Re: RES: Priority to a mission critical transaction

Mark Lewis wrote:

>On Wed, 2006-11-29 at 08:25 -0500, Brian Hurt wrote:
>...
>
>
>>I have the same question. I've done some embedded real-time
>>programming, so my innate reaction to priority inversions is that
>>they're evil. But, especially given priority inheritance, is there any
>>situation where priority inversion provides *worse* performance than
>>running everything at the same priority? I can easily come up with
>>situations where it devolves to that case- where all processes get
>>promoted to the same high priority. But I can't think of one where
>>using priorities makes things worse, and I can think of plenty where it
>>makes things better.
>>
>>
>...
>
>It can make things worse when there are at least 3 priority levels
>involved. The canonical sequence looks as follows:
>
>LOW: Aquire a lock
>MED: Start a long-running batch job that hogs CPU
>HIGH: Wait on lock held by LOW task
>
>at this point, the HIGH task can't run until the LOW task releases its
>lock. but the LOW task can't run to completion and release its lock
>until the MED job completes.
>
>
>
>(random musing): I wonder if PG could efficiently be made to temporarily
>raise the priority of any task holding a lock that a high priority task
>waits on. I guess that would just make it so that instead of HIGH tasks
>being effectively reduced to LOW, then LOW tasks could be promoted to
>HIGH.
>
>
>

I thought that was what priority inheritance did- once HIGH blocks on a
lock held by LOW, LOW gets it's priority raised to that of HIGH. Then
LOW takes precedence over MED. If LOW blocks on a lock held by MED when
it has the same priority of HIGH, MED gets it's priority raised to
HIGH. Note that now all three processes are running with HIGH priority-
but is this any different from the default case of running them as the
same priority? This is what I was talking about when I said I could
imagine priority inheritance "devolving" to the single priority case.

Of course, this is a little tricky to implement. I haven't looked at
how difficult it'd be within Postgres.

Brian