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

From: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, David Steele <david(at)pgmasters(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>
Subject: Re: Fix checkpoint skip logic on idle systems by tracking LSN progress
Date: 2016-12-02 04:20:15
Message-ID: CAB7nPqS-nFO2xKa6R4B-u4KqU_OCPLMGPv+kkAG9Q7zunDtq6w@mail.gmail.com
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On Wed, Nov 30, 2016 at 7:53 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> 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.

Right, this should be reworded a bit better to mention both. Done as attached.
--
Michael

Attachment Content-Type Size
hs-checkpoints-v19.patch invalid/octet-stream 23.5 KB

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