Thread: Possible memory leak with SQL function?
Hello list, Is the following known behaviour, or should I put some time in writing a self contained test case? We have a function that takes a value and returns a ROW type. With the function implemented in language SQL, when executing this function in a large transaction, memory usage of the backend process increases. MemoryContextStats showed a lot of SQL function data. Debugging init_sql_fcache() showed that it was for the same function oid each time, and the oid was the function from value to ROW type. When the function is implemented in PL/pgSQL, the memory usage was much less. I'm sorry I cannot be more specific at the moment, such as what is 'much less' memory with a PL/pgSQl function, and are there as many SQL function data's as calls to the SQL function, because I would have to write a test case for this. I was just wondering, if this is known behavior of SQL functions vs PL/pgSQL functions, or could it be a bug? -- Yeb Havinga http://www.mgrid.net/ Mastering Medical Data
On Thu, Sep 12, 2013 at 5:29 AM, Yeb Havinga <yebhavinga@gmail.com> wrote: > Is the following known behaviour, or should I put some time in writing a > self contained test case? > > We have a function that takes a value and returns a ROW type. With the > function implemented in language SQL, when executing this function in a > large transaction, memory usage of the backend process increases. > MemoryContextStats showed a lot of SQL function data. Debugging > init_sql_fcache() showed that it was for the same function oid each time, > and the oid was the function from value to ROW type. > > When the function is implemented in PL/pgSQL, the memory usage was much > less. > > I'm sorry I cannot be more specific at the moment, such as what is 'much > less' memory with a PL/pgSQl function, and are there as many SQL function > data's as calls to the SQL function, because I would have to write a test > case for this. I was just wondering, if this is known behavior of SQL > functions vs PL/pgSQL functions, or could it be a bug? It sounds like a bug to me, although I can't claim to know everything there is to know about this topic. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
On 2013-09-13 18:32, Robert Haas wrote:
> On Thu, Sep 12, 2013 at 5:29 AM, Yeb Havinga <yebhavinga@gmail.com> wrote:
>> Is the following known behaviour, or should I put some time in writing a
>> self contained test case?
>>
>> We have a function that takes a value and returns a ROW type. With the
>> function implemented in language SQL, when executing this function in a
>> large transaction, memory usage of the backend process increases.
>> MemoryContextStats showed a lot of SQL function data. Debugging
>> init_sql_fcache() showed that it was for the same function oid each time,
>> and the oid was the function from value to ROW type.
>>
>> When the function is implemented in PL/pgSQL, the memory usage was much
>> less.
>>
>> I'm sorry I cannot be more specific at the moment, such as what is 'much
>> less' memory with a PL/pgSQl function, and are there as many SQL function
>> data's as calls to the SQL function, because I would have to write a test
>> case for this. I was just wondering, if this is known behavior of SQL
>> functions vs PL/pgSQL functions, or could it be a bug?
> It sounds like a bug to me, although I can't claim to know everything
> there is to know about this topic.
>
I spent some time writing a test case, but failed to make a test case
that showed the memory difference I described upthread, in contrast, in
the test below, the SQL function actually shows a smaller memory
footprint than the plpgsql counterpart. This test case only demonstrates
that in a long running transaction, calling sql or plpgsql functions
causes increasing memory usage that is not released until after commit.
callit.sql:
----------
DO
$$
DECLARE b text; i int;
BEGIN
-- SELECT 'a' into b; -- memory constant i := fp('a'); -- memory increases
-- i := fs('a'); -- memory increases but slow
END;
$$ LANGUAGE plpgsql;
-------------
sqlvsplpgsql.sql:
-------------
CREATE OR REPLACE FUNCTION fp (a text) RETURNS int AS $$
DECLARE result int;
BEGIN SELECT 10 INTO result; RETURN result;
END;
$$
LANGUAGE plpgsql;
CREATE OR REPLACE FUNCTION fs (a text) RETURNS int AS $$ SELECT 10;
$$
LANGUAGE sql;
\i callit.sql
-------------
rm /tmp/ff /tmp/ff2 ; cp callit.sql /tmp/ff ; cat /tmp/ff /tmp/ff >>
/tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff; cat /tmp/ff /tmp/ff >>
/tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >>
/tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >>
/tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >>
/tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >>
/tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >>
/tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff
psql -1 postgres -f /tmp/ff
Then watch htop in another terminal.
--
Yeb Havinga
http://www.mgrid.net/
Mastering Medical Data
<p dir="ltr">Noah, this is the kind of memory leak I was referring to which would be nice if valgrind could help with.
I'mnot sure exactly what that would look like though, I've never tried writing support code for valgrind to deal with
customallocators.<p dir="ltr">-- <br /> greg<div class="gmail_quote">On 16 Sep 2013 15:38, "Yeb Havinga" <<a
href="mailto:yebhavinga@gmail.com">yebhavinga@gmail.com</a>>wrote:<br type="attribution" /><blockquote
class="gmail_quote"style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> On 2013-09-13 18:32, Robert
Haaswrote:<br /><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
OnThu, Sep 12, 2013 at 5:29 AM, Yeb Havinga <<a href="mailto:yebhavinga@gmail.com"
target="_blank">yebhavinga@gmail.com</a>>wrote:<br /><blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px#ccc solid;padding-left:1ex"> Is the following known behaviour, or should I put some time in
writinga<br /> self contained test case?<br /><br /> We have a function that takes a value and returns a ROW type. With
the<br/> function implemented in language SQL, when executing this function in a<br /> large transaction, memory usage
ofthe backend process increases.<br /> MemoryContextStats showed a lot of SQL function data. Debugging<br />
init_sql_fcache()showed that it was for the same function oid each time,<br /> and the oid was the function from value
toROW type.<br /><br /> When the function is implemented in PL/pgSQL, the memory usage was much<br /> less.<br /><br />
I'msorry I cannot be more specific at the moment, such as what is 'much<br /> less' memory with a PL/pgSQl function,
andare there as many SQL function<br /> data's as calls to the SQL function, because I would have to write a test<br />
casefor this. I was just wondering, if this is known behavior of SQL<br /> functions vs PL/pgSQL functions, or could it
bea bug?<br /></blockquote> It sounds like a bug to me, although I can't claim to know everything<br /> there is to
knowabout this topic.<br /><br /></blockquote> I spent some time writing a test case, but failed to make a test case
thatshowed the memory difference I described upthread, in contrast, in the test below, the SQL function actually shows
asmaller memory footprint than the plpgsql counterpart. This test case only demonstrates that in a long running
transaction,calling sql or plpgsql functions causes increasing memory usage that is not released until after commit.<br
/><br/> callit.sql:<br /> ----------<br /> DO<br /> $$<br /> DECLARE b text;<br /> i int;<br /> BEGIN<br />
-- SELECT 'a' into b; -- memory constant<br /> i := fp('a'); -- memory increases<br /> -- i := fs('a'); -- memory
increasesbut slow<br /> END;<br /> $$ LANGUAGE plpgsql;<br /> -------------<br /><br /> sqlvsplpgsql.sql:<br />
-------------<br/> CREATE OR REPLACE FUNCTION fp (a text)<br /> RETURNS int<br /> AS $$<br /> DECLARE result int;<br
/>BEGIN<br /> SELECT 10 INTO result;<br /> RETURN result;<br /> END;<br /> $$<br /> LANGUAGE
plpgsql;<br/> CREATE OR REPLACE FUNCTION fs (a text)<br /> RETURNS int<br /> AS $$<br /> SELECT 10;<br /> $$<br />
LANGUAGEsql;<br /> \i callit.sql<br /> -------------<br /><br /><br /> rm /tmp/ff /tmp/ff2 ; cp callit.sql /tmp/ff ;
cat/tmp/ff /tmp/ff >> /tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff; cat /tmp/ff /tmp/ff >> /tmp/ff2;
cat/tmp/ff2 /tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >> /tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat
/tmp/ff/tmp/ff >> /tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >> /tmp/ff2; cat
/tmp/ff2/tmp/ff2 >> /tmp/ff;cat /tmp/ff /tmp/ff >> /tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff;cat
/tmp/ff/tmp/ff >> /tmp/ff2; cat /tmp/ff2 /tmp/ff2 >> /tmp/ff<br /><br /> psql -1 postgres -f /tmp/ff<br
/><br/> Then watch htop in another terminal.<br /><br /><br /> -- <br /> Yeb Havinga<br /><a
href="http://www.mgrid.net/"target="_blank">http://www.mgrid.net/</a><br /> Mastering Medical Data<br /><br /><br /><br
/>-- <br /> Sent via pgsql-hackers mailing list (<a href="mailto:pgsql-hackers@postgresql.org"
target="_blank">pgsql-hackers@postgresql.org</a>)<br/> To make changes to your subscription:<br /><a
href="http://www.postgresql.org/mailpref/pgsql-hackers"
target="_blank">http://www.postgresql.org/<u></u>mailpref/pgsql-hackers</a><br/></blockquote></div>
Yeb Havinga <yebhavinga@gmail.com> writes: > On 2013-09-13 18:32, Robert Haas wrote: >> On Thu, Sep 12, 2013 at 5:29 AM, Yeb Havinga <yebhavinga@gmail.com> wrote: >>> We have a function that takes a value and returns a ROW type. With the >>> function implemented in language SQL, when executing this function in a >>> large transaction, memory usage of the backend process increases. >>> When the function is implemented in PL/pgSQL, the memory usage was much >>> less. > I spent some time writing a test case, but failed to make a test case > that showed the memory difference I described upthread, in contrast, in > the test below, the SQL function actually shows a smaller memory > footprint than the plpgsql counterpart. This test case only demonstrates > that in a long running transaction, calling sql or plpgsql functions > causes increasing memory usage that is not released until after commit. I looked into this, and found that what the test is showing is that use of a "simple" expression in a plpgsql DO block leaks some memory that's not reclaimed till end of transaction; see analysis at http://www.postgresql.org/message-id/7438.1384273112@sss.pgh.pa.us You had > -- SELECT 'a' into b; -- memory constant > i := fp('a'); -- memory increases > -- i := fs('a'); -- memory increases but slow The SELECT doesn't leak because it's not a simple expression. The other two cases exhibit what's basically the same leak, though the SQL-function case leaks less memory per iteration and probably takes more cycles to do it, as a consequence of inlining the function's constant result into the calling expression. I'm not sure whether we're going to put much effort into fixing this leak; this usage pattern seems outside what DO blocks are intended for. (If you're going to execute the same code over and over again, it makes a whole lot more sense to define it as a real function, to avoid parsing overhead. Or just put it inside a loop in the DO text.) But anyway, the bottom line is that this test case doesn't seem to have much to do with your original problem with SQL functions. Can you have another go at recreating that leak? regards, tom lane