There's a ticket open for this: https://issues.apache.org/jira/browse/CASSANDRA-2521. Vote on it if you think its important.
-ryan On Wed, Jun 15, 2011 at 7:34 PM, Jeffrey Kesselman <jef...@gmail.com> wrote: > 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 > <tmarthinus...@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 <kamios...@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 <jef...@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 <jbel...@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 >>> >> <konstant...@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" <jef...@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 <jbel...@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 <jef...@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 <jbel...@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 >>> >>>>>> <jef...@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" <jbel...@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. >
