Bug #38924 LOCK_mdl needs to cool down
Submitted: 20 Aug 2008 22:21 Modified: 17 Sep 2009 11:47
Reporter: Neel Nadgir
Status: Analyzing
Category:Server: Locking Severity:S5 (Performance)
Version:6.0.7-alpha OS:Any
Assigned to: Alexey Stroganov Target Version:
Tags: mdl, performance

[20 Aug 2008 22:21] Neel Nadgir 
Description:
LOCK_mdl is hot for simple sysbench read-only benchmarks. Instrumenting the code with
timing calls shows that around 10% of the query execution time is spent acquiring
LOCK_mdl. 

How to repeat:
Build mysql 6.0.7-alpha, and instrument calls to acquire and release LOCK_mdl in
sql/mdl.cc. Then run sysbench readonly test on a system with > 8 cpus with threads >
number of cpus. Alternatively (on opensolaris), you can estimate time taken by
mdl_acquire_shared_lock() using dtrace, or get a rough estimate of the contention using
plockstat.
[17 Sep 2009 11:47] Philip Stoev 
Ranger, kindly please take ownership over this benchmarking issue related to sysbench and
metadata locking. Please try to repeat this bug and obtain a performance profile. In
addition, Kostja said the following:

(12:44:56 PM) kostja: I need a profile, though.
(12:45:11 PM) kostja: I would like to understand what action to take:
(12:45:18 PM) kostja: - to reduce the body of the critical section
(12:45:21 PM) kostja: - to add spin loops
(12:45:27 PM) kostja: - to split the mutex
(12:45:30 PM) kostja: and so and so
(12:45:43 PM) kostja: and I need to understand how critical it is:
(12:45:51 PM) kostja: is the situation, overall, worse or better than in 5.1?
(12:46:04 PM) kostja: adding this information to the bug report
[20 Jan 11:17] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/97520

3057 Konstantin Osipov	2010-01-20
      Bug#46272/Bug#38924 review fixes in progress.
      Rename a few members, realign comments to fit 66 characters, 
      update comments.
[20 Jan 11:42] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/97529

3058 Konstantin Osipov	2010-01-20
      Bug#46272/Bug#38924 review fixes in progress.
      Update MDL_key class to be more compact.
      Simplify the comparison method, used when sorting MDL_keys.
[20 Jan 12:20] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/97535

3059 Konstantin Osipov	2010-01-20
      Bug#46272/Bug#38924 review fixes in progress.
      Misc fixes.
[20 Jan 13:31] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/97548

3060 Konstantin Osipov	2010-01-20
      Bug#46272/Bug#38924 review fixes in progress.
      Introduce class MDL_map to hold all MDL locks
      taken in the server.
[20 Jan 14:44] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/97578

3061 Konstantin Osipov	2010-01-20
      Bug#46272/Bug#38924 review fixes in progress.
      Misc. fixes.