Re: Pre-allocation of shared memory ...

I have added the following sentence to the docs too:

Note, you will need enough swap space to cover all your memory
needs.

I still wish Linux would just fail the fork/malloc when memory is low,
rather than requiring swap for everything _or_ overcommitting. I wonder
if making a unified buffer cache just made that too hard to do.

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

Andrew Dunstan wrote:
>
> On this machine (RH9, kernel 2.4.20-18.9) the docs say (in
> /usr/src/linux-2.4/Documentation/vm/overcommit-accounting ):
>
> -----------------
> The Linux kernel supports four overcommit handling modes
>
> 0 - Heuristic overcommit handling. Obvious overcommits of
> address space are refused. Used for a typical system. It
> ensures a seriously wild allocation fails while allowing
> overcommit to reduce swap usage
>
> 1 - No overcommit handling. Appropriate for some scientific
> applications
>
> 2 - (NEW) strict overcommit. The total address space commit
> for the system is not permitted to exceed swap + half ram.
> In almost all situations this means a process will not be
> killed while accessing pages but only by malloc failures
> that are reported back by the kernel mmap/brk code.
>
> 3 - (NEW) paranoid overcommit The total address space commit
> for the system is not permitted to exceed swap. The machine
> will never kill a process accessing pages it has mapped
> except due to a bug (ie report it!)
> ----------------------
>
> So maybe
>
> sysctl -w vm.overcommit_memory=3
>
> is what's needed? I guess you might pay a performance hit for doing that,
> though.
>
> andrew
>
> > > Yeah, I see it in the Mandrake kernel. But it's not in stock 2.4.19,
> > > so you can't assume everybody has it.
> > >
> >
> > We had this problem on a recent version of good old Slackware.
> > I think we also had it on RedHat 8 or so.
> >
> > Doing this kind of killing is definitely a bad habit. I thought it had
> > it had to do with something else so my proposal for pre-allocation
> > seems to be pretty obsolete ;).
> >
> > Thanks a lot.
> >
> > Hans
>
>
>
>
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