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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] Updated: (CASSANDRA-1214) Force linux to not swap the JVM
Date Wed, 18 Aug 2010 19:09:23 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-1214?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jonathan Ellis updated CASSANDRA-1214:
--------------------------------------

    Attachment: 1214-v4.txt

v4 includes the ivy changes to download jna at build time.

Again, the relevant text from http://www.apache.org/legal/3party.html is, "LGPL v2.1-licensed
works must not be included in Apache products, although they may be listed as system requirements
or distributed elsewhere as optional works."  We are not including jna, nor are we even requiring
it [although it explicitly states it would be fine to do so].  The only restriction is on
distributing the lgpl work itself, so while Hadoop is welcome to pile additional restrictions
on themselves this is fine for us, since (and perhaps this wasn't clear) dependencies we pull
in with ivy are build-time only, and are not distributed with our source or binary artifacts.

(FWIW it is also fine for an apache-licensed debian package, to declare a dependency on an
lgpl one.)

> Force linux to not swap the JVM
> -------------------------------
>
>                 Key: CASSANDRA-1214
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1214
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: James Golick
>             Fix For: 0.6.5
>
>         Attachments: 1214-v3.txt, 1214-v4.txt, mlockall-jna.patch.txt, Read Throughput
with mmap.jpg, trunk-1214.txt
>
>
> The way mmap()'d IO is handled in cassandra is dangerous. It allocates potentially massive
buffers without any care for bounding the total size of the program's buffers. As the node's
dataset grows, this *will* lead to swapping and instability.
> This is a dangerous and wrong default for a couple of reasons.
> 1) People are likely to test cassandra with the default settings. This issue is insidious
because it only appears when you have sufficient data in a certain node, there is absolutely
no way to control it, and it doesn't at all respect the memory limits that you give to the
JVM.
> That can all be ascertained by reading the code, and people should certainly do their
homework, but nevertheless, cassandra should ship with sane defaults that don't break down
when you cross some magic unknown threshold.
> 2) It's deceptive. Unless you are extremely careful with capacity planning, you will
get bit by this. Most people won't really be able to use this in production, so why get them
excited about performance that they can't actually have?

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