On Sat, Feb 18, 2012 at 7:35 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Feb 14, 2012 at 3:25 PM, Greg Smith <greg@2ndquadrant.com> wrote:
>> On 02/14/2012 01:45 PM, Greg Smith wrote:
>>>
>>> scale=1000, db is 94% of RAM; clients=4
>>> Version TPS
>>> 9.0 535
>>> 9.1 491 (-8.4% relative to 9.0)
>>> 9.2 338 (-31.2% relative to 9.1)
>>
>> A second pass through this data noted that the maximum number of buffers
>> cleaned by the background writer is <=2785 in 9.0/9.1, while it goes as high
>> as 17345 times in 9.2. The background writer is so busy now it hits the
>> max_clean limit around 147 times in the slower[1] of the 9.2 runs. That's
>> an average of once every 4 seconds, quite frequent. Whereas max_clean
>> rarely happens in the comparable 9.0/9.1 results. This is starting to point
>> my finger more toward this being an unintended consequence of the background
>> writer/checkpointer split.
>
> I guess the question that occurs to me is: why is it busier?
>
> It may be that the changes we've made to reduce lock contention are
> allowing foreground processes to get work done faster. When they get
> work done faster, they dirty more buffers, and therefore the
> background writer gets busier. Also, if the background writer is more
> reliably cleaning pages even during checkpoints, that could have the
> same effect. Backends write fewer of their own pages, therefore they
> get more real work done, which of course means dirtying more pages.
The checkpointer/bgwriter split allows the bgwriter to do more work,
which is the desired outcome, not an unintended consequence.
The general increase in performance means there is more work to do. So
both things mean there is more bgwriter activity.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
