Обсуждение: Memory for BYTEA returned by C function is not released until connection is dropped
Memory for BYTEA returned by C function is not released until connection is dropped
От
John Leiseboer
Дата:
I have written a number of functions in C that return BYTEA type. I have compiled and run on both Windows and Linux, 32-bitand 64-bit, PostgreSQL versions 9.3 and 9.4. My functions return BYTEA data to the caller. The problem is that memory usage grows until there is no memory left on thehost, at which point an error is returned. If I drop the connection (e.g. by quitting from pqsql), the memory is returned. I wrote the following minimal function to test palloc() and BYTEA return behaviour, and found that this minimal program alsoexhibits the unbounded memory growth problem. C source code: PG_FUNCTION_INFO_V1(test_palloc); Datum test_palloc() { bytea *test_ret; int test_len = 1024; test_ret = (bytea *)palloc(test_len + VARHDRSZ); SET_VARSIZE(test_ret, test_len + VARHDRSZ); PG_RETURN_BYTEA_P(test_ret); } Function definition: CREATE OR REPLACE FUNCTION test_palloc() RETURNS BYTEA AS E'<path to shared library>', test_palloc' LANGUAGE C IMMUTABLESTRICT; psql commands to reproduce the problem: \o out.txt SELECT ids.*, test_palloc() FROM GENERATE_SERIES(1, 1000000) ids; At the completion of the above command, host memory will have been consumed but not released back to the system. After quittingpsql (\q), memory is released. Is this expected behaviour or a bug? Am I doing something wrong? How can I return a BYTEA type from a C library functionwithout having to drop the connection in order to recover the allocated memory that is returned to the caller? Regards, John
John Leiseboer <jleiseboer@bigpond.com> writes: > I have written a number of functions in C that return BYTEA type. I have compiled and run on both Windows and Linux, 32-bitand 64-bit, PostgreSQL versions 9.3 and 9.4. > My functions return BYTEA data to the caller. The problem is that memory usage grows until there is no memory left on thehost, at which point an error is returned. If I drop the connection (e.g. by quitting from pqsql), the memory is returned. > I wrote the following minimal function to test palloc() and BYTEA return behaviour, and found that this minimal programalso exhibits the unbounded memory growth problem. > C source code: > PG_FUNCTION_INFO_V1(test_palloc); > Datum test_palloc() > { > bytea *test_ret; > int test_len = 1024; > test_ret = (bytea *)palloc(test_len + VARHDRSZ); > SET_VARSIZE(test_ret, test_len + VARHDRSZ); > PG_RETURN_BYTEA_P(test_ret); > } > Function definition: > CREATE OR REPLACE FUNCTION test_palloc() RETURNS BYTEA AS E'<path to shared library>', test_palloc' LANGUAGE C IMMUTABLESTRICT; > psql commands to reproduce the problem: > \o out.txt > SELECT ids.*, test_palloc() FROM GENERATE_SERIES(1, 1000000) ids; > At the completion of the above command, host memory will have been consumed but not released back to the system. Afterquitting psql (\q), memory is released. Well, first off, it's not that function that is eating memory: if you've got it marked as IMMUTABLE then it will in fact only be executed *once* per query. I don't see any evidence of memory leakage on the server at all when trying a comparable query. (Disclaimer: I just used "repeat('x', 1024)" rather than bothering to compile up a .so. But I do not think it behaves differently.) What I do see bloating is psql, which is absorbing a query result of about 1GB (1000000 1kB-sized rows) and then spending a lot of cycles to pretty-print that while writing it to out.txt. On my Linux box, psql does release the memory back to the kernel when that's over. But that would depend on the behavior of libc, so it's quite plausible that some other platforms would not. The way to avoid this is to not ask the client program to absorb the whole 1GB-sized query result at once. You could use a cursor and FETCH a few thousand rows at a time. (In reasonably recent versions of psql, "\set FETCH_COUNT" can do that for you automatically, at the cost of possibly less pretty output formatting.) Or use COPY, which is implemented in more of a streaming style to begin with. But at any rate, bottom line is that your problem is client-side not server-side, and no amount of fooling with the function innards will change it. regards, tom lane
Re: Memory for BYTEA returned by C function is not released until connection is dropped
От
John Leiseboer
Дата:
Tom Lane [mailto:tgl@sss.pgh.pa.us] writes: > But at any rate, bottom line is that your problem is client-side not server-side, and no amount of fooling with the functioninnards will change it. I wish it were. While monitoring memory on Linux and Windows machines I see that psql memory usage hardly changes, but PostgreSQLserver memory usage increases steadily until the query stops. PostgreSQL server memory usage stays high until afterthe client drops the connection. This is definitely a case of the server holding onto memory until the client dropsthe connection. In other case, when I let the query continue until memory is exhausted, the PostgreSQL server crashes with "out of memory"error, not the client. When does the PostgreSQL server call pfree() after a C function has returned to the caller? All I've found in books and Googlesearches is: "What makes palloc() special is that it allocates the memory in the current context and the whole memory is freed in onego when the context is destroyed." What "context"? The connection? The transaction? A SQL statement? The function call? John