Re: Priorities for users or queries? - Mailing list pgsql-general

Hi Jan,

That makes sense.  Does that mean that a low-priority "road-block" can
cause a deadlock or just an very long one lock?

Benjamin

Jan Wieck wrote:
> On 2/11/2007 1:02 PM, Benjamin Arai wrote:
>> Hi Magnus,
>>
>> Think this can be avoided as long the the queries executed on the
>> lower priority process never lock anything important.  In my case, I
>> would alway be doing inserts with the lower priority process, so
>> inversion should never occur.  On the other hand if some lock occur
>> somewhere else specific to Postgres then there may be an issue.  Are
>> there any other tables locked by the the Postgres process other than
>> those locks explicitly set by the query?
>
> If you assume that the logical row level locks, placed by such low
> priority "road-block", would be the important thing to watch out for,
> you are quite wrong. Although Postgres appears to avoid blocking
> readers by concurrent updates using MVCC, this isn't entirely true.
> The moment one updating backend needs to scribble around in any heap
> or index block, it needs an exclusive lock on that block until it is
> done with that. It will not hold that block level lock until the end
> of its transaction, but it needs to hold it until the block is in a
> consistent state again. That means that the lower the priority of
> those updating processes, the more exclusively locked shared buffers
> you will have in the system, with the locking processes currently not
> getting the CPU because of their low priority.
>
>
> Jan
>
>>
>> Benjamin
>>
>> Magnus Hagander wrote:
>>> Most likely, you do not want to do this. You *can* do it, but you are
>>> quite likely to suffer from priority inversion
>>> (http://en.wikipedia.org/wiki/Priority_inversion)
>>>
>>> //Magnus
>>>
>>>
>>> Adam Rich wrote:
>>>
>>>> There is a function pg_backend_pid() that will return the PID for
>>>> the current session.  You could call this from your updating app
>>>> to get a pid to feed to the NICE command.
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: pgsql-general-owner@postgresql.org
>>>> [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Benjamin Arai
>>>> Sent: Saturday, February 10, 2007 6:56 PM
>>>> To: pgsql-general@postgresql.org; pgsql-admin@postgresql.org
>>>> Subject: [GENERAL] Priorities for users or queries?
>>>>
>>>>
>>>> Hi,
>>>>
>>>> Is there a way to give priorities to queries or users?  Something
>>>> similar to NICE in Linux.  My goal is to give the updating
>>>> (backend) application a very low priority and give the web
>>>> application a high priority to avoid disturbing the user experience.
>>>>
>>>> Thanks in advance!
>>>>
>>>> Benjamin
>>>>
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>>>
>>>
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>