Re: [GENERAL] Upgrade to dual processor machine?

ok, now i subscribed to performance, too ;-)

> Well, now it gets more complicated.   You need to determine:
> A) The median processing time of each of those requests.
> B) The amount of Sort_mem actually required for each request.

as I am dealing with postgres for a webserver the median processing
time of each request has got to be <1 sec. how can i measure
the amount of sort_mem needed?

at the highest there are perhaps 20 concurrent database requests
at the same time. i have enabled 64 maximum connections, because
i have apache configured to use persistent database connections
using mod_perl and pg.pm. I set Apache to run MaxClients at 40
(there could additionally be some manual psql connections)

> Um, you do know that I'm talking about *database* requests -- that is,
> queries -- and not web page requests, yes?  If you're using server-side
> caching, there can be a *huge* difference.

yes, I did understand that. And when I measured 5.000 requests per minute
I looked at the pgsql.log (after enabling the debug options and counting
all the queries within minute). so, server-side caching does not appear
within these 5.000 requests... that's for sure.

> If you have 5000 requests per minute, and only 64 connections, then I
> can hypothesize that:
> 1) you are doing some kind of connection pooling;
> 2) those are exclusively *read-only* requests;
> 3) those are very simple requests, or at least processed very quickly

1) correct
2) no, not exclusively - but as it's a webserver-application (www.city-map.de)
most users just read from the database, while they always do an update
to raise some statistics (page-views counters etc.) - furthermore, there is
an internal content-management system where about 100 editors do inserts and
updates. but there are of course more visitors (>10.000 daily) than editors.
3) yes, many requests are very simple for better performance in a web-application

Swapping does never happen so far.


--

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Henrik Steffen
Geschäftsführer

top concepts Internetmarketing GmbH
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----- Original Message -----
From: "Josh Berkus" <josh@agliodbs.com>
To: "Henrik Steffen" <steffen@city-map.de>; <josh@agliodbs.com>
Cc: <pgsql-general@postgresql.org>; <pgsql-performance@postgresql.org>
Sent: Wednesday, November 13, 2002 6:05 PM
Subject: Re: [GENERAL] Upgrade to dual processor machine?


> Henrik,
>
> First off, I'm moving this discussion to the PGSQL-PERFORMANCE list,
> where it belongs.   To subscribe, send the message "subscribe
> pgsql-perform your@email.address" to "majordomo@postgresql.org".
>
> > This is was I figured out now:
> >
> > 1) RAM available: 1024 MB, there's nothing else but postgres on this
> >    machine, so if I calculate 128 MB for Linux, there are 896 MB left
> >    for Postgres.
> >
> > 2) 70 % of 896 MB is 627 MB
> >
> > Now, if I follow your instructions:
> >
> > 250K +
> > 8.2K * 128 (shared_buffers) = 1049,6K +
> > 14.2K * 64 (max_connections) = 908,8K +
> > 1024K * 5000 (average number of requests per minute) = 5120000K
> > ===============================================================
> > 5122208.4K ==> 5002.16 MB
> >
> > this is a little bit more than I have available, isn't it? :(((
> >
> > sure that this has got to be the "average number of requests per
> > minute"
> > and not "per second" ? seems so much, doesn't it?
> >
> > what am I supposed to do now?
>
> Well, now it gets more complicated.   You need to determine:
> A) The median processing time of each of those requests.
> B) The amount of Sort_mem actually required for each request.
>
> I reccommend "per minute" because that's an easy over-estimate ... few
> requests last a full minute, and as a result
> average-requests-per-minute gives you a safe guage of maximum
> concurrent requests (in transactional database environments), which is
> really what we are trying to determine.
>
> Um, you do know that I'm talking about *database* requests -- that is,
> queries -- and not web page requests, yes?  If you're using server-side
> caching, there can be a *huge* difference.
>
> If you have 5000 requests per minute, and only 64 connections, then I
> can hypothesize that:
> 1) you are doing some kind of connection pooling;
> 2) those are exclusively *read-only* requests;
> 3) those are very simple requests, or at least processed very quickly.
>
> If all of the above is true, then you can probably base you calculation
> on requests-per-second, rather than requests-per-minute.
>
> Then, of course, it becomes an interactive process.  You change the
> settings, re-start the database server, and watch the memory used by
> the postgreSQL processes.   Your goal is to have that memory usage
> hover around 700mb during heavy usage periods (any less, and you are
> throttling the database through scarcity of RAM)  but to never, ever,
> force usage of Swap memory, which will slow down the server 10-fold.
>
> If you see the RAM only at half that, but the processor at 90%+, then
> you should consider upgrading your processor.  But you're more likely
> to run out of RAM first.    I believe that you haven't already because
> with your low shared-buffer settings, most of the potential sort_mem is
> going unused.
>
> BTW, if you are *really* getting 5000 queries per minute, I would
> strongly reccomend doubling your RAM.
>
> -Josh Berkus
>
>
> >
> > ----- Original Message -----
> > From: "Josh Berkus" <josh@agliodbs.com>
> > To: <pgsql-general@postgresql.org>
> > Cc: <steffen@city-map.de>
> > Sent: Tuesday, November 12, 2002 9:05 PM
> > Subject: Re: Upgrade to dual processor machine?
> >
> >
> > Heinrik,
> >
> > "So, where do i find and change shmmax shmall settings ??
> > What should I put there?
> >
> > What is a recommended value for shared buffers in postgresql.conf ?"
> >
> > There is no "recommended value."   You have to calculate this
> > relatively:
> >
> > 1) Figure out how much RAM your server has available for PostgreSQL.
> >  For
> > example, I have one server on which I allocate 256 mb for Apache, 128
> > mb for
> > linux, and thus have 512mb available for Postgres.
> >
> > 2) Calculate out the memory settings to use 70% of that amount of Ram
> > in
> > regular usage.   Please beware that sort_mem is *not* shared, meaning
> > that it
> > will be multiplied by the number of concurrent requests requiring
> > sorting.
> > Thus, your calculation (in K) should be:
> >
> > 250K  +
> > 8.2K * shared_buffers +
> > 14.2K * max_connections +
> > sort_mem * average number of requests per minute
> > =====================================
> > memory available to postgresql in K * 0.7
> >
> > You will also have to set SHMMAX and SHMMALL to accept this memory
> > allocation.
> > Since shmmax is set in bytes, then I generally feel safe making it:
> > 1024 * 0.5 * memory available to postgresql in K
> >
> > Setting them is done simply:
> > $ echo 134217728 >/proc/sys/kernel/shmall
> > $ echo 134217728 >/proc/sys/kernel/shmmax
> >
> > This is all taken from the postgresql documentation, with some
> > experience:
> >
> http://www.us.postgresql.org/users-lounge/docs/7.2/postgres/runtime.html
> >
> > --
> > -Josh Berkus
> >  Aglio Database Solutions
> >  San Francisco
>
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