Re: Reducing bgwriter wakeups

On 20.02.2012 00:18, Robert Haas wrote:
> On Sun, Feb 19, 2012 at 4:11 PM, Simon Riggs<simon(at)2ndquadrant(dot)com> wrote:
>> On Sun, Feb 19, 2012 at 8:15 PM, Robert Haas<robertmhaas(at)gmail(dot)com> wrote:
>>> On Sun, Feb 19, 2012 at 1:53 PM, Simon Riggs<simon(at)2ndquadrant(dot)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.

That seems like swinging the pendulum too much in the other direction,
as others have noted. A simple thing you could do, however, is to only
wake up bgwriter every 10 dirtied pages in the backend or something like
that. That would reduce the wakeups by a factor of 10. Would that be
useful? It's not actually clear to me what the problem you're trying to
solve is.

>>>> 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.

Well, as long as the OS has some clean buffers, as it presumably does if
the system has been idle for a while, bgwriter will catch up very
quickly by simply dumping a large number of dirty pages to the OS. Also,
as the code stands, bgwriter still wakes up every 10 seconds even when
no-one signals it, which makes this a much less likely to happen.

Nevertheless, I also feel that it would be better for bgwriter to be a
bit more proactive than that.

> 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.

True. (but as noted above, bgwriter still wakes up every 10 seconds so
this isn't really an issue at the moment)

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com