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
