0004_m_000000_0: at >>>>>> org.apache.mahout.clustering.iterator.CIMapper.map(CIMapper.java:18) >>>>>> >>>>>> Not sure if anyone knows if it is inherently thread safe to process >>>>>> multiple input key, val pair to the mapper simultaneously ? >>>>>> >>>>>> Thanks >>>>>> >>>>>> Yunming >>>
