Re: AutoVacuum starvation from sinval messages

On Thu, Nov 8, 2012 at 6:23 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> On 8 November 2012 20:36, Jeff Janes <jeff.janes@gmail.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.

-- 
Robert Haas
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