Mike Rylander wrote:
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.)
I'm seeing the same thing on one of our 8.1 servers. Summing RSS from
`ps` or RES from `top` accounts for about 1 GB, but `free` says:
total used free shared buffers cached
Mem: 4060968 3870328 190640 0 14788 432048
-/+ buffers/cache: 3423492 637476
Swap: 2097144 175680 1921464
That's 3.4 GB/170 MB in RAM/swap, up from 2.7 GB/0 last Thursday, 2.2
GB/0 last Monday, or 1.9 GB after a reboot ten days ago. Stopping
Postgres brings down the number, but not all the way -- it drops to
about 2.7 GB, even though the next most memory-intensive process is
`ntpd` at 5 MB. (Before Postgres starts, there's less than 30 MB of
stuff running.) The only way I've found to get this box back to normal
is to reboot it.
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.
I can't offer any explanation for why this server is starting to swap --
where'd the memory go? -- but I know it started after upgrading to
PostgreSQL 8.1. I'm not saying it's something in the PostgreSQL code,
but this server definitely didn't do this in the months under 7.4.
Mike: is your system AMD64, by any chance? The above system is, as is
another similar story I heard.
--Will Glynn
Freedom Healthcare
