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From Jeffrey Kesselman <jef...@gmail.com>
Subject Re: Forcing Cassandra to free up some space
Date Thu, 16 Jun 2011 02:34:26 GMT
The GC cleanup approach, if depending on specific objects being GCd,
is fundamentally flawed.

I brought this up earlier, won't restart that thread.  It should be in
the archives.


On Wed, Jun 15, 2011 at 10:17 PM, Terje Marthinussen
<tmarthinussen@gmail.com> wrote:
> Watching this on a node here right now and it sort of shows how bad this can
> get.
> This node still has 109GB free disk by the way...
> INFO [CompactionExecutor:5] 2011-06-16 09:11:59,164 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:5] 2011-06-16 09:12:23,929 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:5] 2011-06-16 09:12:46,489 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:17:53,299 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:18:17,782 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:18:42,078 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:19:06,984 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:19:32,079 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:19:57,265 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:20:22,706 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:20:47,331 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:21:13,062 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:21:38,288 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:22:03,500 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:22:29,407 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:22:55,577 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:23:20,951 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:23:46,448 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:3] 2011-06-16 09:24:12,030 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [ScheduledTasks:1] 2011-06-16 09:29:29,494 GCInspector.java (line 128)
> GC for ParNew: 392 ms, 398997776 reclaimed leaving 2334786808 used; max is
> 10844635136
>  INFO [ScheduledTasks:1] 2011-06-16 09:29:32,831 GCInspector.java (line 128)
> GC for ParNew: 737 ms, 332336832 reclaimed leaving 2473311448 used; max is
> 10844635136
>  INFO [CompactionExecutor:6] 2011-06-16 09:48:00,633 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:6] 2011-06-16 09:48:26,119 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:6] 2011-06-16 09:48:49,002 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:6] 2011-06-16 10:10:20,196 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:6] 2011-06-16 10:10:45,322 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:6] 2011-06-16 10:11:07,619 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:7] 2011-06-16 11:01:45,562 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:7] 2011-06-16 11:02:10,236 StorageService.java
> (line 2071) requesting GC to free disk space
>  INFO [CompactionExecutor:7] 2011-06-16 11:05:31,297 StorageService.java
> (line 2071) requesting GC to free disk space
> If I look at the data dir, I see 46 *Compacted files which makes up an
> additional 137GB of space.
> The oldest of these Compacted files dates back to Jun 16th 01:26.
> If these got deleted, there should actually be enough disk for the node to
> run a full compaction run if needed.
> Either the GC cleanup tactic is seriously flawed or  we have a potential bug
> keeping references far longer than needed?
> Terje
>
>
> On Wed, Jun 15, 2011 at 11:50 PM, Shotaro Kamio <kamioshot@gmail.com> wrote:
>>
>> We've encountered the situation that compacted sstable files aren't
>> deleted after node repair. Even when gc is triggered via jmx, it
>> sometimes leaves compacted files. In a case, a lot of files are left.
>> Some files stay more than 10 hours already. There is no guarantee that
>> gc will cleanup all compacted sstable files.
>>
>> We have a great interest on the following ticket.
>> https://issues.apache.org/jira/browse/CASSANDRA-2521
>>
>>
>> Regards,
>> Shotaro
>>
>>
>> On Fri, May 27, 2011 at 11:27 AM, Jeffrey Kesselman <jeffpk@gmail.com>
>> wrote:
>> > Im also not sure that will guarantee all space is cleaned up.  It
>> > really depends on what you are doing inside Cassandra.  If you have
>> > your on garbage collect that is just in some way tied to the gc run,
>> > then it will run when  it runs.
>> >
>> > If otoh you are associating records in your storage with specific
>> > objects in memory and using one of the post-mortem hooks (finalize or
>> > PhantomReference) to tell you to clean up that particular record then
>> > its quite possible they wont all get cleaned up.  In general hotspot
>> > does not find and clean every candidate object on every GC run.  It
>> > starts with the easiest/fastest to find and then sees what more it
>> > thinks it needs to do to create enough memory for anticipated near
>> > future needs.
>> >
>> > On Thu, May 26, 2011 at 10:16 PM, Jonathan Ellis <jbellis@gmail.com>
>> > wrote:
>> >> 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
>> >>
>> >
>> >
>> >
>> > --
>> > It's always darkest just before you are eaten by a grue.
>> >
>>
>>
>>
>> --
>> Shotaro Kamio
>
>



-- 
It's always darkest just before you are eaten by a grue.

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