Hello,
Please see some comments inline.
Best Regards,
Alexei Kornienko
On 03/21/2014 11:11 PM, Joe Gordon wrote:
On Fri, Mar 21, 2014 at 4:04 AM, Sean Dague <s...@dague.net
<mailto:s...@dague.net>> wrote:
On 03/20/2014 06:18 PM, Joe Gordon wrote:
>
>
>
> On Thu, Mar 20, 2014 at 3:03 PM, Alexei Kornienko
> <alexei.kornie...@gmail.com <mailto:alexei.kornie...@gmail.com>
<mailto:alexei.kornie...@gmail.com
<mailto:alexei.kornie...@gmail.com>>> wrote:
>
> Hello,
>
> We've done some profiling and results are quite interesting:
> during 1,5 hour ceilometer inserted 59755 events (59755 calls to
> record_metering_data)
> this calls resulted in total 2591573 SQL queries.
>
> And the most interesting part is that 291569 queries were
ROLLBACK
> queries.
> We do around 5 rollbacks to record a single event!
>
> I guess it means that MySQL backend is currently totally
unusable in
> production environment.
>
>
> It should be noticed that SQLAlchemy is horrible for performance, in
> nova we usually see sqlalchemy overheads of well over 10x (time
> nova.db.api call vs the time MySQL measures when slow log is
recording
> everything).
That's not really a fair assessment. Python object inflation takes
time.
I do get that there is SQLA overhead here, but even if you trimmed it
out you would not get the the mysql query time.
To give an example from nova:
doing a nova list with no servers:
stack@devstack:~/devstack$ nova --timing list
| GET
http://10.0.0.16:8774/v2/a82ededa9a934b93a7184d06f302d745/servers/detail
| 0.0817470550537 |
So nova command takes 0.0817470550537 seconds.
Inside the nova logs (when putting a timer around all nova.db.api
calls [1] ), nova.db.api.instance_get_all_by_filters takes 0.06 seconds:
2014-03-21 20:58:46.760 DEBUG nova.db.api
[req-91879f86-7665-4943-8953-41c92c42c030 demo demo]
'instance_get_all_by_filters' 0.06 seconds timed
/mnt/stack/nova/nova/db/api.py:1940
But the sql slow long reports the same query takes only 0.001006
seconds with a lock_time of 0.000269 for a total of 0.00127 seconds.
# Query_time: 0.001006 Lock_time: 0.000269 Rows_sent: 0
Rows_examined: 0
So in this case only 2% of the time
that nova.db.api.instance_get_all_by_filters takes is spent inside of
mysql. Or to put it differently
nova.db.api.instance_get_all_by_filters is 47 times slower then the
raw DB call underneath.
Yes I agree that that turning raw sql data into python objects should
take time, but I just don't think it should take 98% of the time.
If you would open actual code of nova.db.api.instance_get_all_by_filters
-
https://github.com/openstack/nova/blob/master/nova/db/sqlalchemy/api.py#L1817
You will find out that python code is actually doing lot's of things:
1) setup join conditions
2) create query filters
3) doing some heavy matching, loops in exact_filter, regex_filter,
tag_filter
This code won't go away with python objects since it's related to
busyness logic.
I think that it's quite hypocritical to say that the problem is "turning
raw sql data into python objects"
[1]
https://github.com/jogo/nova/commit/7743ee366bbf8746f1c0f634f29ebf73bff16ea1
That being said, having Ceilometer's write path be highly tuned
and not
use SQLA (and written for every back end natively) is probably
appropriate.
While I like this idea, they loose free postgresql support by dropping
SQLA. But that is a solvable problem.
-Sean
--
Sean Dague
Samsung Research America
s...@dague.net <mailto:s...@dague.net> / sean.da...@samsung.com
<mailto:sean.da...@samsung.com>
http://dague.net
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