PostgreSQL performance on FreeBSD

[Sean Chittenden]

Small nit:

PostgreSQL used SYSV because it allowed for the detection of dead processes.  If you `kill -9`’ed a process, PostgreSQL can detect that and then shut down and perform an automatic recovery.  In this regard, sysv is pretty clever.  The move to POSIX shared mem was done for a host of reasons, but it means that you don’t have to adjust your SYSV limits.  My understanding from a few years ago is that there is still a ~64KB SYSV memory segment that is still used to act as the latch to signal if a process was killed, but all of the shared buffers are stored in posix mmap’ed regions.

At this point in time this could be replaced with kqueue(2) EVFILT_PROC, but no one has done that yet.

-sc



--
Sean Chittenden
sean at chittenden.org

> On Jun 22, 2016, at 07:26 , Maxim Sobolev <sobomax at freebsd.org> wrote:
> 
> Konstantin,
> 
> Not if you do sem_unlink() immediately, AFAIK. And that's what PG does. So
> the window of opportunity for the leakage is quite small, much smaller than
> for SYSV primitives. Sorry for missing your status update message, I've
> missed it somehow.
> 
> ----
>                mySem = sem_open(semname, O_CREAT | O_EXCL,
>                                                 (mode_t) IPCProtection,
> (unsigned) 1);
> 
> #ifdef SEM_FAILED
>                if (mySem != (sem_t *) SEM_FAILED)
>                        break;
> #else
>                if (mySem != (sem_t *) (-1))
>                        break;
> #endif
> 
>                /* Loop if error indicates a collision */
>                if (errno == EEXIST || errno == EACCES || errno == EINTR)
>                        continue;
> 
>                /*
>                 * Else complain and abort
>                 */
>                elog(FATAL, "sem_open(\"%s\") failed: %m", semname);
>        }
> 
>        /*
>         * Unlink the semaphore immediately, so it can't be accessed
> externally.
>         * This also ensures that it will go away if we crash.
>         */
>        sem_unlink(semname);
> 
>        return mySem;
> ----
> 
> -Max
> 
> On Wed, Jun 22, 2016 at 3:02 AM, Konstantin Belousov <kostikbel at gmail.com>
> wrote:
> 
>> On Tue, Jun 21, 2016 at 12:48:00PM -0700, Maxim Sobolev wrote:
>>> Thanks, Konstantin for the great work, we are definitely looking forward
>> to
>>> get all those improvements to be part of the default FreeBSD kernel/port.
>>> Would be nice if you can post an update some day later as to what's
>>> integrated and what's not.
>> I did posted the update several days earlier.  Since you replying to this
>> thread, it would be not unreasonable to read recent messages that were
>> sent.
>> 
>>> 
>>> Just in case, I've opened #14206 with PG to switch us to using POSIX
>>> semaphores by default. Apart from the mentioned performance benefits,
>> SYSV
>>> semaphores are PITA to deal with as they come in very limited quantities
>> by
>>> default. Also they might stay around if PG dies/gets nuked and prevent it
>>> from starting again due to overflow. We've got some quite ugly code to
>>> clean up those using ipcrm(1) in our build scripts to deal with just
>> that.
>>> I am happy that code could be retired now.
>> 
>> Named semaphores also stuck around if processes are killed without cleanup.
>> 
>> 
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