The purpose of commit 8a54e12a38 was to
fix this, and it sufficed when the PREPARE TRANSACTION completed before
the CIC looked for lock conflicts. Otherwise, things still broke. As
before, in a cluster having used CIC while having enabled prepared
transactions, queries that use the resulting index can silently fail to
find rows. It may be necessary to reindex to recover from past
occurrences; REINDEX CONCURRENTLY suffices. Fix this for future index
builds by making CIC wait for arbitrarily-recent prepared transactions
and for ordinary transactions that may yet PREPARE TRANSACTION. As part
of that, have PREPARE TRANSACTION transfer locks to its dummy PGPROC
before it calls ProcArrayClearTransaction(). Back-patch to 9.6 (all
supported versions).
Andrey Borodin, reviewed (in earlier versions) by Andres Freund.
Discussion: https://postgr.es/m/01824242-AA92-4FE9-9BA7-AEBAFFEA3D0C@yandex-team.ru
This commit adds new column "waitstart" into pg_locks view. This column
reports the time when the server process started waiting for the lock
if the lock is not held. This information is useful, for example, when
examining the amount of time to wait on a lock by subtracting
"waitstart" in pg_locks from the current time, and identify the lock
that the processes are waiting for very long.
This feature uses the current time obtained for the deadlock timeout
timer as "waitstart" (i.e., the time when this process started waiting
for the lock). Since getting the current time newly can cause overhead,
we reuse the already-obtained time to avoid that overhead.
Note that "waitstart" is updated without holding the lock table's
partition lock, to avoid the overhead by additional lock acquisition.
This can cause "waitstart" in pg_locks to become NULL for a very short
period of time after the wait started even though "granted" is false.
This is OK in practice because we can assume that users are likely to
look at "waitstart" when waiting for the lock for a long time.
The first attempt of this patch (commit 3b733fcd04) caused the buildfarm
member "rorqual" (built with --disable-atomics --disable-spinlocks) to report
the failure of the regression test. It was reverted by commit 890d2182a2.
The cause of this failure was that the atomic variable for "waitstart"
in the dummy process entry created at the end of prepare transaction was
not initialized. This second attempt fixes that issue.
Bump catalog version.
Author: Atsushi Torikoshi
Reviewed-by: Ian Lawrence Barwick, Robert Haas, Justin Pryzby, Fujii Masao
Discussion: https://postgr.es/m/a96013dc51cdc56b2a2b84fa8a16a993@oss.nttdata.com
This commit adds new column "waitstart" into pg_locks view. This column
reports the time when the server process started waiting for the lock
if the lock is not held. This information is useful, for example, when
examining the amount of time to wait on a lock by subtracting
"waitstart" in pg_locks from the current time, and identify the lock
that the processes are waiting for very long.
This feature uses the current time obtained for the deadlock timeout
timer as "waitstart" (i.e., the time when this process started waiting
for the lock). Since getting the current time newly can cause overhead,
we reuse the already-obtained time to avoid that overhead.
Note that "waitstart" is updated without holding the lock table's
partition lock, to avoid the overhead by additional lock acquisition.
This can cause "waitstart" in pg_locks to become NULL for a very short
period of time after the wait started even though "granted" is false.
This is OK in practice because we can assume that users are likely to
look at "waitstart" when waiting for the lock for a long time.
Bump catalog version.
Author: Atsushi Torikoshi
Reviewed-by: Ian Lawrence Barwick, Robert Haas, Justin Pryzby, Fujii Masao
Discussion: https://postgr.es/m/a96013dc51cdc56b2a2b84fa8a16a993@oss.nttdata.com
In a cluster having used CREATE INDEX CONCURRENTLY while having enabled
prepared transactions, queries that use the resulting index can silently
fail to find rows. Fix this for future CREATE INDEX CONCURRENTLY by
making it wait for prepared transactions like it waits for ordinary
transactions. This expands the VirtualTransactionId structure domain to
admit prepared transactions. It may be necessary to reindex to recover
from past occurrences. Back-patch to 9.5 (all supported versions).
Andrey Borodin, reviewed (in earlier versions) by Tom Lane and Michael
Paquier.
Discussion: https://postgr.es/m/2E712143-97F7-4890-B470-4A35142ABC82@yandex-team.ru
The SimpleLruTruncate() header comment states the new coding rule. To
achieve this, add locktype "frozenid" and two LWLocks. This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors. Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit. If a user's physical
replication primary logged ": apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms. At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point. One can test a cluster for
this data loss by running VACUUM FREEZE in every database. Back-patch
to 9.5 (all supported versions).
Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com
Until now, only selected bulk operations (e.g. COPY) did this. If a
given relfilenode received both a WAL-skipping COPY and a WAL-logged
operation (e.g. INSERT), recovery could lose tuples from the COPY. See
src/backend/access/transam/README section "Skipping WAL for New
RelFileNode" for the new coding rules. Maintainers of table access
methods should examine that section.
To maintain data durability, just before commit, we choose between an
fsync of the relfilenode and copying its contents to WAL. A new GUC,
wal_skip_threshold, guides that choice. If this change slows a workload
that creates small, permanent relfilenodes under wal_level=minimal, try
adjusting wal_skip_threshold. Users setting a timeout on COMMIT may
need to adjust that timeout, and log_min_duration_statement analysis
will reflect time consumption moving to COMMIT from commands like COPY.
Internally, this requires a reliable determination of whether
RollbackAndReleaseCurrentSubTransaction() would unlink a relation's
current relfilenode. Introduce rd_firstRelfilenodeSubid. Amend the
specification of rd_createSubid such that the field is zero when a new
rel has an old rd_node. Make relcache.c retain entries for certain
dropped relations until end of transaction.
Bump XLOG_PAGE_MAGIC, since this introduces XLOG_GIST_ASSIGN_LSN.
Future servers accept older WAL, so this bump is discretionary.
Kyotaro Horiguchi, reviewed (in earlier, similar versions) by Robert
Haas. Heikki Linnakangas and Michael Paquier implemented earlier
designs that materially clarified the problem. Reviewed, in earlier
designs, by Andrew Dunstan, Andres Freund, Alvaro Herrera, Tom Lane,
Fujii Masao, and Simon Riggs. Reported by Martijn van Oosterhout.
Discussion: https://postgr.es/m/20150702220524.GA9392@svana.org
Until now, only selected bulk operations (e.g. COPY) did this. If a
given relfilenode received both a WAL-skipping COPY and a WAL-logged
operation (e.g. INSERT), recovery could lose tuples from the COPY. See
src/backend/access/transam/README section "Skipping WAL for New
RelFileNode" for the new coding rules. Maintainers of table access
methods should examine that section.
To maintain data durability, just before commit, we choose between an
fsync of the relfilenode and copying its contents to WAL. A new GUC,
wal_skip_threshold, guides that choice. If this change slows a workload
that creates small, permanent relfilenodes under wal_level=minimal, try
adjusting wal_skip_threshold. Users setting a timeout on COMMIT may
need to adjust that timeout, and log_min_duration_statement analysis
will reflect time consumption moving to COMMIT from commands like COPY.
Internally, this requires a reliable determination of whether
RollbackAndReleaseCurrentSubTransaction() would unlink a relation's
current relfilenode. Introduce rd_firstRelfilenodeSubid. Amend the
specification of rd_createSubid such that the field is zero when a new
rel has an old rd_node. Make relcache.c retain entries for certain
dropped relations until end of transaction.
Back-patch to 9.5 (all supported versions). This introduces a new WAL
record type, XLOG_GIST_ASSIGN_LSN, without bumping XLOG_PAGE_MAGIC. As
always, update standby systems before master systems. This changes
sizeof(RelationData) and sizeof(IndexStmt), breaking binary
compatibility for affected extensions. (The most recent commit to
affect the same class of extensions was
089e4d405d0f3b94c74a2c6a54357a84a681754b.)
Kyotaro Horiguchi, reviewed (in earlier, similar versions) by Robert
Haas. Heikki Linnakangas and Michael Paquier implemented earlier
designs that materially clarified the problem. Reviewed, in earlier
designs, by Andrew Dunstan, Andres Freund, Alvaro Herrera, Tom Lane,
Fujii Masao, and Simon Riggs. Reported by Martijn van Oosterhout.
Discussion: https://postgr.es/m/20150702220524.GA9392@svana.org
This is required as it is no safer for two related processes to extend the
same relation at a time than for unrelated processes to do the same. We
don't acquire a heavyweight lock on any other object after relation
extension lock which means such a lock can never participate in the
deadlock cycle. So, avoid checking wait edges from this lock.
This provides an infrastructure to allow parallel operations like insert,
copy, etc. which were earlier not possible as parallel group members won't
conflict for relation extension lock.
Author: Dilip Kumar, Amit Kapila
Reviewed-by: Amit Kapila, Kuntal Ghosh and Sawada Masahiko
Discussion: https://postgr.es/m/CAD21AoCmT3cFQUN4aVvzy5chw7DuzXrJCbrjTU05B+Ss=Gn1LA@mail.gmail.com
relation extension lock.
The only exception to the rule is that we can try to acquire the same
relation extension lock more than once. This is allowed as we are not
creating any new lock for this case. This restriction implies that the
relation extension lock won't ever participate in the deadlock cycle
because we can never wait for any other heavyweight lock after acquiring
this lock.
Such a restriction is okay for relation extension locks as unlike other
heavyweight locks these are not held till the transaction end. These are
taken for a short duration to extend a particular relation and then
released.
Author: Dilip Kumar, with few changes by Amit Kapila
Reviewed-by: Amit Kapila, Kuntal Ghosh and Sawada Masahiko
Discussion: https://postgr.es/m/CAD21AoCmT3cFQUN4aVvzy5chw7DuzXrJCbrjTU05B+Ss=Gn1LA@mail.gmail.com
The description of the lock type for speculative insertions was
incorrect, being copy-pasted from another one.
As discussed, also move the description for all the fields of lock tag
types from the structure listing lock tag types to the set of macros
setting each LOCKTAG.
Author: John Naylor
Discussion: https://postgr.es/m/CACPNZCtA0-ybaC4fFfaDq_8p_TUOLvGxZH9Dm-=TMHZJarBa7Q@mail.gmail.com
This uses the progress reporting infrastructure added by c16dc1aca5,
adding support for CREATE INDEX and CREATE INDEX CONCURRENTLY.
There are two pieces to this: one is index-AM-agnostic, and the other is
AM-specific. The latter is fairly elaborate for btrees, including
reportage for parallel index builds and the separate phases that btree
index creation uses; other index AMs, which are much simpler in their
building procedures, have simplistic reporting only, but that seems
sufficient, at least for non-concurrent builds.
The index-AM-agnostic part is fairly complete, providing insight into
the CONCURRENTLY wait phases as well as block-based progress during the
index validation table scan. (The index validation index scan requires
patching each AM, which has not been included here.)
Reviewers: Rahila Syed, Pavan Deolasee, Tatsuro Yamada
Discussion: https://postgr.es/m/20181220220022.mg63bhk26zdpvmcj@alvherre.pgsql
Opening a relation with no lock at all is unsafe; there's no guarantee
that we'll see a consistent state of the relevant catalog entries.
While use of MVCC scans to read the catalogs partially addresses that
complaint, it's still possible to switch to a new catalog snapshot
partway through loading the relcache entry. Moreover, whether or not
you trust the reasoning behind sometimes using less than
AccessExclusiveLock for ALTER TABLE, that reasoning is certainly not
valid if concurrent users of the table don't hold a lock corresponding
to the operation they want to perform.
Hence, add some assertion-build-only checks that require any caller
of relation_open(x, NoLock) to hold at least AccessShareLock. This
isn't a full solution, since we can't verify that the lock level is
semantically appropriate for the action --- but it's definitely of
some use, because it's already caught two bugs.
We can also assert that callers of addRangeTableEntryForRelation()
hold at least the lock level specified for the new RTE.
Amit Langote and Tom Lane
Discussion: https://postgr.es/m/16565.1538327894@sss.pgh.pa.us
LockRelationOid and sibling routines supposed that, if our session already
holds the lock they were asked to acquire, they could skip calling
AcceptInvalidationMessages on the grounds that we must have already read
any remote sinval messages issued against the relation being locked.
This is normally true, but there's a critical special case where it's not:
processing inside AcceptInvalidationMessages might attempt to access system
relations, resulting in a recursive call to acquire a relation lock.
Hence, if the outer call had acquired that same system catalog lock, we'd
fall through, despite the possibility that there's an as-yet-unread sinval
message for that system catalog. This could, for example, result in
failure to access a system catalog or index that had just been processed
by VACUUM FULL. This is the explanation for buildfarm failures we've been
seeing intermittently for the past three months. The bug is far older
than that, but commits a54e1f158 et al added a new recursion case within
AcceptInvalidationMessages that is apparently easier to hit than any
previous case.
To fix this, we must not skip calling AcceptInvalidationMessages until
we have *finished* a call to it since acquiring a relation lock, not
merely acquired the lock. (There's already adequate logic inside
AcceptInvalidationMessages to deal with being called recursively.)
Fortunately, we can implement that at trivial cost, by adding a flag
to LOCALLOCK hashtable entries that tracks whether we know we have
completed such a call.
There is an API hazard added by this patch for external callers of
LockAcquire: if anything is testing for LOCKACQUIRE_ALREADY_HELD,
it might be fooled by the new return code LOCKACQUIRE_ALREADY_CLEAR
into thinking the lock wasn't already held. This should be a fail-soft
condition, though, unless something very bizarre is being done in
response to the test.
Also, I added an additional output argument to LockAcquireExtended,
assuming that that probably isn't called by any outside code given
the very limited usefulness of its additional functionality.
Back-patch to all supported branches.
Discussion: https://postgr.es/m/12259.1532117714@sss.pgh.pa.us
Change pg_bsd_indent to follow upstream rules for placement of comments
to the right of code, and remove pgindent hack that caused comments
following #endif to not obey the general rule.
Commit e3860ffa4d wasn't actually using
the published version of pg_bsd_indent, but a hacked-up version that
tried to minimize the amount of movement of comments to the right of
code. The situation of interest is where such a comment has to be
moved to the right of its default placement at column 33 because there's
code there. BSD indent has always moved right in units of tab stops
in such cases --- but in the previous incarnation, indent was working
in 8-space tab stops, while now it knows we use 4-space tabs. So the
net result is that in about half the cases, such comments are placed
one tab stop left of before. This is better all around: it leaves
more room on the line for comment text, and it means that in such
cases the comment uniformly starts at the next 4-space tab stop after
the code, rather than sometimes one and sometimes two tabs after.
Also, ensure that comments following #endif are indented the same
as comments following other preprocessor commands such as #else.
That inconsistency turns out to have been self-inflicted damage
from a poorly-thought-through post-indent "fixup" in pgindent.
This patch is much less interesting than the first round of indent
changes, but also bulkier, so I thought it best to separate the effects.
Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us
The new indent version includes numerous fixes thanks to Piotr Stefaniak.
The main changes visible in this commit are:
* Nicer formatting of function-pointer declarations.
* No longer unexpectedly removes spaces in expressions using casts,
sizeof, or offsetof.
* No longer wants to add a space in "struct structname *varname", as
well as some similar cases for const- or volatile-qualified pointers.
* Declarations using PG_USED_FOR_ASSERTS_ONLY are formatted more nicely.
* Fixes bug where comments following declarations were sometimes placed
with no space separating them from the code.
* Fixes some odd decisions for comments following case labels.
* Fixes some cases where comments following code were indented to less
than the expected column 33.
On the less good side, it now tends to put more whitespace around typedef
names that are not listed in typedefs.list. This might encourage us to
put more effort into typedef name collection; it's not really a bug in
indent itself.
There are more changes coming after this round, having to do with comment
indentation and alignment of lines appearing within parentheses. I wanted
to limit the size of the diffs to something that could be reviewed without
one's eyes completely glazing over, so it seemed better to split up the
changes as much as practical.
Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us
Contention on the relation extension lock can become quite fierce when
multiple processes are inserting data into the same relation at the same
time at a high rate. Experimentation shows the extending the relation
multiple blocks at a time improves scalability.
Dilip Kumar, reviewed by Petr Jelinek, Amit Kapila, and me.
When a process is waiting for a heavyweight lock, we will now indicate
the type of heavyweight lock for which it is waiting. Also, you can
now see when a process is waiting for a lightweight lock - in which
case we will indicate the individual lock name or the tranche, as
appropriate - or for a buffer pin.
Amit Kapila, Ildus Kurbangaliev, reviewed by me. Lots of helpful
discussion and suggestions by many others, including Alexander
Korotkov, Vladimir Borodin, and many others.
This patch introduces "pg_blocking_pids(int) returns int[]", which returns
the PIDs of any sessions that are blocking the session with the given PID.
Historically people have obtained such information using a self-join on
the pg_locks view, but it's unreasonably tedious to do it that way with any
modicum of correctness, and the addition of parallel queries has pretty
much broken that approach altogether. (Given some more columns in the view
than there are today, you could imagine handling parallel-query cases with
a 4-way join; but ugh.)
The new function has the following behaviors that are painful or impossible
to get right via pg_locks:
1. Correctly understands which lock modes block which other ones.
2. In soft-block situations (two processes both waiting for conflicting lock
modes), only the one that's in front in the wait queue is reported to
block the other.
3. In parallel-query cases, reports all sessions blocking any member of
the given PID's lock group, and reports a session by naming its leader
process's PID, which will be the pg_backend_pid() value visible to
clients.
The motivation for doing this right now is mostly to fix the isolation
tests. Commit 38f8bdcac4 lobotomized
isolationtester's is-it-waiting query by removing its ability to recognize
nonconflicting lock modes, as a crude workaround for the inability to
handle soft-block situations properly. But even without the lock mode
tests, the old query was excessively slow, particularly in
CLOBBER_CACHE_ALWAYS builds; some of our buildfarm animals fail the new
deadlock-hard test because the deadlock timeout elapses before they can
probe the waiting status of all eight sessions. Replacing the pg_locks
self-join with use of pg_blocking_pids() is not only much more correct, but
a lot faster: I measure it at about 9X faster in a typical dev build with
Asserts, and 3X faster in CLOBBER_CACHE_ALWAYS builds. That should provide
enough headroom for the slower CLOBBER_CACHE_ALWAYS animals to pass the
test, without having to lengthen deadlock_timeout yet more and thus slow
down the test for everyone else.
Reflow text in lock manager README so that it fits within 80 columns.
Correct some mistakes. Expand the README to explain not only why group
locking exists but also the data structures that support it. Improve
comments related to group locking several files. Change the name of a
macro argument for improved clarity.
Most of these problems were reported by Tom Lane, but I found a few
of them myself.
Robert Haas and Tom Lane
For locking purposes, we now regard heavyweight locks as mutually
non-conflicting between cooperating parallel processes. There are some
possible pitfalls to this approach that are not to be taken lightly,
but it works OK for now and can be changed later if we find a better
approach. Without this, it's very easy for parallel queries to
silently self-deadlock if the user backend holds strong relation locks.
Robert Haas, with help from Amit Kapila. Thanks to Noah Misch and
Andres Freund for extensive discussion of possible issues with this
approach.
Some frontend code like e.g. pg_xlogdump or pg_resetxlog, has to use
backend headers. Unfortunately until now that code includes most of the
locking code. It's generally not nice to expose such low level details,
but de6fd1c898 made that a hard problem. We fall back to defining
'inline' away if the compiler doesn't support it - that can cause linker
errors like on buildfarm animal pademelon if a inline function
references backend only code.
To fix that problem separate definitions from lock.h that are required
from frontend code into lockdefs.h and use it in the relevant
places. I've only removed the minimal amount of necessary definitions
for now - it might turn out that we want more for other reasons.
To avoid such details being exposed again put some checks against being
included from frontend code into atomics.h, lock.h, lwlock.h and
s_lock.h. It's otherwise fairly easy to indirectly include these
headers.
Discussion: 20150806070902.GE12214@awork2.anarazel.de
The newly added ON CONFLICT clause allows to specify an alternative to
raising a unique or exclusion constraint violation error when inserting.
ON CONFLICT refers to constraints that can either be specified using a
inference clause (by specifying the columns of a unique constraint) or
by naming a unique or exclusion constraint. DO NOTHING avoids the
constraint violation, without touching the pre-existing row. DO UPDATE
SET ... [WHERE ...] updates the pre-existing tuple, and has access to
both the tuple proposed for insertion and the existing tuple; the
optional WHERE clause can be used to prevent an update from being
executed. The UPDATE SET and WHERE clauses have access to the tuple
proposed for insertion using the "magic" EXCLUDED alias, and to the
pre-existing tuple using the table name or its alias.
This feature is often referred to as upsert.
This is implemented using a new infrastructure called "speculative
insertion". It is an optimistic variant of regular insertion that first
does a pre-check for existing tuples and then attempts an insert. If a
violating tuple was inserted concurrently, the speculatively inserted
tuple is deleted and a new attempt is made. If the pre-check finds a
matching tuple the alternative DO NOTHING or DO UPDATE action is taken.
If the insertion succeeds without detecting a conflict, the tuple is
deemed inserted.
To handle the possible ambiguity between the excluded alias and a table
named excluded, and for convenience with long relation names, INSERT
INTO now can alias its target table.
Bumps catversion as stored rules change.
Author: Peter Geoghegan, with significant contributions from Heikki
Linnakangas and Andres Freund. Testing infrastructure by Jeff Janes.
Reviewed-By: Heikki Linnakangas, Andres Freund, Robert Haas, Simon Riggs,
Dean Rasheed, Stephen Frost and many others.
This improves on commit bbfd7edae5 by
making two simple changes:
* pg_attribute_noreturn now takes parentheses, ie pg_attribute_noreturn().
Likewise pg_attribute_unused(), pg_attribute_packed(). This reduces
pgindent's tendency to misformat declarations involving them.
* attributes are now always attached to function declarations, not
definitions. Previously some places were taking creative shortcuts,
which were not merely candidates for bad misformatting by pgindent
but often were outright wrong anyway. (It does little good to put a
noreturn annotation where callers can't see it.) In any case, if
we would like to believe that these macros can be used with non-gcc
compilers, we should avoid gratuitous variance in usage patterns.
I also went through and manually improved the formatting of a lot of
declarations, and got rid of excessively repetitive (and now obsolete
anyway) comments informing the reader what pg_attribute_printf is for.
Until now __attribute__() was defined to be empty for all compilers but
gcc. That's problematic because it prevents using it in other compilers;
which is necessary e.g. for atomics portability. It's also just
generally dubious to do so in a header as widely included as c.h.
Instead add pg_attribute_format_arg, pg_attribute_printf,
pg_attribute_noreturn macros which are implemented in the compilers that
understand them. Also add pg_attribute_noreturn and pg_attribute_packed,
but don't provide fallbacks, since they can affect functionality.
This means that external code that, possibly unwittingly, relied on
__attribute__ defined to be empty on !gcc compilers may now run into
warnings or errors on those compilers. But there shouldn't be many
occurances of that and it's hard to work around...
Discussion: 54B58BA3.8040302@ohmu.fi
Author: Oskari Saarenmaa, with some minor changes by me.
This makes it possible to store lwlocks as part of some other data
structure in the main shared memory segment, or in a dynamic shared
memory segment. There is still a main LWLock array and this patch does
not move anything out of it, but it provides necessary infrastructure
for doing that in the future.
This change is likely to increase the size of LWLockPadded on some
platforms, especially 32-bit platforms where it was previously only
16 bytes.
Patch by me. Review by Andres Freund and KaiGai Kohei.
When acquiring a lock in fast-path mode, we must reset the locallock
object's lock and proclock fields to NULL. They are not necessarily that
way to start with, because the locallock could be left over from a failed
lock acquisition attempt earlier in the transaction. Failure to do this
led to all sorts of interesting misbehaviors when LockRelease tried to
clean up no-longer-related lock and proclock objects in shared memory.
Per report from Dan Wood.
In passing, modify LockRelease to elog not just Assert if it doesn't find
lock and proclock objects for a formerly fast-path lock, matching the code
in FastPathGetRelationLockEntry and LockRefindAndRelease. This isn't a
bug but it will help in diagnosing any future bugs in this area.
Also, modify FastPathTransferRelationLocks and FastPathGetRelationLockEntry
to break out of their loops over the fastpath array once they've found the
sole matching entry. This was inconsistently done in some search loops
and not others.
Improve assorted related comments, too.
Back-patch to 9.2 where the fast-path mechanism was introduced.
This patch introduces two additional lock modes for tuples: "SELECT FOR
KEY SHARE" and "SELECT FOR NO KEY UPDATE". These don't block each
other, in contrast with already existing "SELECT FOR SHARE" and "SELECT
FOR UPDATE". UPDATE commands that do not modify the values stored in
the columns that are part of the key of the tuple now grab a SELECT FOR
NO KEY UPDATE lock on the tuple, allowing them to proceed concurrently
with tuple locks of the FOR KEY SHARE variety.
Foreign key triggers now use FOR KEY SHARE instead of FOR SHARE; this
means the concurrency improvement applies to them, which is the whole
point of this patch.
The added tuple lock semantics require some rejiggering of the multixact
module, so that the locking level that each transaction is holding can
be stored alongside its Xid. Also, multixacts now need to persist
across server restarts and crashes, because they can now represent not
only tuple locks, but also tuple updates. This means we need more
careful tracking of lifetime of pg_multixact SLRU files; since they now
persist longer, we require more infrastructure to figure out when they
can be removed. pg_upgrade also needs to be careful to copy
pg_multixact files over from the old server to the new, or at least part
of multixact.c state, depending on the versions of the old and new
servers.
Tuple time qualification rules (HeapTupleSatisfies routines) need to be
careful not to consider tuples with the "is multi" infomask bit set as
being only locked; they might need to look up MultiXact values (i.e.
possibly do pg_multixact I/O) to find out the Xid that updated a tuple,
whereas they previously were assured to only use information readily
available from the tuple header. This is considered acceptable, because
the extra I/O would involve cases that would previously cause some
commands to block waiting for concurrent transactions to finish.
Another important change is the fact that locking tuples that have
previously been updated causes the future versions to be marked as
locked, too; this is essential for correctness of foreign key checks.
This causes additional WAL-logging, also (there was previously a single
WAL record for a locked tuple; now there are as many as updated copies
of the tuple there exist.)
With all this in place, contention related to tuples being checked by
foreign key rules should be much reduced.
As a bonus, the old behavior that a subtransaction grabbing a stronger
tuple lock than the parent (sub)transaction held on a given tuple and
later aborting caused the weaker lock to be lost, has been fixed.
Many new spec files were added for isolation tester framework, to ensure
overall behavior is sane. There's probably room for several more tests.
There were several reviewers of this patch; in particular, Noah Misch
and Andres Freund spent considerable time in it. Original idea for the
patch came from Simon Riggs, after a problem report by Joel Jacobson.
Most code is from me, with contributions from Marti Raudsepp, Alexander
Shulgin, Noah Misch and Andres Freund.
This patch was discussed in several pgsql-hackers threads; the most
important start at the following message-ids:
AANLkTimo9XVcEzfiBR-ut3KVNDkjm2Vxh+t8kAmWjPuv@mail.gmail.com1290721684-sup-3951@alvh.no-ip.org1294953201-sup-2099@alvh.no-ip.org1320343602-sup-2290@alvh.no-ip.org1339690386-sup-8927@alvh.no-ip.org4FE5FF020200002500048A3D@gw.wicourts.gov4FEAB90A0200002500048B7D@gw.wicourts.gov
In situations where there are over 8MB of empty pages at the end of
a table, the truncation work for trailing empty pages takes longer
than deadlock_timeout, and there is frequent access to the table by
processes other than autovacuum, there was a problem with the
autovacuum worker process being canceled by the deadlock checking
code. The truncation work done by autovacuum up that point was
lost, and the attempt tried again by a later autovacuum worker. The
attempts could continue indefinitely without making progress,
consuming resources and blocking other processes for up to
deadlock_timeout each time.
This patch has the autovacuum worker checking whether it is
blocking any other thread at 20ms intervals. If such a condition
develops, the autovacuum worker will persist the work it has done
so far, release its lock on the table, and sleep in 50ms intervals
for up to 5 seconds, hoping to be able to re-acquire the lock and
try again. If it is unable to get the lock in that time, it moves
on and a worker will try to continue later from the point this one
left off.
While this patch doesn't change the rules about when and what to
truncate, it does cause the truncation to occur sooner, with less
blocking, and with the consumption of fewer resources when there is
contention for the table's lock.
The only user-visible change other than improved performance is
that the table size during truncation may change incrementally
instead of just once.
This problem exists in all supported versions but is infrequently
reported, although some reports of performance problems when
autovacuum runs might be caused by this. Initial commit is just the
master branch, but this should probably be backpatched once the
build farm and general developer usage confirm that there are no
surprising effects.
Jan Wieck
Bruce Momjian
Noah Misch
Fujii Masao
Amit Kapila
Peter Eisentraut
Michael Paquier
Tom Lane
Alvaro Herrera
Robert Haas
Andres Freund
Heikki Linnakangas
Peter Eisentraut
Kevin Grittner