Re: AutoVacuum starvation from sinval messages

On 9 November 2012 13:42, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Nov 8, 2012 at 6:23 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>>> On 8 November 2012 20:36, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>>>> It does not seem outrageous to me that there would be real-world
>>>> conditions in which invalidations would be sent more than once a
>>>> minute over prolonged periods, so this total starvation seems like a
>>>> bug.
>>
>>> Yes, its a bug, but do you really believe the above? In what cases?
>>
>> It doesn't take a whole lot of DDL to provoke an sinval overrun, if
>> the recipient process is just sitting idle and not servicing the
>> messages. I think Jeff's concern is entirely valid.
>
> So, do we need a sinval overrun or just a sinval message to provoke
> starvation? The former would be bad but the latter would be really,
> really bad. IIRC the queue has 4K entries, and IIRC a single DDL
> operation might provoke a couple of sinvals, but I'm thinking that
> somebody would probably have to be creating >1024 temp tables a minute
> to overrun the queue, which is very possible but not necessarily
> common. OTOH, creating 1 temp table a minute would hit a much broader
> swath of users.

The point is moot because latches don't work that way anymore.

--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services