T Jake Luciani commented on CASSANDRA-8449:

Patch is here https://github.com/tjake/cassandra/tree/mmap-uncompressed

> Allow zero-copy reads again
> ---------------------------
>                 Key: CASSANDRA-8449
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8449
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: T Jake Luciani
>            Priority: Minor
>              Labels: performance
>             Fix For: 3.0
> We disabled zero-copy reads in CASSANDRA-3179 due to in flight reads 
> accessing a ByteBuffer when the data was unmapped by compaction.  Currently 
> this code path is only used for uncompressed reads.
> The actual bytes are in fact copied to the client output buffers for both 
> netty and thrift before being sent over the wire, so the only issue really is 
> the time it takes to process the read internally.  
> This patch adds a slow network read test and changes the tidy() method to 
> actually delete a sstable once the readTimeout has elapsed giving plenty of 
> time to serialize the read.
> Removing this copy causes significantly less GC on the read path and improves 
> the tail latencies:
> http://cstar.datastax.com/graph?stats=c0c8ce16-7fea-11e4-959d-42010af0688f&metric=gc_count&operation=2_read&smoothing=1&show_aggregates=true&xmin=0&xmax=109.34&ymin=0&ymax=5.5

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