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Cover heap_page_prune_opt()'s cleanup lock tactic in README.

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Jeff Janes, reviewed by Jim Nasby.
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Noah Misch 2016-01-01 21:52:22 -05:00
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commit dfcd9cb302
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src/backend/storage/buffer/README
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@ -81,18 +81,20 @@ it won't be able to actually examine the page until it acquires shared
or exclusive content lock.
Rule #5 only affects VACUUM operations. Obtaining the
necessary lock is done by the bufmgr routine LockBufferForCleanup().
It first gets an exclusive lock and then checks to see if the shared pin
count is currently 1. If not, it releases the exclusive lock (but not the
caller's pin) and waits until signaled by another backend, whereupon it
tries again. The signal will occur when UnpinBuffer decrements the shared
pin count to 1. As indicated above, this operation might have to wait a
good while before it acquires lock, but that shouldn't matter much for
concurrent VACUUM. The current implementation only supports a single
waiter for pin-count-1 on any particular shared buffer. This is enough
for VACUUM's use, since we don't allow multiple VACUUMs concurrently on a
single relation anyway.
Obtaining the lock needed under rule #5 is done by the bufmgr routines
LockBufferForCleanup() or ConditionalLockBufferForCleanup(). They first get
an exclusive lock and then check to see if the shared pin count is currently
1. If not, ConditionalLockBufferForCleanup() releases the exclusive lock and
then returns false, while LockBufferForCleanup() releases the exclusive lock
(but not the caller's pin) and waits until signaled by another backend,
whereupon it tries again. The signal will occur when UnpinBuffer decrements
the shared pin count to 1. As indicated above, this operation might have to
wait a good while before it acquires lock, but that shouldn't matter much for
concurrent VACUUM. The current implementation only supports a single waiter
for pin-count-1 on any particular shared buffer. This is enough for VACUUM's
use, since we don't allow multiple VACUUMs concurrently on a single relation
anyway. Anyone wishing to obtain a cleanup lock outside of recovery or a
VACUUM must use the conditional variant of the function.
Buffer Manager's Internal Locking