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Added more comments to THD wait service

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This commit is contained in:
Mikael Ronstrom
2010-10-27 20:35:01 +02:00
parent 4dff0296f4
commit dea3152c92
1 changed files with 13 additions and 0 deletions
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13
include/mysql/service_thd_wait.h
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@ -50,6 +50,19 @@
extern "C" { extern "C" {
#endif #endif
/*
One should only report wait events that could potentially block for a
long time. A mutex wait is too short of an event to report. The reason
is that an event which is reported leads to a new thread starts
executing a query and this has a negative impact of usage of CPU caches
and thus the expected gain of starting a new thread must be higher than
the expected cost of lost performance due to starting a new thread.
Good examples of events that should be reported are waiting for row locks
that could easily be for many milliseconds or even seconds and the same
holds true for global read locks, table locks and other meta data locks.
Another event of interest is going to sleep for an extended time.
*/
typedef enum _thd_wait_type_e { typedef enum _thd_wait_type_e {
THD_WAIT_SLEEP= 1, THD_WAIT_SLEEP= 1,
THD_WAIT_DISKIO= 2, THD_WAIT_DISKIO= 2,