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https://issues.apache.org/jira/browse/CASSANDRA-8670?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14390598#comment-14390598
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Benedict commented on CASSANDRA-8670:
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bq. Also on trying to understand code cache pollution. How much do you really
know about how many instructions are emitted when the JVM can inline and
duplicate stuff out the yin yang?
Well, there are a lot of heuristics to apply, that are admittedly limited and
imperfect. But in general: hotspot won't inline megamorphic call sites, and
even bimorphic callsites are unlikely to be (i think probably never) inlined,
only given a static despatch fast path. These heuristics are enough to guide
decisions around this sufficiently well in my experience. If there are multiple
implementations viable at any moment, then the callsite will not be inlined, at
most its location will be, and even if it _is_ inlined, this doesn't
necessarily pollute the code cache, since inlined methods are small (as are
most methods) and adjacent occupancy of the cache is essentially free, unless
the overall method size exceeds the cache line boundary.
In general my view, the simplest heuristic is: if the benefit is small, and it
increases the number of active call sites, then let's not. This works from a
code management as well as a cache pollution perspective at once.
> Large columns + NIO memory pooling causes excessive direct memory usage
> -----------------------------------------------------------------------
>
> Key: CASSANDRA-8670
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8670
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Ariel Weisberg
> Assignee: Ariel Weisberg
> Fix For: 3.0
>
> Attachments: OutputStreamBench.java, largecolumn_test.py
>
>
> If you provide a large byte array to NIO and ask it to populate the byte
> array from a socket it will allocate a thread local byte buffer that is the
> size of the requested read no matter how large it is. Old IO wraps new IO for
> sockets (but not files) so old IO is effected as well.
> Even If you are using Buffered{Input | Output}Stream you can end up passing a
> large byte array to NIO. The byte array read method will pass the array to
> NIO directly if it is larger than the internal buffer.
> Passing large cells between nodes as part of intra-cluster messaging can
> cause the NIO pooled buffers to quickly reach a high watermark and stay
> there. This ends up costing 2x the largest cell size because there is a
> buffer for input and output since they are different threads. This is further
> multiplied by the number of nodes in the cluster - 1 since each has a
> dedicated thread pair with separate thread locals.
> Anecdotally it appears that the cost is doubled beyond that although it isn't
> clear why. Possibly the control connections or possibly there is some way in
> which multiple
> Need a workload in CI that tests the advertised limits of cells on a cluster.
> It would be reasonable to ratchet down the max direct memory for the test to
> trigger failures if a memory pooling issue is introduced. I don't think we
> need to test concurrently pulling in a lot of them, but it should at least
> work serially.
> The obvious fix to address this issue would be to read in smaller chunks when
> dealing with large values. I think small should still be relatively large (4
> megabytes) so that code that is reading from a disk can amortize the cost of
> a seek. It can be hard to tell what the underlying thing being read from is
> going to be in some of the contexts where we might choose to implement
> switching to reading chunks.
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