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https://issues.apache.org/jira/browse/CASSANDRA-11870?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15338935#comment-15338935
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Mahdi Mohammadi commented on CASSANDRA-11870:
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Shouldn't we set a default value for `max_direct_memory_in_mb` in Config.java?
> Consider allocating direct buffers bypassing ByteBuffer.allocateDirect
> ----------------------------------------------------------------------
>
> Key: CASSANDRA-11870
> URL: https://issues.apache.org/jira/browse/CASSANDRA-11870
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Robert Stupp
> Assignee: Robert Stupp
> Priority: Minor
> Fix For: 3.x
>
>
> As outlined in CASSANDRA-11818, {{ByteBuffer.allocateDirect}} uses
> {{Bits.reserveMemory}}, which is there to respect the JVM setting
> {{-XX:MaxDirectMemorySize=...}}.
> {{Bits.reserveMemory}} first tries an "optimistic" {{tryReserveMemory}} and
> exits immediately on success. However, if that somehow doesn't succeed, it
> triggers a {{System.gc()}}, which is bad IMO (however, kind of how direct
> buffers work in Java). After that GC it sleeps and tries to reserve the
> memory up to 9 times - up to 511 ms - and then throws
> {{OutOfMemoryError("Direct buffer memory")}}.
> This is unnecessary for us since we always immediately "free" direct buffers
> as soon as we no longer need them.
> Proposal: Manage direct-memory reservations in our own code and skip
> {{Bits.reserveMemory}} that way.
> (However, Netty direct buffers are not under our control.)
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