$Header: /cvsroot/pgsql/src/backend/storage/lmgr/README,v 1.3 1998/07/06 18:16:07 momjian Exp $
There are two fundemental lock structures. Lock methods describe the
locking behavior. We currently only support multi-level locking. Lock
modes describe the mode of the lock(read/write or shared/exclusive).
See src/tools/backend/index.html and src/include/storage/lock.h for more
details.
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The lock manager's LOCK:
tag -
The key fields that are used for hashing locks in the shared memory
lock hash table. This is kept as a separate struct to ensure that we
always zero out the correct number of bytes. This is a problem as
part of the tag is an itempointer which is 6 bytes and causes 2
additional bytes to be added as padding.
tag.relId -
Uniquely identifies the relation that the lock corresponds to.
tag.dbId -
Uniquely identifies the database in which the relation lives. If
this is a shared system relation (e.g. pg_user) the dbId should be
set to 0.
tag.tupleId -
Uniquely identifies the block/page within the relation and the
tuple within the block. If we are setting a table level lock
both the blockId and tupleId (in an item pointer this is called
the position) are set to invalid, if it is a page level lock the
blockId is valid, while the tuleId is still invalid. Finally if
this is a tuple level lock (we currently never do this) then both
the blockId and tupleId are set to valid specifications. This is
how we get the appearance of a multi-level lock table while using
only a single table (see Gray's paper on 2 phase locking if
you are puzzled about how multi-level lock tables work).
mask -
This field indicates what types of locks are currently held in the
given lock. It is used (against the lock table's conflict table)
to determine if the new lock request will conflict with existing
lock types held. Conficts are determined by bitwise AND operations
between the mask and the conflict table entry for the given lock type
to be set. The current representation is that each bit (1 through 5)
is set when that lock type (WRITE, READ, WRITE INTENT, READ INTENT, EXTEND)
has been acquired for the lock.
waitProcs -
This is a shared memory queue of all process structures corresponding to
a backend that is waiting (sleeping) until another backend releases this
lock. The process structure holds the information needed to determine
if it should be woken up when this lock is released. If, for example,
we are releasing a read lock and the process is sleeping trying to acquire
a read lock then there is no point in waking it since the lock being
released isn't what caused it to sleep in the first place. There will
be more on this below (when I get to releasing locks and waking sleeping
process routines).
nHolding -
Keeps a count of how many times this lock has been attempted to be
acquired. The count includes attempts by processes which were put
to sleep due to conflicts. It also counts the same backend twice
if, for example, a backend process first acquires a read and then
acquires a write.
holders -
Keeps a count of how many locks of each type have been attempted. Only
elements 1 through MAX_LOCK_TYPES are used as they correspond to the lock
type defined constants (WRITE through EXTEND). Summing the values of
holders should come out equal to nHolding.
nActive -
Keeps a count of how many times this lock has been succesfully acquired.
This count does not include attempts that were rejected due to conflicts,
but can count the same backend twice (e.g. a read then a write -- since
its the same transaction this won't cause a conflict)
activeHolders -
Keeps a count of how locks of each type are currently held. Once again
only elements 1 through MAX_LOCK_TYPES are used (0 is not). Also, like
holders, summing the values of activeHolders should total to the value
of nActive.
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