Hi!
On Feb 17, 2009, at 6:23 AM, MARK CALLAGHAN wrote:
the THD object caches the pointer to the HASH entry for the user and
the global counter value at which the pointer is valid. When FLUSH
Yep. You can think for the most part that when we say Session... you
can think THD (though there are some obvious differences in ours since
they are connected to the executing thread). When the session is
created we just tell it the key it is to use in scoreboard.
As far as per table stats go... I've been wondering about just leaving
this to the engines and instead requesting data from them. I've not
really put enough thoughts into this yet (this is for work past April).
to check that the pointer is still valid and update fields. Most of
the field updates could be done lock-free except for one. We have a
Concurrent_connections field that must be correct. Of course atomic
ops could provide that.
I want to avoid all locks for update and for session creation (aka...
people write apps that connect/disconnect... I want minimal penalty
for doing that). For update that means a read will be dirty, but I
believe that should be fine. For update this is not an issue since a
session is unique within the system.
I've not entirely thought of a good method yet for getting the slot in
the scoreboard for the connection, other then using the atomic value
of "connection number" and relating this back to the scoreboard. That
still requires an ATOMIC,... and those are not entirely free from cost.
BTW there is still in Drizzle a silly lock for SHOW PROCESSLIST to
show the current "SQL" being executed. There is some refactoring I
need to do to get rid of that.
Cheers,
-Brian
--
_______________________________________________________
Brian "Krow" Aker, brian at tangent.org
Seattle, Washington
http://krow.net/ <-- Me
http://tangent.org/ <-- Software
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You can't grep a dead tree.
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