On 12/8/05, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> 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.
Right, I can definitely see that happening. Some backends are upwards
of 200M, some are just a few since they haven't been touched yet.
>
> 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.
Absolutely ...
>
> I'd trust the totals reported by free and dstat a lot more than summing
> per-process numbers from ps or top.
>
And there's the part that's confusing me: the numbers for used memory
produced by free and dstat, after subtracting the buffers/cache
amounts, are /larger/ than those that ps and top report. (top says the
same thing as ps, on the whole.)
> > 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.
>
Neither can I, which is why I don't blame it. ;) I'm just reporting
when/where I noticed the issue.
> regards, tom lane
>
--
Mike Rylander
mrylander@gmail.com
GPLS -- PINES Development
Database Developer
http://open-ils.org
