From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mike Rylander <mrylander@gmail.com>
Cc: pgsql general <pgsql-general@postgresql.org>
Subject: Re: Memory Leakage Problem
Date: Wed, 07 Dec 2005 23:38:53 -0500
Message-ID: <6353.1134016733@sss.pgh.pa.us> (raw)
In-Reply-To: <b918cf3d0512071946m3cbfc630y679e0350a7f6b864@mail.gmail.com>
References: <c10e7feb0512060122u33a9c8eakb178525f3834b00@mail.gmail.com>
<1133891282.11803.7.camel@state.g2switchworks.com>
<c10e7feb0512061816o21305ac7n576694f2624dc6ed@mail.gmail.com>
<1133991204.11803.43.camel@state.g2switchworks.com>
<c10e7feb0512071825h30c93c00kdf5ea2a11e613f0b@mail.gmail.com>
<b918cf3d0512071946m3cbfc630y679e0350a7f6b864@mail.gmail.com>
Mike Rylander <mrylander@gmail.com> writes:
> To cut to the chase, here are
> some numbers for everyone to digest:
> total gnu ps resident size
> # ps ax -o rss|perl -e '$x += $_ for (<>);print "$x\n";'
> 5810492
> total gnu ps virual size
> # ps ax -o vsz|perl -e '$x += $_ for (<>);print "$x\n";'
> 10585400
> total gnu ps "if all pages were dirtied and swapped" size
> # ps ax -o size|perl -e '$x += $_ for (<>);print "$x\n";'
> 1970952
I wouldn't put any faith in those numbers at all, because you'll be
counting the PG shared memory multiple times.
On the Linux versions I've used lately, ps and top report a process'
memory size as including all its private memory, plus all the pages
of shared memory that it has touched since it started. So if you run
say a seqscan over a large table in a freshly-started backend, the
reported memory usage will ramp up from a couple meg to the size of
your shared_buffer arena plus a couple meg --- but in reality the
space used by the process is staying constant at a couple meg.
Now, multiply that effect by N backends doing this at once, and you'll
have a very skewed view of what's happening in your system.
I'd trust the totals reported by free and dstat a lot more than summing
per-process numbers from ps or top.
> Now, I'm not blaming Pg for the apparent discrepancy in calculated vs.
> reported-by-free memory usage, but I only noticed this after upgrading
> to 8.1.
I don't know of any reason to think that 8.1 would act differently from
older PG versions in this respect.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, mrylander@gmail.com, pgsql-general@postgresql.org
Subject: Re: Memory Leakage Problem
In-Reply-To: <6353.1134016733@sss.pgh.pa.us>
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