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ChangeSet@stripped, 2007-08-14 15:52:46+02:00, thek@adventure.(none) +1 -0
Bug #21074 Large query_cache freezes mysql server sporadically under heavy load
Invaldating a subset of a sufficiently large query cache can take a long time.
During this time the server is efficiently frozen and no other operation can
be executed. This patch addresses this problem by setting a time limit on
how long time a dictionary access request can take before giving up on the
attempt. This patch does not work for query cache invalidations issued by
DROP, ALTER or RENAME TABLE operations.
sql/sql_cache.cc@stripped, 2007-08-14 15:52:44+02:00, thek@adventure.(none) +27 -1
Changed mutex locking to a timed spinn lock. If access to query cache dictionary
takes "a long time" (currently more than 0.1 seconds) the system fall backs on
ordinary statement execution.
diff -Nrup a/sql/sql_cache.cc b/sql/sql_cache.cc
--- a/sql/sql_cache.cc 2007-05-10 14:54:53 +02:00
+++ b/sql/sql_cache.cc 2007-08-14 15:52:44 +02:00
@@ -1023,6 +1023,13 @@ Query_cache::send_result_to_client(THD *
Query_cache_block_table *block_table, *block_table_end;
ulong tot_length;
Query_cache_query_flags flags;
+ const uint spin_treshold= 50000;
+ const double lock_time_treshold= 0.1; /* Time in seconds */
+ uint spin_count= 0;
+ int lock_status= 0;
+ ulong new_time= 0;
+ ulong stop_time= 0;
+
DBUG_ENTER("Query_cache::send_result_to_client");
/*
@@ -1069,7 +1076,26 @@ Query_cache::send_result_to_client(THD *
}
}
- STRUCT_LOCK(&structure_guard_mutex);
+ stop_time= my_clock()+(ulong)lock_time_treshold*CLOCKS_PER_SEC;
+ while ((lock_status= pthread_mutex_trylock(&structure_guard_mutex)) == EBUSY
+ && spin_count < spin_treshold
+ && new_time < stop_time)
+ {
+ spin_count++;
+ if (spin_count%5)
+ new_time= my_clock();
+ pthread_yield();
+ }
+
+ if (lock_status != 0)
+ {
+ /*
+ Query cache is too busy doing something else.
+ Fall back on ordinary statement execution.
+ */
+ DBUG_RETURN(0);
+ }
+
if (query_cache_size == 0 || flush_in_progress)
{
DBUG_PRINT("qcache", ("query cache disabled"));
