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Re: Freeing unused pages between two revisions...
Emmanuel Lécharny wrote:
we have had long discussions with Howard on LMDB about the release of
pages that are not anymore in use. This is a critical part of a MVCC
based B-tree, as each update will copy many pages, and we want to re-use
the copied pages.
The problem is that those copied pages may be still in use by older
readers. Let's quickly explain how it all works.
This is a good analysis, but I have to state that the investigation down
this path has stalled for a major reason:
The current reader/writer interaction depends on the fact that we only
need to know the oldest reader, and that we can be lazy about
determining what the oldest reader is. Once you start looking at
intervals between ordered ages of readers, that means you require
continuously up to date reader status, which means you require highly
consistent updating and reading of the readers table. That imposes
memory consistency guarantees that we currently don't require, and
adding those guarantees has a significant performance cost.
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/