Hello.

I´ve been trying to find some information about changes in the way IDS
9.40 handles LRU Queues. It seems that there is a new algorithm that
allows a performance enhancement regarding Low and High Priority
Buffers.

The 9.40 documentation does not mention anything, but in a white paper
it is described as an obvious advantage. It mentions the possibility
to shift high priority buffers with low priority buffers based on the
hit count.

Can anyone give a light on this subject?

Thanks

RDT.

The old algorithm gave pages priority based on assumptions about
importance giving data pages low, index leaf pages medium-low, index
branch pages medium-high and root nodes high (in later versions).

The problem with this is that in practice many indexes were typically
created on heavily hit tables to improve performance but this meant
that there could often be as many pages associated with a table
holding index pages as there were the actual data. There was no limit
on the percentage of the buffers that could be assigned to the various
priorities. Therefore as pages can only be replaced by pages of the
same or higher priority the buffers could then become flooded with
index pages leading to frantic thrashing of the few remaining pages
assigned to data.

To overcome this problem 9.40 uses only two priorities (high and low)
and dynamically controls the percentage assigned to each through
maintanence of an Access Frequency Threshold. Access Frequency is
simply a count of the number of times a page is accessed by a user or
the system so it is essentially a vote on relative importance being
reset to 0 for each page at checkpoints. The threshold starts at 36
and is then adjusted in 10% increments at checkpoints to ensure that
an approximate ratio of 3:1 high to low is maintained. The High band
works as a First In First Out so that once the steady state of 3:1 is
reached pages can only be promoted to High by demoting the first page
that entered the high band.

Hope this helps,
Andy

Andrew

Thank you. Helped a lot.

RDT.

andrew.watson (AT) ardenta (DOT) com (Andy Watson) wrote in message news:<5708c605.0309090050.66dcb609 (AT) posting (DOT) google.com>...

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