diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/backend/storage/buffer/README | 26 |
1 files changed, 14 insertions, 12 deletions
diff --git a/src/backend/storage/buffer/README b/src/backend/storage/buffer/README index 45c5c83a41..c4a7668533 100644 --- a/src/backend/storage/buffer/README +++ b/src/backend/storage/buffer/README @@ -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 |