On Tue, Aug 23, 2011 at 5:25 AM, Robert Haas
<robertmhaas@gmail.com> wrote:
I've been giving this quite a bit more thought, and have decided to
abandon the scheme described above, at least for now. It has the
advantage of avoiding virtually all locking, but it's extremely
inefficient in its use of memory in the presence of long-running
transactions. For example, if there's an open transaction that's been
sitting around for 10 million transactions or so and has an XID
assigned, any new snapshot is going to need to probe into the big
array for any XID in that range. At 8 bytes per entry, that means
we're randomly accessing about ~80MB of memory-mapped data. That
seems problematic both in terms of blowing out the cache and (on small
machines) possibly even blowing out RAM. Nor is that the worst case
scenario: a transaction could sit open for 100 million transactions.
First i respectfully disagree with you on the point of 80MB. I would say that its very rare that a small system( with <1 GB RAM ) might have a long running transaction sitting idle, while 10 million transactions are sitting idle. Should an optimization be left, for the sake of a very small system to achieve high enterprise workloads?
Second, if we make use of the memory mapped files, why should we think, that all the 80MB of data will always reside in memory? Won't they get paged out by the operating system, when it is in need of memory? Or do you have some specific OS in mind?
Thanks,
Gokul.