Re: Separating bgwriter and checkpointer

On Tue, Sep 20, 2011 at 9:06 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> On 20.09.2011 10:48, Simon Riggs wrote:
>>
>> On Fri, Sep 16, 2011 at 2:38 AM, Fujii Masao<masao(dot)fujii(at)gmail(dot)com>
>>  wrote:
>>>
>>> On Fri, Sep 16, 2011 at 7:53 AM, Simon Riggs<simon(at)2ndquadrant(dot)com>
>>>  wrote:
>>>>
>>>> This patch splits bgwriter into 2 processes: checkpointer and
>>>> bgwriter, seeking to avoid contentious changes. Additional changes are
>>>> expected in this release to build upon these changes for both new
>>>> processes, though this patch stands on its own as both a performance
>>>> vehicle and in some ways a refcatoring to simplify the code.
>>>
>>> I like this idea to simplify the code. How much performance gain can we
>>> expect by this patch?
>>
>> On heavily I/O bound systems, this is likely to make a noticeable
>> difference, since bgwriter reduces I/O in user processes.
>
> Hmm. If the system is I/O bound, it doesn't matter which process performs
> the I/O. It's still the same amount of I/O in total, and in an I/O bound
> system, that's what determines the overall throughput.

That's true, but not relevant.

The bgwriter avoids I/O, if it is operating correctly. This patch
ensures it continues to operate even during heavy checkpoints. So it
helps avoid extra I/O during a period of very high I/O activity.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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