Re: issue with gininsert under very high load

On 02/12/2014 04:04 PM, Heikki Linnakangas wrote:
> On 02/12/2014 10:50 PM, Andres Freund wrote:
>> On February 12, 2014 9:33:38 PM CET, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
>>>> On 2014-02-12 14:39:37 -0500, Andrew Dunstan wrote:
>>>>> On investigation I found that a number of processes were locked
>>> waiting for
>>>>> one wedged process to end its transaction, which never happened
>>> (this
>>>>> transaction should normally take milliseconds). oprofile revealed
>>> that
>>>>> postgres was spending 87% of its time in s_lock(), and strace on the
>>> wedged
>>>>> process revealed that it was in a tight loop constantly calling
>>> select(). It
>>>>> did not respond to a SIGTERM.
>>>
>>>> That's a deficiency of the gin fastupdate cache: a) it bases it's
>>> size
>>>> on work_mem which usually makes it *far* too big b) it doesn't
>>> perform the
>>>> cleanup in one go if it can get a suitable lock, but does independent
>>>> locking for each entry. That usually leads to absolutely horrific
>>>> performance under concurreny.
>>>
>>> I'm not sure that what Andrew is describing can fairly be called a
>>> concurrent-performance problem. It sounds closer to a stuck lock.
>>> Are you sure you've diagnosed it correctly?
>>
>> No. But I've several times seen similar backtraces where it wasn't
>> actually stuck, just livelocked. I'm just on my mobile right now, but
>> afair Andrew described a loop involving lots of semaphores and
>> spinlock, that shouldn't be the case if it were actually stuck.
>> If there dozens of processes waiting on the same lock, cleaning up a
>> large amount of items one by one, it's not surprising if its
>> dramatically slow.
>
> Perhaps we should use a lock to enforce that only one process tries to
> clean up the pending list at a time.
>

Is that going to serialize all these inserts?

cheers

andrew