[jira] [Commented] (HBASE-10191) Move large arena storage off heap

["Andrew Purtell (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-10191?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13906456#comment-13906456 ] 

Andrew Purtell commented on HBASE-10191:
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I'm looking at Netty 4's netty-buffer module (http://netty.io/4.0/api/io/netty/buffer/package-summary.html), which has some nice properties, including composite buffers, arena allocation, dynamic buffer resizing, and reference counting, never mind dev and testing by another community. I also like it because you can plug in your own allocators and specialize the abstract ByteBuf base type. More on this later.

When I get closer to seeing what exactly needs to be done I will post a design doc. Current thinking follows. Below the term 'buffer' currently means Netty ByteBufs or derived classes backed by off-heap allocated direct buffers.

*Write*

When coming in from RPC, cells are laid out by codecs into cellbocks in buffers and the cellblocks/buffers are handed to the memstore. Netty's allocation arenas replace the MemstoreLAB. The memstore data structure evolves into an index over cellblocks.

Per [~mcorgan]'s comment above, we should think about how the memstore index can be built with fewer object allocations than the number of cells in the memstore, yet be in the ballpark with efficiency of concurrent access. A tall order. CSLM wouldn't be the right choice as it allocates at least one list entry per key, but we could punt and use it initially and make a replacement datastructure as a follow on task.

*Read*

We feed down buffers to HDFS to fill with file block data. We pick which pool to get a buffer from for a read depending on family caching strategy. Pools could be backed by arenas that match up with LRU policy strata, with a common pool/arena for noncaching reads. (Or for noncaching reads, can we optionally use a new API for getting buffers up from HDFS, perhaps backed by the pinned shared RAM cache, since we know we will be referring to the contents only briefly?) It will be important to get reference counting right as we will be servicing scans while attempting to evict. Related, eviction of a block may not immediately return a buffer to a pool, if there is more than one block in a buffer.

We maintain new metrics on numbers of buffers allocated, stats on arenas, stats on wastage and internal fragmentation of the buffers, etc, and use these to guide optimizations and refinements.

> 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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