Re: Reducing bgwriter wakeups

On Sun, Feb 19, 2012 at 4:11 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On Sun, Feb 19, 2012 at 8:15 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Sun, Feb 19, 2012 at 1:53 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>> Recent changes for power reduction mean that we now issue a wakeup
>>> call to the bgwriter every time we set a hint bit.
>>>
>>> However cheap that is, its still overkill.
>>>
>>> My proposal is that we wakeup the bgwriter whenever a backend is
>>> forced to write a dirty buffer, a job the bgwriter should have been
>>> doing.
>>>
>>> This significantly reduces the number of wakeup calls and allows the
>>> bgwriter to stay asleep even when very light traffic happens, which is
>>> good because the bgwriter is often the last process to sleep.
>>>
>>> Seems useful to have an explicit discussion on this point, especially
>>> in view of recent performance results.
>>
>> I don't see what this has to do with recent performance results, so
>> please elaborate.  Off-hand, I don't see any point in getting cheap.
>> It seems far more important to me that the background writer become
>> active when needed than that we save some trivial amount of power by
>> waiting longer before activating it.
>
> Then you misunderstand, since I am advocating waking it when needed.

Well, I guess that depends on when it's actually needed.  You haven't
presented any evidence one way or the other.

I mean, let's suppose that a sudden spike of activity hits a
previously-idle system.  If we wait until all of shared_buffers is
dirty before waking up the background writer, it seems possible that
the background writer is going to have a hard time catching up.  If we
wake it immediately, we don't have that problem.

Also, in general, I think that it's not a good idea to let dirty data
sit in shared_buffers forever.  I'm unhappy about the change this
release cycle to skip checkpoints if we've written less than a full
WAL segment, and this seems like another step in that direction.  It's
exposing us to needless risk of data loss.  In 9.1, if you process a
transaction and, an hour later, the disk where pg_xlog is written
melts into a heap of molten slag, your transaction will be there, even
if you end up having to run pg_resetxlog.  In 9.2, it may well be that
xlog contains the only record of that transaction, and you're hosed.
The more work we do to postpone writing the data until the absolutely
last possible moment, the more likely it is that it won't be on disk
when we need it.

--
Robert Haas
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