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https://issues.apache.org/jira/browse/DRILL-5275?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15873878#comment-15873878
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Paul Rogers commented on DRILL-5275:
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During the merge phase, CPU becomes the bottleneck. Performance monitoring, on
a sort of 18 GB of data with 3GB of memory available, shows that the disk reads
bounce around from around 60 MB/s to 10 MB/s, but CPU stays at 100%. Since most
time in execution is in the sort operator, this suggests that the merge
operation (using a priority queue) may be the bottleneck.
That said, during the sort/spill phase, the above change does improve write
performance of spill batches.
> Sort spill serialization is slow due to repeated buffer allocations
> -------------------------------------------------------------------
>
> Key: DRILL-5275
> URL: https://issues.apache.org/jira/browse/DRILL-5275
> Project: Apache Drill
> Issue Type: Bug
> Affects Versions: 1.10.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Fix For: 1.10.0
>
>
> Drill provides a sort operator that spills to disk. The spill and read
> operations use the serialization code in the
> {{VectorAccessibleSerializable}}. This code, in turn, uses the
> {{DrillBuf.getBytes()}} method to write to an output stream. (Yes, the "get"
> method writes, and the "write" method reads...)
> The DrillBuf method turns around and calls the UDLE method that does:
> {code}
> byte[] tmp = new byte[length];
> PlatformDependent.copyMemory(addr(index), tmp, 0, length);
> out.write(tmp);
> {code}
> That is, for each write the code allocates a heap buffer. Since Drill buffers
> can be quite large (4, 8, 16 MB or larger), the above rapidly fills the heap
> and causes GC.
> The result is slow performance. On a Mac, with an SSD that can do 700 MB/s of
> I/O, we get only about 40 MB/s. Very likely because of excessive CPU cost and
> GC.
> The solution is to allocate a single read or write buffer, then use that same
> buffer over and over when reading or writing. This must be done in
> {{VectorAccessibleSerializable}} as it is a per-thread class that has
> visibility to all the buffers to be written.
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