Re: Final background writer cleanup for 8.3

"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Greg Smith <gsmith(at)gregsmith(dot)com> writes:
>> In the interest of closing work on what's officially titled the "Automatic
>> adjustment of bgwriter_lru_maxpages" patch, I wanted to summarize where I
>> think this is at ...
>
>> 2) Having backends write their own buffers out does not significantly
>> degrade performance, as those turn into cached OS writes which generally
>> execute fast enough to not be a large drag on the backend.
>
> [ itch... ] That assumption scares the heck out of me. It is doubtless
> true in a lightly loaded system, but once the kernel is under any kind
> of memory pressure I think it's completely wrong. I think designing the
> system around this assumption will lead to something that performs great
> as long as you're not pushing it hard.

I think Heikki's experiments showed it wasn't true for at least some kinds of
heavy loads. However I would expect it to depend heavily on just what kind of
load the machine is under. At least if it's busy writing then I would expect
it to throttle writes. Perhaps in TPCC there are enough reads to throttle the
write rate to something the kernel can buffer.

> If you're still fiddling with it then you probably aren't going to get
> it right in the next few days. Perhaps you should think about whether
> this can be left out entirely for 8.3 and revisited later.

How does all of this relate to your epiphany that we should just have bgwriter
be a full clock sweep ahead clock hand without retracing its steps?

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com