maria-developers team mailing list archive

[Jacob Mathew <jacob.mathew@xxxxxxxxxxx>]

Hi Mattias,

There is an instance of Spider for each partition in the Spider node, so
each instance of Spider handles exactly one partition/shard.  So please
elaborate on what you mean by Spider opening/closing specific partitions.

Note that if we push the partitioning logic into each storage engine, then
there could be one instance of Spider in the Spider node that handles all
partitions.

Thanks,
Jacob

Jacob B. Mathew
Spider and Server Developer
MariaDB Corporation
+1 408 655 8999  (mobile)
jacob.b.mathew    (Skype)
jacob.mathew@xxxxxxxxxxx


On Thu, Apr 26, 2018 at 3:59 AM, Mattias Jonsson <
mattias.jonsson@xxxxxxxxxxx> wrote:

> Hi Holyfoot, Sergei and Jacob,
>
> Thank you for looking into this.
>
> Jacob there are no Jira ticket (that I am aware of) also I do see it as
> two different problems with the same source.
> 1) Spider engine does not handle opening/closing specific partitions (as I
> understand it currently relies on opening/closing all partitions at once in
> order). Currently this is a crashing bug!
> 2) MDEV-11084 makes explicit partition selection work like FLUSH TABLE,
> due to the closing partitions part.
>
> I think it would be nice if you could create tickets for those two issues
> (or at least for the first one).
>
> Regards
> Mattias
>
> On Thu, Apr 26, 2018 at 11:26 AM, Alexey Botchkov <holyfoot@xxxxxxxxxxx>
> wrote:
>
>> No, there's no JIRA task for that problem.
>> So i guess you can create one :)
>>
>> Best regards.
>> HF
>>
>> On Thu, Apr 26, 2018 at 4:50 AM, Jacob Mathew <jacob.mathew@xxxxxxxxxxx>
>> wrote:
>>
>>> Hi Mattias and Holyfoot,
>>>
>>> Is there a Jira bug for this problem?  If not I can create a bug for it
>>> and assign it to Holyfoot.
>>>
>>> Thanks,
>>> Jacob
>>>
>>> Jacob B. Mathew
>>> Spider and Server Developer
>>> MariaDB Corporation
>>> +1 408 655 8999  (mobile)
>>> jacob.b.mathew    (Skype)
>>> jacob.mathew@xxxxxxxxxxx
>>>
>>>
>>> On Tue, Apr 17, 2018 at 9:11 AM, Mattias Jonsson <
>>> mattias.jonsson@xxxxxxxxxxx> wrote:
>>>
>>>> Hi Holyfoot,
>>>>
>>>> On Mon, Apr 16, 2018 at 3:38 PM, Alexey Botchkov <holyfoot@xxxxxxxxxxxx>
>>>> wrote:
>>>> > Hi, Mattias, guys!
>>>> >
>>>> > While investigating the crash, i'd like to discuss that
>>>> >
>>>> >> it seems to close partitions whenever it
>>>> >> is not used in a statement (i.e. require it to be reopened in the
>>>> next
>>>> >> statement that would use another partition
>>>> >
>>>> >
>>>> > Yes, it does that, handling statements with the specified 'PARTITION'
>>>> > option.
>>>> > The patch supposed to solve the problem when there are too many
>>>> partitions
>>>> > opened,
>>>> > so i think it must close the unused partitions sometime.
>>>> > No, it doesn't have to happen that often. I planned to check the
>>>> > table_open_cache
>>>> > variable before the forced close. But decided not to do that
>>>> initially - as
>>>> > it simplified testing,
>>>> > and i thought if someone uses the PARTITION option, he would stick to
>>>> using
>>>> > this partition
>>>> > anyway. And  i forgot about that issue.
>>>>
>>>> The reason for not closing partitions in this case is that it turns
>>>> the 'SELECT col FROM t PARTITION(p)' almost into a 'FLUSH TABLES t'
>>>> which is kind of unexpected.
>>>> I did a test mixing PK selects with and without 'PARTITION (p)' clause
>>>> and that shows it will close all but 1 partition and then (re)open all
>>>> but 1 partition.
>>>>
>>>> Think of the case when a server runs in production with a heavily
>>>> partitioned table serving simple PK queries and then someone runs a
>>>> query with explicit PARTITION selection, then it will introduce a
>>>> short stall. First for the query itself (closing all but 1 partitions)
>>>> and then for the next simple PK query using the same table (opening
>>>> all but 1 partition).
>>>>
>>>> As I read the bug report: the reporter wants to avoid opening all
>>>> partitions. Not that it keeps the partitions open in the table open
>>>> cache (which is an issue on the architectural level of partitioning
>>>> not really fitting into the open table cache).
>>>>
>>>> I attached a diff with the test and results (I also added handler
>>>> status variables to show my point). The diff is against
>>>> b4a2baffa82e5c07b96a1c752228560dcac1359b.
>>>>
>>>> Here is the part of the result file that shows what I mean with extra
>>>> comments prepended by MJ>:
>>>> CREATE TABLE t1 (a int PRIMARY KEY)
>>>> ENGINE = InnoDB
>>>> PARTITION BY HASH (a) PARTITIONS 1000;
>>>> INSERT INTO t1 VALUES (0), (1), (2), (3);
>>>> FLUSH STATUS;
>>>>
>>>> SELECT a FROM t1 PARTITION(p0) WHERE a = 0;
>>>> a
>>>> 0
>>>> SHOW SESSION STATUS
>>>> WHERE VARIABLE_NAME LIKE 'HANDLER_%' AND VALUE > 0;
>>>> Variable_name   Value
>>>> MJ> Here it closes all but one partitions.
>>>> Handler_close   999
>>>>
>>>> SELECT a FROM t1 WHERE a = 0;
>>>> a
>>>> 0
>>>> SHOW SESSION STATUS
>>>> WHERE VARIABLE_NAME LIKE 'HANDLER_%' AND VALUE > 0;
>>>> Variable_name   Value
>>>> Handler_close   1000
>>>> Handler_external_lock   2006
>>>> MJ> and here it re-opens those 'all but one' partitions.
>>>> Handler_open    999
>>>>
>>>>
>>>> Regards
>>>> Mattias
>>>> >
>>>> >
>>>> > Best regards.
>>>> > HF
>>>> >
>>>> >
>>>> >
>>>> >
>>>> > 13.04.2018 19:07, Mattias Jonsson wrote:
>>>> >>
>>>> >> Hi MariaDB Devs,
>>>> >>
>>>> >> I tried to evaluate spider engine and found an issue where it
>>>> crashes,
>>>> >> most likely due to MDEV-11084 (Stacktrace and reproducible test case
>>>> >> attached).
>>>> >>
>>>> >> That also leads me to wonder about the performance for partitioned
>>>> >> tables after MDEV-11084, when it seems to close partitions whenever
>>>> it
>>>> >> is not used in a statement (i.e. require it to be reopened in the
>>>> next
>>>> >> statement that would use another partition, effectively not using the
>>>> >> open table cache).
>>>> >>
>>>> >> I cannot see anything mentioned in the final commit message hinting
>>>> on
>>>> >> that it closes partitions not used in the current query, but in the
>>>> >> previous patches it was mentioned without any reason.
>>>> >>
>>>> >> Why does it close the already opened partitions?
>>>> >> https://github.com/MariaDB/server/blob/10.3/sql/ha_partition
>>>> .cc#L8365
>>>> >>
>>>> >> I would not mind opening the partitions only when they are to be used
>>>> >> (even though there are engines that need to be tested more), but
>>>> >> closing them makes no sense to me performance wise. Also notice that
>>>> >> the partitions first will be put back into the open table cache and
>>>> >> then on the next query the non-used partitions will be closed and the
>>>> >> needed ones be (re)-opened.
>>>> >>
>>>> >> Regards
>>>> >> Mattias Jonsson
>>>> >
>>>> >
>>>> >
>>>> >
>>>>
>>>
>>>
>>
>
>
> --
> Mattias Jonsson
> IT Team Lead - Development
>
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