Обсуждение: Re: [GENERAL] Current best practice for maximum shared_bufferssettings on big hardware?

A few years ago, I was working with "big" servers. At least, they were
big for that age, with *128G* of RAM!!!1 Holy mackeral, right?!!?

Anyway, at that time, I tried allocating 64G to shared buffers and we
had a bunch of problems with inconsistent performance, including "stall"
periods where the database would stop responding for 2 or 3 seconds.
After trying all sorts of tuning options that didn't help, the problem
finally went away after reducing shared_buffers to 32G. I speculated, at
the time, that the shared buffer code hit performance issues managing
that much memory, but I never had the opportunity to really follow up
on it.

Now, this was back in 2012 or thereabouts. Seems like another lifetime.
Probably PostgreSQL 9.2 at that time.

Nowadays, 128G is a "medium sized" server. I just got access to one
with 775G. It would appear that I could order from Dell with 1.5T of
RAM if I'm willing to sell my house ...

Yet, all the docs and advice I'm able to find online seem to have been
written pre 2008 and say things like "if your server has more than 1G
of RAM ..."

I feel like it's time for a documentation update ;) But I, personally
don't have the experience recently enough to know what sort of
recommendations to make.

What are people's experience with modern versions of Postgres on hardware
this size? Do any of the experts have specific recommendations on large
shared_buffers settings? Any developers care to comment on any work
that's been done since 2012 to make large values work better?

--
Bill Moran <wmoran@potentialtech.com>


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On Wed, 24 May 2017 21:02:45 +0800
stevenchang1213 <stevenchang1213@gmail.com> wrote:

> hello, at most 40% total memory, official doc also says so.

The docs say, "it is unlikely that an allocation of more than 40% of RAM
to shared_buffers will work better than a smaller amount" which is a far
cry from defining a maximum amount, or even warning that there are dangers
for high values. Also, that is in the same paragraph that starts with
"If you have a dedicated database server with 1GB or more of RAM ..." so
I don't need to check the RCS logs to predict that that paragraph hasn't
been updated in a while.

> you can testify it using pg_prewarm and pgfincore.

Not sure how those tools are going to predict whether I'm going to see
database stalls or other performance inversions from adding more
shared_buffers. For the purposes of seeing if shared_buffers are being
used effectively, I need only graph the block hits and misses to see
that a huge number of cache pages are satisfying requests, but also that
the amount of cache misses is still high enough for me to know that my
working set does _not_ fit in shared_buffers. What I _don't_ know is
whether increasing shared_buffers (to say, 128G on a 750G machine) is
still going to result in the same, weird performance inversion I saw
back in the 9.2 days.

> btw, numa supported? if so, extra care is necessary when starting db cluster.

Egad how I hate Linux's default NUMA policy. But I do know how to manage
it, and it's not part of the issue.

> 從我的 Samsung Galaxy 智慧型手機傳送。
> -------- 原始訊息 --------自： Bill Moran <wmoran@potentialtech.com> 日期： 2017/5/24  20:24  (GMT+08:00) 至：
pgsql-general@postgresql.org主旨： [GENERAL] Current best practice for maximum shared_buffers settings on big hardware?  
>
> A few years ago, I was working with "big" servers. At least, they were
> big for that age, with *128G* of RAM!!!1 Holy mackeral, right?!!?
>
> Anyway, at that time, I tried allocating 64G to shared buffers and we
> had a bunch of problems with inconsistent performance, including "stall"
> periods where the database would stop responding for 2 or 3 seconds.
> After trying all sorts of tuning options that didn't help, the problem
> finally went away after reducing shared_buffers to 32G. I speculated, at
> the time, that the shared buffer code hit performance issues managing
> that much memory, but I never had the opportunity to really follow up
> on it.
>
> Now, this was back in 2012 or thereabouts. Seems like another lifetime.
> Probably PostgreSQL 9.2 at that time.
>
> Nowadays, 128G is a "medium sized" server. I just got access to one
> with 775G. It would appear that I could order from Dell with 1.5T of
> RAM if I'm willing to sell my house ...
>
> Yet, all the docs and advice I'm able to find online seem to have been
> written pre 2008 and say things like "if your server has more than 1G
> of RAM ..."
>
> I feel like it's time for a documentation update ;) But I, personally
> don't have the experience recently enough to know what sort of
> recommendations to make.
>
> What are people's experience with modern versions of Postgres on hardware
> this size? Do any of the experts have specific recommendations on large
> shared_buffers settings? Any developers care to comment on any work
> that's been done since 2012 to make large values work better?
>
> --
> Bill Moran <wmoran@potentialtech.com>
>
>
> --
> Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general


--
Bill Moran <wmoran@potentialtech.com>