• Type: Umbrella Umbrella
    • Status: Open
    • Priority: Major Major
    • Resolution: Unresolved
    • Affects Version/s: None
    • Fix Version/s: None
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      Even with the improved G1 GC in Java 7, Java processes that want to address large regions of memory while also providing low high-percentile latencies continue to be challenged. Fundamentally, a Java server process that has high data throughput and also tight latency SLAs will be stymied by the fact that the JVM does not provide a fully concurrent collector. There is simply not enough throughput to copy data during GC under safepoint (all application threads suspended) within available time bounds. This is increasingly an issue for HBase users operating under dual pressures: 1. tight response SLAs, 2. the increasing amount of RAM available in "commodity" server configurations, because GC load is roughly proportional to heap size.

      We can address this using parallel strategies. We should talk with the Java platform developer community about the possibility of a fully concurrent collector appearing in OpenJDK somehow. Set aside the question of if this is too little too late, if one becomes available the benefit will be immediate though subject to qualification for production, and transparent in terms of code changes. However in the meantime we need an answer for Java versions already in production. This requires we move the large arena allocations off heap, those being the blockcache and memstore. On other JIRAs recently there has been related discussion about combining the blockcache and memstore (HBASE-9399) and on flushing memstore into blockcache (HBASE-5311), which is related work. We should build off heap allocation for memstore and blockcache, perhaps a unified pool for both, and plumb through zero copy direct access to these allocations (via direct buffers) through the read and write I/O paths. This may require the construction of classes that provide object views over data contained within direct buffers. This is something else we could talk with the Java platform developer community about - it could be possible to provide language level object views over off heap memory, on heap objects could hold references to objects backed by off heap memory but not vice versa, maybe facilitated by new intrinsics in Unsafe. Again we need an answer for today also. We should investigate what existing libraries may be available in this regard. Key will be avoiding marshalling/unmarshalling costs. At most we should be copying primitives out of the direct buffers to register or stack locations until finally copying data to construct protobuf Messages. A related issue there is HBASE-9794, which proposes scatter-gather access to KeyValues when constructing RPC messages. We should see how far we can get with that and also zero copy construction of protobuf Messages backed by direct buffer allocations. Some amount of native code may be required.

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            • Assignee:
              Andrew Purtell
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