Обсуждение: Memory leaks using refcursors
Hi all, We have a problem with an application which uses heavily ref cursors to fetch results from the database. We often have the following case: - BEGIN - lots of queries using ref cursors (it can run for an hour for example). We close each result set when we don't use it anymore - COMMIT Even if we close the result sets, the memory of the PostgreSQL backend is not released until the transaction is commited at the end of the script. This is really a problem for us as PostgreSQL starts swapping during the transaction. The code used to manipulate the result set is: CallableStatement myCallableStatement = myConnexion.prepareCall("{? = Call cp_TestPostgresStandAlone()}"); myCallableStatement.registerOutParameter(1, Types.OTHER); myCallableStatement.execute(); ResultSet myResultSet = (ResultSet) myCallableStatement.getObject(1); /* simple manipulations of the result set */ myResultSet.close(); myCallableStatement.close(); After this two close() calls, the memory used by the cursor should be released and it's not the case. The stored proc is something like: CREATE OR REPLACE FUNCTION cp_TestPostgresStandAlone() RETURNS refcursor AS $BODY$ DECLARE stock_cursor refcursor; BEGIN OPEN stock_cursor FOR SELECT COUNT(*) FROM TestPostgresStandAlone; RETURN stock_cursor; END; $BODY$ LANGUAGE 'plpgsql' VOLATILE; AFAICS in the source code, the refcursor of a result set is not closed when we close the result set and I don't think PostgreSQL closes it by itself before the final commit. Is there any way to fix this behaviour? Thierry (in CC:) wrote a self contained test case to reproduce this memory leak. We can post it to the list if necessary. Thanks for your help. -- Guillaume
Just a post to answer to the question of Dave Cramer on IRC and have a comprehensive thread: 23:07 <dcramer> how would it know the cursor was open ? AFAICS there's code in the driver to deal with the refcursor, especially to get the cursor name and fetch all the results to a result set. Isn't there a way to keep the cursor name and close it automatically when the statement or the resulting result set is closed? I don't see an easy way to do it in the application directly (but any tip is welcome) and ISTM it should be dealt by the driver directly to prevent the memory usage of the backend growing dramatically during the transaction. -- Guillaume
"Guillaume Smet" <guillaume.smet@gmail.com> writes: > Isn't there a way to keep the cursor name and close it automatically > when the statement or the resulting result set is closed? While I agree that the driver probably should do that, I'm not sure why you are having a problem. A cursor doesn't normally accumulate its result set in memory on the backend side, so I wouldn't have expected all that big a memory leak. What queries are you executing? regards, tom lane
Hi Tom, On 1/17/07, Tom Lane <tgl@sss.pgh.pa.us> wrote: > While I agree that the driver probably should do that, I'm not sure why > you are having a problem. A cursor doesn't normally accumulate its > result set in memory on the backend side, so I wouldn't have expected > all that big a memory leak. What queries are you executing? I don't know exactly how much queries are executed during this transaction but it's a lot of queries and they are _all_ executed through stored proc returning cursors. The self contained test case is available here: http://people.openwide.fr/~gsmet/postgresql/refcursor_problem/ and especially the stored proc is in the readme file along with all the instructions to run the test case. I just run a test case with 10k iterations and the backend which executes the queries uses 236 MB of memory before the commit. -- Guillaume
Tom,
The driver ends up issuing a
fetch all in <unnamed portal 1>
which if I try to close gives an error
cursor "<unnamed portal 1>" does not exist
so my guess is the portal is gone... what else might be keeping server memory ???
I thought about this and long running transactions and if this were really a problem slony would certainly blow up, as it has transactions that run days.
Dave
On 17-Jan-07, at 5:31 PM, Tom Lane wrote:
"Guillaume Smet" <guillaume.smet@gmail.com> writes:Isn't there a way to keep the cursor name and close it automaticallywhen the statement or the resulting result set is closed?While I agree that the driver probably should do that, I'm not sure whyyou are having a problem. A cursor doesn't normally accumulate itsresult set in memory on the backend side, so I wouldn't have expectedall that big a memory leak. What queries are you executing?regards, tom lane---------------------------(end of broadcast)---------------------------TIP 6: explain analyze is your friend
Hi all, Summary of our discussion with Dave on IRC. The following patch provided by Dave fixed the problem for the first 31 iterations of our test but failed after that because the garbage collector calls the close() method a second time when the object is dereferenced (finalize() method of AbstractJDBC2Statement). So we have to be sure that when the close() method is called a second time, it doesn't fail. On 1/18/07, Dave Cramer <davec@postgresintl.com> wrote: > + > + if ( this.refCursorName != null ) > + { > + connection.execSQLUpdate("close \"" + refCursorName +'"'); I added a refCursorName = null; here to be sure the close cursor is not executed twice and it works correctly. > + } I don't have the memory leaks anymore. I'll ask the people developing the real application to test this patch to be sure everything is now OK and I'll post the confirmation here. Dave, thanks a lot for your help. -- Guillaume
Hi again, In fact, there is still a remaining problem. People who developed this application sometimes commit the transaction before closing the result set in their code. With the current driver, it's not a problem as it didn't close the cursor at all so the cursor is closed at the end of the transaction and that's all. But with this patch, we have the following case: - begin - open result set -> open the cursor - commit -> close the cursor - close the result set -> try to close the cursor -> exception and backend in an error state if autocommit is false They fixed the order of commit/close in their application but IMHO, it's a bad idea to introduce this sort of regression. A try/catch is probably not a good idea because it leaves the backend in an error state if the CLOSE query is executed in a transaction (if autocommit is false for example). Any ideas on how we can solve this problem? I can't find any way to check if a cursor is still alive without throwing an error. -- Guillaume
On 18-Jan-07, at 12:42 PM, Guillaume Smet wrote: > Hi again, > > In fact, there is still a remaining problem. People who developed this > application sometimes commit the transaction before closing the result > set in their code. > With the current driver, it's not a problem as it didn't close the > cursor at all so the cursor is closed at the end of the transaction > and that's all. > > But with this patch, we have the following case: > - begin > - open result set -> open the cursor > - commit -> close the cursor > - close the result set -> try to close the cursor -> exception and > backend in an error state if autocommit is false > > They fixed the order of commit/close in their application but IMHO, > it's a bad idea to introduce this sort of regression. > > A try/catch is probably not a good idea because it leaves the backend > in an error state if the CLOSE query is executed in a transaction (if > autocommit is false for example). > > Any ideas on how we can solve this problem? I can't find any way to > check if a cursor is still alive without throwing an error. There's one important piece of information I believe might be relevant here. The application is using XA. I'm not convinced that there isn't a subtle bug in the server's XA implementation that may be keeping track of the open cursor. Under the covers this is using the unnamed cursor, and there is no close cursor being issued, The cursor is implicitly closed by opening another unnamed cursor. I've asked Guillaume to test this hypothesis with his test case that does not use XA to see if the memory "leak" still exists without XA Dave > > -- > Guillaume > > ---------------------------(end of > broadcast)--------------------------- > TIP 1: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to majordomo@postgresql.org so that > your > message can get through to the mailing list cleanly >
Hi Dave, On 1/19/07, Dave Cramer <pg@fastcrypt.com> wrote: > I've asked Guillaume to test this hypothesis with his test case that > does not use XA to see if the memory "leak" still exists without XA Perhaps I was not clear when I explained the remaining problem. I don't have any memory leak left. The attached file is a test case which shows the new problem. The commit is done before closing the resultset and so the close() method can't find the portal which results in an exception (the portal is closed on commit). It's probably not something we should do but before there was no error in this case and now it throws an exception which is not really clear for the end user: An exception has occured.ERROR: cursor "<unnamed portal 1>" does not exist org.postgresql.util.PSQLException: ERROR: cursor "<unnamed portal 1>" does not exist -- Guillaume
Hi Guilluame On 19-Jan-07, at 8:26 AM, Guillaume Smet wrote: > Hi Dave, > > On 1/19/07, Dave Cramer <pg@fastcrypt.com> wrote: >> I've asked Guillaume to test this hypothesis with his test case that >> does not use XA to see if the memory "leak" still exists without XA > > Perhaps I was not clear when I explained the remaining problem. I > don't have any memory leak left. The attached file is a test case > which shows the new problem. You were exceptionally clear when you described the problem you are now having. Apparently I have not been clear. I don't believe the solution is for the result set to close the unnamed portal when it is closed. As you can see there is a catch-22 situation which you described clearly. I believe the problem is in the server's XA code somehow not closing unnamed parameters properly. If you could run the code I sent you and tell me if it causes a leak, then that will confirm it. Dave > > The commit is done before closing the resultset and so the close() > method can't find the portal which results in an exception (the portal > is closed on commit). > It's probably not something we should do but before there was no error > in this case and now it throws an exception which is not really clear > for the end user: > An exception has occured.ERROR: cursor "<unnamed portal 1>" does > not exist > org.postgresql.util.PSQLException: ERROR: cursor "<unnamed portal 1>" > does not exist > > -- > Guillaume > > ---------------------------(end of > broadcast)--------------------------- > TIP 6: explain analyze is your friend >
On 1/19/07, Dave Cramer <pg@fastcrypt.com> wrote: > I believe the problem is in the server's XA code somehow not closing > unnamed parameters properly. > > If you could run the code I sent you and tell me if it causes a leak, > then that will confirm it. Sorry, I did not understand what you've asked me. I just run the test without the XA code and it has the memory leak with the old 408 driver. IMHO, it's quite logical as this memory leak has nothing to do with the transaction management as all the operations are in a big transaction. When I close the transaction, XA or not XA, the backend used by the connection returns to a normal memory usage. -- Guillaume
On 19-Jan-07, at 11:07 AM, Guillaume Smet wrote: > On 1/19/07, Dave Cramer <pg@fastcrypt.com> wrote: >> I believe the problem is in the server's XA code somehow not closing >> unnamed parameters properly. >> >> If you could run the code I sent you and tell me if it causes a leak, >> then that will confirm it. > > Sorry, I did not understand what you've asked me. I just run the test > without the XA code and it has the memory leak with the old 408 > driver. > IMHO, it's quite logical as this memory leak has nothing to do with > the transaction management as all the operations are in a big > transaction. > When I close the transaction, XA or not XA, the backend used by the > connection returns to a normal memory usage. So I've sent Guillaume a driver which closes the refcursor directly after reading the contents for testing. So far so good. Dave > > -- > Guillaume > > ---------------------------(end of > broadcast)--------------------------- > TIP 5: don't forget to increase your free space map settings >
Hello, Can someone explain why I cann't get bytea data of more than 1998 bytes? Below 1998 size is ok but I got exception if the size is at and above 1998. I used jdbc-8.1-407.jdbc3.jar. //setting bytea value. Work fine byte[] bdata = new byte[1998]; for (int b = 0; b < bdata.length; b++) { bdata[b] = (byte) (b % Byte.MAX_VALUE); } ByteArrayInputStream is = new ByteArrayInputStream(bdata); prepstmt.setBinaryStream(1, is, bdata.length); //getting bytea value. Got error ByteArrayInputStream s = (ByteArrayInputStream) rs .getBinaryStream(1); Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1998 at org.postgresql.util.PGbytea.toBytes(PGbytea.java:39) at org.postgresql.jdbc2.AbstractJdbc2ResultSet.getBytes(AbstractJdbc2Result Set.java:2048) at org.postgresql.jdbc2.AbstractJdbc2ResultSet.getBinaryStream(AbstractJdbc 2ResultSet.java:2168) Thanks Leon Do > -----Original Message----- > From: pgsql-jdbc-owner@postgresql.org > [mailto:pgsql-jdbc-owner@postgresql.org] On Behalf Of Dave Cramer > Sent: Friday, January 19, 2007 2:13 PM > To: Guillaume Smet > Cc: PostgreSQL JDBC; Tom Lane > Subject: Re: [JDBC] Memory leaks using refcursors > > > On 19-Jan-07, at 11:07 AM, Guillaume Smet wrote: > > > On 1/19/07, Dave Cramer <pg@fastcrypt.com> wrote: > >> I believe the problem is in the server's XA code somehow > not closing > >> unnamed parameters properly. > >> > >> If you could run the code I sent you and tell me if it > causes a leak, > >> then that will confirm it. > > > > Sorry, I did not understand what you've asked me. I just > run the test > > without the XA code and it has the memory leak with the old 408 > > driver. > > IMHO, it's quite logical as this memory leak has nothing to do with > > the transaction management as all the operations are in a big > > transaction. > > When I close the transaction, XA or not XA, the backend used by the > > connection returns to a normal memory usage. > > So I've sent Guillaume a driver which closes the refcursor > directly after reading the contents for testing. > > So far so good. > > Dave > > > > -- > > Guillaume > > > > ---------------------------(end of > > broadcast)--------------------------- > > TIP 5: don't forget to increase your free space map settings > > > > > ---------------------------(end of > broadcast)--------------------------- > TIP 4: Have you searched our list archives? > > http://archives.postgresql.org >
On Fri, 19 Jan 2007, Do, Leon (Leon) wrote: > Hello, > When starting a new thread, please don't reply to some other random message on the list. > Can someone explain why I cann't get bytea data of more than 1998 bytes? > Below 1998 size is ok but I got exception if the size is at and above > 1998. I used jdbc-8.1-407.jdbc3.jar. > Could you provide a complete test case. What you've written below isn't entirely clear about what's going on. I believe bytea decoding is done correctly. Perhaps you are doing something like calling getBinaryStream on a column that is not bytea? Kris Jurka
Sorry, it is my mistake. It is wrong data type. Thanks Leon Do > -----Original Message----- > From: pgsql-jdbc-owner@postgresql.org > [mailto:pgsql-jdbc-owner@postgresql.org] On Behalf Of Kris Jurka > Sent: Sunday, January 21, 2007 12:10 AM > To: Do, Leon (Leon) > Cc: PostgreSQL JDBC > Subject: Re: [JDBC] Max size for bytea type > > > > On Fri, 19 Jan 2007, Do, Leon (Leon) wrote: > > > Hello, > > > > When starting a new thread, please don't reply to some other > random message on the list. > > > Can someone explain why I cann't get bytea data of more > than 1998 bytes? > > Below 1998 size is ok but I got exception if the size is at > and above > > 1998. I used jdbc-8.1-407.jdbc3.jar. > > > > Could you provide a complete test case. What you've written > below isn't entirely clear about what's going on. I believe > bytea decoding is done correctly. Perhaps you are doing > something like calling getBinaryStream on a column that is not bytea? > > Kris Jurka > > > ---------------------------(end of > broadcast)--------------------------- > TIP 3: Have you checked our extensive FAQ? > > http://www.postgresql.org/docs/faq >
On Mon, 22 Jan 2007, Do, Leon (Leon) wrote: > Sorry, it is my mistake. It is wrong data type. > Still there is a TODO item here. The driver shouldn't bail out with an ArrayIndexOutOfBoundsException. It might be best to fail immediately for a getBytes call on something that isn't bytea data. It's conceivable that you there is bytea escaped data in a text field, but really, what are the odds (and use cases) of that? Kris Jurka
Yes, we need a TODO item on the bytea parser. We need to be able to handle rainny scenarios when data is bad. Thanks Leon Do > -----Original Message----- > From: Kris Jurka [mailto:books@ejurka.com] > Sent: Thursday, February 01, 2007 1:32 PM > To: Do, Leon (Leon) > Cc: PostgreSQL JDBC > Subject: Re: [JDBC] Max size for bytea type > > > > On Mon, 22 Jan 2007, Do, Leon (Leon) wrote: > > > Sorry, it is my mistake. It is wrong data type. > > > > Still there is a TODO item here. The driver shouldn't bail > out with an ArrayIndexOutOfBoundsException. It might be best > to fail immediately for a getBytes call on something that > isn't bytea data. It's conceivable that you there is bytea > escaped data in a text field, but really, what are the odds > (and use cases) of that? > > Kris Jurka > >
On Thu, 1 Feb 2007, Kris Jurka wrote: > On Mon, 22 Jan 2007, Do, Leon (Leon) wrote: > >> Sorry, it is my mistake. It is wrong data type. >> > > Still there is a TODO item here. The driver shouldn't bail out with an > ArrayIndexOutOfBoundsException. It might be best to fail immediately for a > getBytes call on something that isn't bytea data. It's conceivable that you > there is bytea escaped data in a text field, but really, what are the odds > (and use cases) of that? > Checking the code behind this, I see: AbstractJdbc2ResultSet.java:2150 if (fields[columnIndex - 1].getOID() == Oid.BYTEA) { return trimBytes(columnIndex,PGbytea.toBytes(this_row[columnIndex - 1])); } else { return trimBytes(columnIndex, this_row[columnIndex - 1]); } So the check I suggested already exists and I don't understand how you got this error. Do you still have your test case handy? Kris Jurka
On Fri, 19 Jan 2007, Dave Cramer wrote: > So I've sent Guillaume a driver which closes the refcursor directly after > reading the contents for testing. > > So far so good. > So what's the resolution on this? Should this fix be committed? What are the downsides? Kris Jurka
Kris, I don't see a downside to it. Once we read the data from the cursor we have no more use for it. AFACT it should be committed. Dave On 18-Feb-07, at 7:20 PM, Kris Jurka wrote: > > > On Fri, 19 Jan 2007, Dave Cramer wrote: > >> So I've sent Guillaume a driver which closes the refcursor >> directly after reading the contents for testing. >> >> So far so good. >> > > So what's the resolution on this? Should this fix be committed? > What are the downsides? > > Kris Jurka > > ---------------------------(end of > broadcast)--------------------------- > TIP 4: Have you searched our list archives? > > http://archives.postgresql.org >
Hi Kris, I told Dave on IRC that it was OK for us. The memory leak is fixed and we don't have any regression on the application tested. -- Guillaume