Re: reprise on Linux overcommit handling

Thanks. Interesting. Hard to imagine what they were thinking when they
put this code in.

---------------------------------------------------------------------------

Andrew Dunstan wrote:
> The current developer docs say this:
>
> -------------------
> Linux has poor default memory overcommit behavior. Rather than failing if it
> can not reserve enough memory, it returns success, but later fails when the
> memory can't be mapped and terminates the application with kill -9. To
> prevent unpredictable process termination, use:
>
> sysctl -w vm.overcommit_memory=3
> ---------------------
>
> This would be true if the kernel being used had the paranoid mode compiled
> in. This is not true, AFAICS, of either the stock 2.4 kernels nor of the
> latest RH kernels. It is true of 2.4.21 *with* the -ac4 (and posibly earlier
> -ac*) patch. In fact, Alan's patch apparently allows tuning of the amount of
> overcommitting allowed. As I read the kernel source I got from RH today
> (2.4.20-19.9), doing this will in fact make the kernel freely allow
> overcommiting of memory, rather than it trying in a rather unsatisfactory
> way to avoid it. IOW, with many kernels the advice would make things worse,
> not better - e.g. the RH source says this in mm/mmap.c:
>
> if (sysctl_overcommit_memory)
> return 1;
>
>
> Rather than give bad advice, it might be better to advise users (1) to run
> Pg on machines that are likely to be stable and not run into OOM situations,
> and (2) to check with their vendors about proper overcommit handling.
>
> Personally, my advice would be to avoid Linux for mission critical apps
> until this is fixed, but that's just my opinion, and I'm happily developing
> on Linux, albeit for something that is not mission critical.
>
> cheers
>
> andrew
>
>
>
>
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