On 2011-01-06 03:10, Robert Haas wrote:
On Wed, Jan 5, 2011 at 3:22 PM, Jesper Krogh<jes...@krogh.cc> wrote:
Given a crash-safe visibility map, what purpuse does the PD_ALL_VISIBLE bit
serve?
If we modify a page on which PD_ALL_VISIBLE isn't set, we don't
attempt to update the visibility map. In theory, this is an important
optimization to reduce contention on the visibility map page, since
there are something like 64K heap pages per visibility map page. In
practice, I'm not sure in what workloads it matters or by how much
If that is the only reason, I would suggest just making the visibillity map
pages more sparse. If you just had 500 or 1K heap pages per visibillity map
page, then it would not change a thing for the performance. With 1K heap
pages
per VM map page a relation with 100GB of data would have a VM map of
the VM map of 100MB, even 100 heap pages per VM-map page would still
enable the database to have visibillity information of 100GB data stored
in 1GB
of memory.
But it would need testing and benchmarking to find out.
Jesper
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Jesper
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