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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Forcing Cassandra to free up some space
Date Fri, 27 May 2011 02:16:48 GMT
In summary, system.gc works fine unless you've deliberately done
something like setting the -XX:-DisableExplicitGC flag.

On Thu, May 26, 2011 at 5:58 PM, Konstantin  Naryshkin
<konstantinn@a-bb.net> wrote:
> So, in summary, there is no way to predictably and efficiently tell Cassandra to get
rid of all of the extra space it is using on disk?
>
> ----- Original Message -----
> From: "Jeffrey Kesselman" <jeffpk@gmail.com>
> To: user@cassandra.apache.org
> Sent: Thursday, May 26, 2011 8:57:49 PM
> Subject: Re: Forcing Cassandra to free up some space
>
> Which JVM?  Which collector?  There have been and continue to be many.
>
> Hotspot itself supports a number of different collectors with
> different behaviors.   Many of them do not collect every candidate on
> every gc, but merely the easiest ones to find.  This is why depending
> on finalizers is a *bad* idea in java code.  They may well never get
> run.  (Finalizer is one of a few features the Sun Java team always
> regretted putting in Java to start with.  It has caused quite a few
> application problems over the years)
>
> The really important thing is that NONE of these behaviors of the
> colelctors are guaranteed by specification not to change from version
> to version.  Basing your code on non-specified behaviors is a good way
> to hit mysterious failures on updates.
>
> For instance, in the mid 90s, IBM had a mode of their Vm called
> "infinite heap."  it *never* garbage collected, even if you called
> System.gc.  Instead it just threw away address space and counted on
> the total memory needs for the life of the program being less then the
> total addressable space of the processor.
>
> It was *very* fast for certain kinds of applications.
>
> Far from being pedantic, not depending on undocumented behavior is
> simply good engineering.
>
>
> On Thu, May 26, 2011 at 4:51 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
>> I've read the relevant source. While you're pedantically correct re
>> the spec, you're wrong as to what the JVM actually does.
>>
>> On Thu, May 26, 2011 at 3:14 PM, Jeffrey Kesselman <jeffpk@gmail.com> wrote:
>>> Some references...
>>>
>>> "An object enters an unreachable state when no more strong references
>>> to it exist. When an object is unreachable, it is a candidate for
>>> collection. Note the wording: Just because an object is a candidate
>>> for collection doesn't mean it will be immediately collected. The JVM
>>> is free to delay collection until there is an immediate need for the
>>> memory being consumed by the object."
>>>
>>> http://java.sun.com/docs/books/performance/1st_edition/html/JPAppGC.fm.html#998394
>>>
>>> and "Calling the gc method suggests that the Java Virtual Machine
>>> expend effort toward recycling unused objects"
>>>
>>> http://download.oracle.com/javase/6/docs/api/java/lang/System.html#gc()
>>>
>>> It goes on to say that the VM will make a "best effort", but "best
>>> effort" is *deliberately* left up to the definition of the gc
>>> implementor.
>>>
>>> I guess you missed the many lectures I have given on this subject over
>>> the years at Java One Conferences....
>>>
>>> On Thu, May 26, 2011 at 3:53 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
>>>> It's a common misunderstanding that system.gc is only a suggestion; on
>>>> any VM you're likely to run Cassandra on, System.gc will actually
>>>> invoke a full collection.
>>>>
>>>> On Thu, May 26, 2011 at 2:18 PM, Jeffrey Kesselman <jeffpk@gmail.com>
wrote:
>>>>> Actually this is no gaurantee.   Its a common misunderstanding that
>>>>> System.gc "forces" gc.  It does not. It is a suggestion only. The vm
always
>>>>> has the option as to when and how much it gcs
>>>>>
>>>>> On May 26, 2011 2:51 PM, "Jonathan Ellis" <jbellis@gmail.com> wrote:
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Jonathan Ellis
>>>> Project Chair, Apache Cassandra
>>>> co-founder of DataStax, the source for professional Cassandra support
>>>> http://www.datastax.com
>>>>
>>>
>>>
>>>
>>> --
>>> It's always darkest just before you are eaten by a grue.
>>>
>>
>>
>>
>> --
>> Jonathan Ellis
>> Project Chair, Apache Cassandra
>> co-founder of DataStax, the source for professional Cassandra support
>> http://www.datastax.com
>>
>
>
>
> --
> It's always darkest just before you are eaten by a grue.
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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