Re: Fix checkpoint skip logic on idle systems by tracking LSN progress

On Mon, Nov 21, 2016 at 11:08 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> On Tue, Nov 15, 2016 at 9:59 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> On Tue, Nov 15, 2016 at 9:27 AM, Kyotaro HORIGUCHI
>> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>> The term "WAL activity' is used in the comment for
>>> GetProgressRecPtr. Its meaning is not clear but not well
>>> defined. Might need a bit detailed explanation about that or "WAL
>>> activity tracking". What do you think about this?
>>>
>>
>> I would have written it as below:
>>
>> GetProgressRecPtr -- Returns the WAL progress. WAL progress is
>> determined by scanning each WALinsertion lock by taking directly the
>> light-weight lock associated to it.
>
> Not sure if that's better.. What about something as fancy as that?
> /*
> - * Get the time of the last xlog segment switch
> + * GetProgressRecPtr -- Returns the newest WAL progress position. WAL
> + * progress is determined by scanning each WALinsertion lock by taking
> + * directly the light-weight lock associated to it. The result of this
> + * routine can be compared with the last checkpoint LSN to check if
> + * a checkpoint can be skipped or not.
> + *
> It may be worth mentioning that the result of this routine is
> basically used for checkpoint skip logic.
>

That's okay, but I think you are using it to skip switch segment stuff
as well. Today, again going through patch, I noticed small anomaly

> + * Switch segment only when WAL has done some progress since the
+ * > last time a segment has switched because of a timeout.

> + if (GetProgressRecPtr() > last_switch_lsn)

Either the above comment is wrong or the code after it has a problem.
last_switch_lsn aka XLogCtl->lastSegSwitchLSN is updated not only for
a timeout but also when there is a lot of WAL activity which makes WAL
Write to cross a segment boundary.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com