Thread: backend freezeing on win32 fixed (I hope ;-) )

Hi,

I think I have fixed the freezing of the postgres backend on Windows NT. Now
it survives 5 regression test in a cycle with some concurrent connections
during running the tests.

Where the problem was (manual backtrace):
- InitPostgres() (utils/init/postinit.c)
- InitProcess() (storage/lmgr/proc.c)
- IpcSemaphoreCreate() (storage/ipc/ipc.c)
- semget() (now in libcygipc - sem.c)
- sem_connect() (sem.c)
- WaitForSingleObject() (win32 system call)
- freezing....

It have looked like a problem with initializing the same semaphore for
second time (they are "preinitialized" for performance reasons in
InitProcGlobal() in storage/lmgr/proc.c)

The fix (made for v6.5.1) is here:
-------------------------------
--- src/backend/storage/lmgr/proc.c.old    Sat Aug 14 16:50:19 1999
+++ src/backend/storage/lmgr/proc.c    Sat Aug 14 16:50:52 1999
@@ -160,6 +160,7 @@         * Pre-create the semaphores for the first maxBackends
processes,         * unless we are running as a standalone backend.         */
+#ifndef __CYGWIN__        if (key != PrivateIPCKey)        {            for (i = 0;
@@ -180,6 +181,7 @@                ProcGlobal->freeSemMap[i] = (1 <<
PROC_NSEMS_PER_SET);            }        }
+#endif /* __CYGWIN__ */    }}
------------------------------- 
            Dan

PS: I have packed the tree after "make install" for 6.5.1 with the patch
above, so it is a "binary distribution".

----------------------------------------------
Daniel Horak
network and system administrator
e-mail: horak@mmp.plzen-city.cz
privat e-mail: dan.horak@email.cz ICQ:36448176
----------------------------------------------

Horak Daniel <horak@mmp.plzen-city.cz> writes:
> I think I have fixed the freezing of the postgres backend on Windows NT. Now
> it survives 5 regression test in a cycle with some concurrent connections
> during running the tests.
> It have looked like a problem with initializing the same semaphore for
> second time (they are "preinitialized" for performance reasons in
> InitProcGlobal() in storage/lmgr/proc.c)

They should never be "initialized a second time".  And the preallocation
is *not* for performance reasons, it is to make sure we can actually get
enough semaphores (rather than dying under load when we fail to get the
N+1'st semaphore when starting the N+1'st backend).

> The fix (made for v6.5.1) is here:
> [ Fix consists of diking out preallocation of semaphores by postmaster ]

I do not like this patch one bit --- I think it is voodoo that doesn't
really have anything to do with the true problem.  I don't know what
the true problem is, mind you, but I don't think this is the way to
fix it.

Is it possible that the CygWin environment doesn't have a correct
emulation of IPC semaphores, such that a sema allocated by one process
(the postmaster) is not available to other procs (the backends)?
That would explain preallocation not working --- but if that's it then
we have major problems in other places, since the code assumes that a
sema once allocated will remain available to later backends.
        regards, tom lane

On Mon, Aug 16, 1999 at 10:07:00AM -0400, Tom Lane wrote:
>Is it possible that the CygWin environment doesn't have a correct
>emulation of IPC semaphores, such that a sema allocated by one process
>(the postmaster) is not available to other procs (the backends)?
>That would explain preallocation not working --- but if that's it then
>we have major problems in other places, since the code assumes that a
>sema once allocated will remain available to later backends.

We don't have correct emulation of IPC semapahores since they are not
implemented at all.  I assume that if you're relying on persistent
semaphores, then some add-on package is being used.

cgf