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https://issues.apache.org/jira/browse/HBASE-10191?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13917682#comment-13917682
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Matt Corgan commented on HBASE-10191:
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{quote}How then to have KeyValues/Cells w/o calling them out as individual
objects? .... For MemStore, once we hit some upper bound – say 64k, 1M? –
'flush' it to an inmemory, sorted, cellblock? Reading, we'd consult the (small)
CSLM memstore and some tiering of cellblocks?{quote}I think there's been talk
of this before, and it makes sense to me. It's basically creating small
in-memory HFiles that can be compacted several times in memory without going to
disk, and holding on to the WAL entries until they do go to disk. We'd get
huge space savings from reduction in objects, references, and repetition via
block encoding. The problem is that if you have hundreds of 1MB in-memory
HFiles, then it becomes too expensive to merge them all (via KVHeap) when
scanning. A possible solution is to subdivide the memstore into "stripes"
(probably smaller than the stripe compaction stripes) and periodically compact
the in-memory stripes. It sounds complicated compared to the current memstore,
but it's probably simpler than other parts of hbase because you don't have to
deal with IOExceptions, retries, etc.
> Move large arena storage off heap
> ---------------------------------
>
> Key: HBASE-10191
> URL: https://issues.apache.org/jira/browse/HBASE-10191
> Project: HBase
> Issue Type: Umbrella
> Reporter: Andrew Purtell
>
> 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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