Continuation of:
http://www.postgresql.org/message-id/1353551097.11440.128.camel@sussancws0025
Rebased patch attached; no other changes.
First of all, I'd like to take a step back and describe the benefit that
I'm trying to achieve. If you have an analytic or data warehouse
workload, then a data load may result in separate writes for
1. Data load itself
2. Setting hint bits
3. Setting PD_ALL_VISIBLE
If you have very low concurrency, short transactions, or data is small
enough to fit in memory; then #2 and #3 might be combined into one
write.
But an analytic or data warehouse workload has long-running queries and
there are typically at least some queries going at any given time. That
means that SELECTs or VACUUMs that happen soon after the data load will
start setting hint bits, and the shared buffers will fill up and those
pages will be evicted (without PD_ALL_VISIBLE because there are still
concurrent queries that can't see the tuples). Then, a later VACUUM will
come along and rewrite the whole table, just to set PD_ALL_VISIBLE.
The visibility map bit and PD_ALL_VISIBLE have the same meaning, so this
proposal is merely to eliminate PD_ALL_VISIBLE and the associated
logic.
The benefits are:
* eliminate extra page writes when the only change is setting
PD_ALL_VISIBLE (when checksums are enabled, this is
particularly expensive because it requires the pages to be
written to WAL as well)
* simplify the code by removing some complex logic that defies
the ordinary rules in transam/README
The costs are:
* during a scan, we need to check the VM so that we don't need
to read the hints on each tuple individually
* inserts, updates, and deletes need to check the VM to know
whether to unset the page's bit
The costs are primarily the need to pin VM pages in situations where we
would have just used PD_ALL_VISIBLE before. Pinning pages requires
locking and maintenance in shared buffers, so this is a real concern.
However, I believe the previous discussion was derailed because of fear
of contention if we pin any pages at all. I'd like this discussion to be
about whether we can spread the cost of pinning a VM page over enough
other work to be negligible.
Heikki pointed out one degenerate case where the cost of pinning pages
showed up:
http://www.postgresql.org/message-id/50FD11C5.1030700@vmware.com
But as he says: "This is a worst-case scenario, and the slowdown isn't
huge, so maybe it's a worthwhile tradeoff. But it shows that removing
PD_ALL_VISIBLE is not completely free."
Also, there is another proposal to eliminate freezing as we know it:
http://www.postgresql.org/message-id/20130523175148.GA29374@alap2.anarazel.de
I don't see any obvious conflict between the two proposals, but they are
related and one may affect the other. They may even be complimentary.
Regards,
Jeff Davis
