The test case problem stemmed from the fact that a debug sync
signal is a global variable that persists until overwritten
by a new signal. This means that if two different signals
are raised in sequence, a thread waiting for the first signal
might miss it if the second signal sets the global variable
before the thread wakes up.
The solution is to deliver a subsequent signal only after the
waiting thread has received it.
mysql-test/t/query_cache_debug.test:
Wait for signal to be delivered.
Only wait for a single debug signal at a time as the signal state
is global. Also, do not activate the query cache debug sync points
if the thread has no associated THD session.
mysql-test/t/query_cache_debug.test:
Only wait for a single debug signal at a time as the signal state
is global.
sql/sql_cache.cc:
Do not execute the debug sync point if the thread has no associated
THD session. This scenario happens for federated threads.
The problem was that threads waiting on the query cache lock
are not easily seen due to the lack of a state indicating that
the thread is waiting on the said lock. This made it difficult
for users to quickly spot (for example, via SHOW PROCESSLIST)
a query cache contention problem.
The solution is to update the thread state when the query cache
lock needs to be acquired. Whenever the lock is to be acquired,
the thread state is updated to "Waiting for query cache lock"
and is reset once the lock is granted or the wait is interrupted.
The intention is to make query cache related hangs more evident.
To further investigate query cache related locking problems, one
may use PERFORMANCE_SCHEMA to track the overhead associated with
the locking bits and determine which particular lock is being a
contention point.
sql/sql_cache.cc:
Set and reset the thread state whenever a attempt to lock the
query cache is made.
Use DEBUG_SYNC instead of the now unnecessary wait_for_kill hack.
This patch corrects a misstake in the test case for bug patch 43658.
There was a race in the test case when the thread id was retrieved from the processlist.
The result was that the same thread id was signalled twice and one thread id wasn't
signalled at all.
The affected platforms appears to be limited to linux.
mysql-test/r/query_cache_debug.result:
There was a race in the test case when the thread id was retrieved from the processlist.
The result was that the same thread id was signalled twice and one thread id wasn't
signalled at all.
mysql-test/t/query_cache_debug.test:
There was a race in the test case when the thread id was retrieved from the processlist.
The result was that the same thread id was signalled twice and one thread id wasn't
signalled at all.
Early patch submitted for discussion.
It is possible for more than one thread to enter the condition
in query_cache_insert(), but the condition predicate is to
signal one thread each time the cache status changes between
the following states: {NO_FLUSH_IN_PROGRESS,FLUSH_IN_PROGRESS,
TABLE_FLUSH_IN_PROGRESS}
Consider three threads THD1, THD2, THD3
THD2: select ... => Got a writer in ::store_query
THD3: select ... => Got a writer in ::store_query
THD1: flush tables => qc status= FLUSH_IN_PROGRESS;
new writers are blocked.
THD2: select ... => Still got a writer and enters cond in
query_cache_insert
THD3: select ... => Still got a writer and enters cond in
query_cache_insert
THD1: flush tables => finished and signal status change.
THD2: select ... => Wakes up and completes the insert.
THD3: select ... => Happily waiting for better times. Why hurry?
This patch is a refactoring of this lock system. It introduces four new methods:
Query_cache::try_lock()
Query_cache::lock()
Query_cache::lock_and_suspend()
Query_cache::unlock()
This change also deprecates wait_while_table_flush_is_in_progress(). All threads are
queued and put on a conditional wait. On each unlock the queue is signalled. This resolve
the issues with left over threads. To assure that no threads are spending unnecessary
time waiting a signal broadcast is issued every time a lock is taken before a full
cache flush.
mysql-test/r/query_cache_debug.result:
* Added test case for bug43758
mysql-test/t/query_cache_debug.test:
* Added test case for bug43758
sql/sql_cache.cc:
* Replaced calls to wait_while_table_flush_is_in_progress() with
calls to try_lock(), lock_and_suspend() and unlock().
* Renamed enumeration Cache_status to Cache_lock_status.
* Renamed enumeration items to UNLOCKED, LOCKED_NO_WAIT and LOCKED.
If the LOCKED_NO_WAIT lock type is used to lock the query cache, other
threads using try_lock() will fail to acquire the lock.
This is useful if the query cache is temporary disabled due to
a full table flush.
sql/sql_cache.h:
* Replaced calls to wait_while_table_flush_is_in_progress() with
calls to try_lock(), lock_and_suspend() and unlock().
* Renamed enumeration Cache_status to Cache_lock_status.
* Renamed enumeration items to UNLOCKED, LOCKED_NO_WAIT and LOCKED.
If the LOCKED_NO_WAIT lock type is used to lock the query cache, other
threads using try_lock() will fail to acquire the lock.
This is useful if the query cache is temporary disabled due to
a full table flush.
The problem is that select queries executed concurrently with
a concurrent insert on a MyISAM table could be cached if the
select started after the query cache invalidation but before
the unlock of tables performed by the concurrent insert. This
race could happen because the concurrent insert was failing
to prevent cache of select queries happening at the same time.
The solution is to add a 'uncacheable' status flag to signal
that a concurrent insert is being performed on the table and
that queries executing at the same time shouldn't cache the
results.
mysql-test/r/query_cache_debug.result:
Add test case result for Bug#41098
mysql-test/t/disabled.def:
Re-enable test case.
mysql-test/t/query_cache_debug.test:
Add test case for Bug#41098
sql/sql_cache.cc:
Debug sync point for regression testing purposes.
sql/sql_insert.cc:
Remove meaningless query cache invalidate. There is already
a preceding invalidate for queries that started before the
concurrent insert.
storage/myisam/ha_myisam.cc:
Check for a active concurrent insert.
storage/myisam/mi_locking.c:
Signal the start of a concurrent insert. Flag is zeroed once
the state is updated back.
storage/myisam/myisamdef.h:
Add flag to signal a active concurrent insert.
Corrects build problems embedded on Windows
Makefile.am:
Install .sym or mysqld-debug if exists
query_cache_debug.test, query_cache_debug.result:
Set more resonable query cache size (bug#35749)
CMakeLists.txt:
Added missing stacktrace.c
mysql-test/r/query_cache_debug.result:
Set more resonable query cache size (bug#35749)
mysql-test/t/query_cache_debug.test:
Set more resonable query cache size (bug#35749)
libmysqld/CMakeLists.txt:
Added missing stacktrace.c
sql/Makefile.am:
Install .sym or mysqld-debug if exists
sql/mysqld.cc:
Corrects build problems embedded on Windows
mysql-test/r/query_cache.result:
Moved test which requires debug binaries to a new file.
mysql-test/t/query_cache.test:
Moved test which requires debug binaries to a new file.
mysql-test/r/query_cache_debug.result:
Moved test which requires debug binaries to a new file.
mysql-test/t/query_cache_debug.test:
Moved test which requires debug binaries to a new file.
