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From sebb <seb...@gmail.com>
Subject Re: doc for jdbc pool configuration
Date Wed, 06 Jan 2016 11:04:31 GMT
On 6 January 2016 at 05:37, Felix Schumacher
<felix.schumacher@internetallee.de> wrote:
>
>
> Am 5. Januar 2016 23:38:45 MEZ, schrieb sebb <sebbaz@gmail.com>:
>>On 5 January 2016 at 21:19, Felix Schumacher
>><felix.schumacher@internetallee.de> wrote:
>>> Am 04.01.2016 um 16:21 schrieb sebb:
>>>>
>>>> On 2 January 2016 at 15:38, Felix Schumacher
>>>> <felix.schumacher@internetallee.de> wrote:
>>>>>
>>>>> Hi all,
>>>>>
>>>>> in our documentation there is one section about the sizing of the
>>pool,
>>>>> which I like to discuss:
>>>>>
>>>>> "If you really want to use shared pooling (why?), then set the max
>>count
>>>>> to
>>>>> the same as the number of threads to ensure threads don't wait on
>>each
>>>>> other."
>>>>>
>>>>> First: I think pooling is a valid option even for user-centric
>>scenarios.
>>>>> Think about simulating the sql requests of an application server.
>>In such
>>>>> a
>>>>> case a pool would have been used, so why not when simulating it?
>>>>>
>>>>> So for this part, I question the part "...(why?)..." and the
>>"really" in
>>>>> front of it.
>>>>
>>>> The problem is that JMeter is then mainly testing the pool
>>>> implementation rather than the server.
>>>> Since the pool implementation cannot at present be replaced, that
>>does
>>>> not seem to be a useful approach.
>>>
>>> In my observations the actual pooling part in jmeter is rather low
>>overhead
>>> compared to jdbc or other jmeter code, so I think the pooling
>>implementation
>>> is not that relevant compared to the fact to use a pool.
>>>
>>> I have done some totally unscientific tests using a local postgresql
>>> instance with a little database and a simple select (with two joins,
>>so a
>>> bit more involved than just "select 1"). The laptop had four cores
>>and
>>> enough memory not to swap.
>>>
>>> For 600 threads 100 loops and different pool sizes I got the
>>following
>>> results:
>>>
>>> pool size 0 (every thread has its own pool)
>>> summary = 120000 in 00:02:28 =  811,0/s Avg:   463 Min:     2 Max:
>>17700
>>> Err:     0 (0,00%)
>>> memory went quite low and load went up to 400
>>>
>>> pool size 64
>>> summary = 120000 in 00:02:22 =  844,8/s Avg:   638 Min:     2 Max:
>>18797
>>> Err:     0 (0,00%)
>>>
>>> pool size 32
>>> summary = 120000 in 00:02:22 =  843,7/s Avg:   633 Min:     2 Max:
>>15679
>>> Err:     0 (0,00%)
>>>
>>> pool size 16
>>> summary = 120000 in 00:02:23 =  840,9/s Avg:   653 Min:     2 Max:
>>13607
>>> Err:     0 (0,00%)
>>>
>>> pool size 8
>>> summary = 120000 in 00:02:22 =  842,8/s Avg:   653 Min:     2 Max:
>>16502
>>> Err:     0 (0,00%)
>>>
>>> pool size 4
>>> summary = 120000 in 00:02:22 =  843,7/s Avg:   674 Min:     2 Max:
>>8114 Err:
>>> 0 (0,00%)
>>>
>>> pool size 2
>>> summary = 120000 in 00:02:49 =  709,6/s Avg:   801 Min:     2 Max:
>>9215 Err:
>>> 0 (0,00%)
>>>
>>> My interpretation is: a pool helps to keep the db happy, but it has
>>to be
>>> sized appropriately (whatever that means)
>>>
>>> I think the result would be even more in favour of the pool, if the
>>db would
>>> not have fitted in RAM and would induce a io bottleneck by the
>>parallel db
>>> processes. Plus, I had to configure my postgresql instance to allow
>>more
>>> than 100 simultaneous connections.
>>
>>I don't doubt that using a pool helps.
>>
>>>>
>>>>
>>>>> Second: When a pool is used (at least the dbcp2 pool) the
>>connections
>>>>> seem
>>>>> to be stored in a stack like construct. So in a uncontended load
>>>>> situation
>>>>> the pool will use only a fraction of the configured size and in a
>>>>> contended
>>>>> situation with really many threads it will probably overload the
>>db.
>>>>
>>>> That suggests that pooling should not be used by JMeter...
>>>
>>> Why? When we are not using a pool, JMeter will overload the db also.
>>>>
>>>>
>>>>> So all in all I would rather change the sentence to something like
>>"If
>>>>> you
>>>>> want to use shared pooling, then set the max count to something
>>>>> sensible".
>>>>
>>>> I think we need to document why pooling is not in general a good
>>idea
>>>> for JMeter tests.
>>>
>>> I would rather document why and when pooling should be used.
>>>
>>> I have at least two use cases where pooling is useful:
>>>  * using the jdbc sampler to fill variables which are then used
>>further in
>>> the test
>>
>>That is possibly a valid use case, however this should be done in a
>>setUp ThreadGroup, so performance is not so much of an issue.
>>Besides, if the setup takes so long that it needs a pool, it should
>>probably be done using a database bulk-load facility.
>
> I would use it like a csv reader on steroids and thus it wouldn't help to put it into
a setup group.

I see. That makes more sense.

So you could configure the pool to have fewer than the number of
threads to reduce the max DB load.
Since the threads are presumably doing other things between JDBC
calls, it's unlikely that they will all be wanting to access the DB
simultaneously, so waits will be uncommon.
In this case the pool implementation is not important so long as it works OK.
Another approach would be to create a new test element and put the
pooling there instead.
That would likely be much easier to use.

For database population, where threads are likely to spend almost all
of their time updating the database, a pool won't really help.
Just configure the thread count to the max desired DB connections.

Pooling still does not make sense for testing app servers unless the
appropriate pool implementation can be specified.

>>
>>>  * simulating the jdbc statements of an application server, which
>>would use
>>> a pool in itself
>>
>>In which case, it is important to be able to use the same pool as the
>>application server.
>
> It would be nicer for sure, but no pool would be even worse.
>
>>
>>>>
>>>>
>>>> However I would prefer to drop pooling support entirely unless the
>>>> pool implementation can be provided by the user.
>>>
>>> The option of making the pool implementation configurable is open to
>>us.
>>>
>>> Regards,
>>>  Felix
>>>>
>>>>
>>>>> Regards,
>>>>>   Felix
>>>
>>>
>

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