Re: vacuumdb causes memory drain.

On 1 May 2018 at 13:05, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote:

> Sushil Shirodkar wrote:
> > After reboot of Linux server this morning, we couldn't reproduce the
> issue. Not sure if this will come up
> > again, and if it is good news or not, will monitor and update forum if
> it reoccurs.
> >
> > Pl. see the output below which I had captured earlier from "free -h"
> before and after
> > running of "vacuumdb".
> >
> > total used free shared
> buff/cache available
> > Mem: 3.7G 158M 3.2G 62M 385M
> 3.3G
> > Swap: 3.9G 0B 3.9G
> >
> > total used free shared
> buff/cache available
> > Mem: 3.7G 169M 138M 117M 3.4G
> 3.2G
> > Swap: 3.9G 0B 3.9G
> >
> >
> > Was running the following command when slowness was observed. The only
> issue I would see is memory
> > getting close to full
>
> That is totally normal.
>
> You can observe that in the second output all the memory is shown as
> "buff/cache",
> that is, it is used for the file system cache rather than remaining unused.
> This memory is available, there is no shortage.
>
> This is the normal state for a Linux system with buffered I/O activity.
>
> > after running the following our application performance would get back
> to normal.
> >
> > sync; echo 1 > /proc/sys/vm/drop_caches
>
> Now that is surprising.
>
> Sure, after running this command your memory will show up as "free" again,
> but I'd expect performance to get worse because no more files are cached.
>
> Next time you run into trouble, try the following commands:
>
> vmstat 1
> iostat -mNx 1
>
> These should show you how your overall CPU, memory and I/O situation is.
> Maybe you can find your performance bottleneck that way.
>
> Yours,
> Laurenz Albe
> --
> Cybertec | https://www.cybertec-postgresql.com
>
>
as another indicator that your not low on memory look at the 'swap' line.
The kernel has not swapped anything out of main memory, so while the 'free'
value might look low, there has been no demand for memory which exceeds
what you have available.

I too find the fact clearing out caches seems to improve performance - that
seems counter to what I'd expect too. What filesystem type is the server
using for PG?

--
regards,

Tim

--
Tim Cross