Re: Support Parallel Query Execution in Executor

On Apr 9, 2006, at 9:26 AM, Martijn van Oosterhout wrote:

> On Sun, Apr 09, 2006 at 08:23:36AM -0700, Myron Scott wrote:
>>> This is the part I'm curious about - is this using the
>>> shared_buffers region
>>> in a circular buffer fashion to store pre-fetched pages?
>>
>> Yes.  That is basically what the slave thread is trying to do.  As
>> well as weed out
>> any tuples/pages that don't need to be looked at due to dead tuples.
>> I did several things to try and insure that a buffer needed by the
>> master thread
>> would not be pulled out of the buffer pool before it was seen by the
>> master.
>> I wanted to do this without holding the buffer pinned, so I did the
>> change to
>> the buffer free list to do this.
>
> Is this necessary? I mean, what's the chance that a page might get
> thrown out early? And if so, what's the chance that page will still be
> in the OS cache?
>
> The cost of fetching a page from the OS is not really much of an
> overhead, so I'd like to know how much benefit these buffer cache  
> hacks
> actually produce.
>

You may be right on this one.  I wanted ensure that I didn't lose
pages I needed.  I may have just added a belt to my
suspenders.  I'll add a switch to turn it off and on and try and
devise a test to see what the costs either way are.


Myron Scott