Okay, this isn't 100% hibernate-related, but I figure you guys think a
lot about stuff like this.
Prevayler's pitch goes something like this: "if all your data fits in
memory, you don't need concurrent transactions, because every
transaction will be demanding the same resource (cpu/memory
bandwidth)." Obviously this doesn't work if your data set is larger
than (or might grow to become larger than) your available memory. I
can see this being a fairly common situation.
What if you were sure that the total amount of data needed to perform
any given transaction could fit in memory? In that case, could you:
1) Load all pages needed by a transaction and pin them in memory
2) Perform the transaction
Any number of transactions could be doing (1) at the same time, but
there would be only a single thread permitted to do (2). Once a
transaction has pinned all the pages it will need, it queues itself
for (2). Once (2) is complete, the pages are unpinned (if the
reference count for the page falls to zero, of course -- multiple
transactions can pin the same page).
This would eliminate the need for fine grained locking and most of the
other stuff that makes databases complex, since only one thread is
running transactions. Adding more transaction threads wouldn't win
you anything, because a transaction on strictly-in-memory data never
blocks.
The downside is that you have to be certain that you'll never run a
query that needs more data than you have memory. OTOH, 4GB of ram is
pretty cheap these days. And if your query needs to pull 4GB of data
off the disk [*], well, it's going to be incredibly slow in the first
place. I don't think too many people run queries like that.
- a
--
"Education is not filling a bucket but lighting a fire." -- WB Yeats
[*] Note that simply selecting a 4GB table doesn't actually need to
pull 4GB off the disk.
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