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Update future-tense comments in README to present tense. Noted by

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Neil Conway.
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Tom Lane 2003-10-31 22:48:08 +00:00
parent 47309464e4
commit 4240d2bffd
1 changed files with 16 additions and 17 deletions
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src/backend/storage/buffer/README
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@ -1,4 +1,4 @@
$Header: /cvsroot/pgsql/src/backend/storage/buffer/README,v 1.3 2001/09/29 04:02:22 tgl Exp $ $Header: /cvsroot/pgsql/src/backend/storage/buffer/README,v 1.4 2003/10/31 22:48:08 tgl Exp $
Notes about shared buffer access rules Notes about shared buffer access rules
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@ -79,20 +79,19 @@ it won't be able to actually examine the page until it acquires shared
or exclusive lock. or exclusive lock.
As of 7.1, the only operation that removes tuples or compacts free space is VACUUM FULL ignores rule #5, because it instead acquires exclusive lock at
(oldstyle) VACUUM. It does not have to implement rule #5 directly, because the relation level, which ensures indirectly that no one else is accessing
it instead acquires exclusive lock at the relation level, which ensures pages of the relation at all.
indirectly that no one else is accessing pages of the relation at all.
To implement concurrent VACUUM we will need to make it obey rule #5 fully. Plain (concurrent) VACUUM must respect rule #5 fully. Obtaining the
To do this, we'll create a new buffer manager operation necessary lock is done by the bufmgr routine LockBufferForCleanup().
LockBufferForCleanup() that gets an exclusive lock and then checks to see It first gets an exclusive lock and then checks to see if the shared pin
if the shared pin count is currently 1. If not, it releases the exclusive count is currently 1. If not, it releases the exclusive lock (but not the
lock (but not the caller's pin) and waits until signaled by another backend, caller's pin) and waits until signaled by another backend, whereupon it
whereupon it tries again. The signal will occur when UnpinBuffer tries again. The signal will occur when UnpinBuffer decrements the shared
decrements the shared pin count to 1. As indicated above, this operation pin count to 1. As indicated above, this operation might have to wait a
might have to wait a good while before it acquires lock, but that shouldn't good while before it acquires lock, but that shouldn't matter much for
matter much for concurrent VACUUM. The current implementation only concurrent VACUUM. The current implementation only supports a single
supports a single waiter for pin-count-1 on any particular shared buffer. waiter for pin-count-1 on any particular shared buffer. This is enough
This is enough for VACUUM's use, since we don't allow multiple VACUUMs for VACUUM's use, since we don't allow multiple VACUUMs concurrently on a
concurrently on a single relation anyway. single relation anyway.