Mailing-List: contact user-help@cassandra.apache.org; run by ezmlm
Precedence: bulk
Reply-To: user@cassandra.apache.org
Received-SPF: pass (nike.apache.org: domain of jeffpk@gmail.com designates
 209.85.214.172 as permitted sender)
MIME-Version: 1.0
In-Reply-To: 
 <CAGBp8g_qF2V_MWLpFb7UTsYG=GMWApHoKSxUE27+s2W0Obdi-Q@mail.gmail.com>
References: 
 <CAGBp8g_qF2V_MWLpFb7UTsYG=GMWApHoKSxUE27+s2W0Obdi-Q@mail.gmail.com>
From: Jeffrey Kesselman <jeffpk@gmail.com>
Date: Fri, 31 Aug 2012 12:50:40 -0400
Message-ID: 
 <CACU4Y-ihnbHeoPjit-qiHyuXE0YjgOuQ7nX4NyCrQS_5Rjdd0Q@mail.gmail.com>
Subject: Re: force gc?
To: user@cassandra.apache.org
Content-Type: multipart/alternative; boundary=bcaec54c527cc86b3b04c8929815

--bcaec54c527cc86b3b04c8929815
Content-Type: text/plain; charset=ISO-8859-1

Cassandra at least used to do disc cleanup as a side effect of
garbage collection through finalizers.  (This is a mistake for the
reason outlined below.)

It is important to understand that you can *never* "force* a gc in java.
Even calling System.gc() is merely a hint to the VM. What you are doing is
telling the VM that you are * willing* to give up some processor time right
now to gc, how much it choses to actually collect or not collect is totally
up to the VM.

The *only* garbage collection guarantee in java is that it will make a
"best effort" to collect what it can to avoid an out of memory exception at
the time that it runs out of memory.  You are not guaranteed when *if
ever*, a given object will actually be collected.  Since finalizers happen
when an object is collected, and not when it becomes a candidate for
collection, the same is true of the finalizer.  You are
not guaranteed when, if ever, it will run.

On Fri, Aug 31, 2012 at 9:03 AM, Alexander Shutyaev <shutyaev@gmail.com>wrote:

> Hi All!
>
> I have a problem with using cassandra. Our application does a lot of
> overwrites and deletes. If I understand correctly cassandra does not
> actually delete these objects until gc_grace seconds have passed. I tried
> to "force" gc by setting gc_grace to 0 on an existing column family and
> running major compaction afterwards. However I did not get disk space back,
> although I'm pretty much sure that my column family should occupy many
> times fewer space. We have also a PostgreSQL db and we duplicate each
> operation with data in both dbs. And the PosgreSQL table is much more
> smaller than the corresponding cassandra's column family. Does anyone have
> any suggestions on how can I analyze my problem? Or maybe I'm doing
> something wrong and there is another way to force gc on an existing column
> family.
>
> Thanks in advance,
> Alexander
>


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

--bcaec54c527cc86b3b04c8929815
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

Cassandra at least used to do disc cleanup as a side effect of garbage=A0co=
llection=A0through finalizers. =A0(This is a mistake for the reason=A0outli=
ned=A0below.)<div><br></div><div>It is important to understand that you can=
 *never* &quot;force* a gc in java. Even calling System.gc() is merely a hi=
nt to the VM. What you are=A0doing=A0is telling the VM that you are *=A0wil=
ling* to give up some processor=A0time=A0right now to gc, how much it chose=
s to actually collect or not collect is totally up to the VM.</div>

<div><br></div><div>The *only* garbage collection=A0guarantee=A0in java is =
that it will make a &quot;best effort&quot; to collect what it can to avoid=
 an out of memory exception at the time that it runs out of memory. =A0You =
are not=A0guaranteed=A0when *if ever*, a given object will actually be coll=
ected. =A0Since finalizers happen when an=A0object=A0is=A0collected, and no=
t when it=A0becomes=A0a candidate for collection, the same is true of the f=
inalizer. =A0You are not=A0guaranteed=A0when, if ever, it will run.<br>

<br><div class=3D"gmail_quote">On Fri, Aug 31, 2012 at 9:03 AM, Alexander S=
hutyaev <span dir=3D"ltr">&lt;<a href=3D"mailto:shutyaev@gmail.com" target=
=3D"_blank">shutyaev@gmail.com</a>&gt;</span> wrote:<br><blockquote class=
=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padd=
ing-left:1ex">

Hi All!<div><br></div><div>I have a problem with using cassandra. Our appli=
cation does a lot of overwrites and deletes. If I understand correctly cass=
andra does not actually delete these objects until gc_grace seconds have pa=
ssed. I tried to &quot;force&quot; gc by setting gc_grace to 0 on an existi=
ng column family and running major compaction afterwards. However I did not=
 get disk space back, although I&#39;m pretty much sure that my column fami=
ly should occupy many times fewer space. We have also a PostgreSQL db and w=
e duplicate each operation with data in both dbs. And the PosgreSQL table i=
s much more smaller than the corresponding cassandra&#39;s column family. D=
oes anyone have any suggestions on how can I analyze my problem? Or maybe I=
&#39;m doing something wrong and there is another way to force gc on an exi=
sting column family.</div>


<div><br></div><div>Thanks in advance,</div><div>Alexander</div>
</blockquote></div><br><br clear=3D"all"><div><br></div>-- <br>It&#39;s alw=
ays darkest just before you are eaten by a grue.<br>
</div>

--bcaec54c527cc86b3b04c8929815--