On Mon, 6 Mar 2000, Gregory Maxwell wrote:
> On Mon, 6 Mar 2000, Paul Jakma wrote:
>
> > another question, forgive me if it's stupid: Why do we even allow
> > overcommited memory? What reason is there is to support app's that
> > malloc() a huge amount of memory without using that memory?
> [snip]
>
> This isn't the only case. Linux uses COW paging on fork. I.e. if an app
i understand this. and i'm wondering would it possible to turn off
overcommit, yet still keep the COW behaviour? eg it should be possible
to mark pages in swap as used in a kernel table, without actually
writing to them yet. ie if we have x pages of swap available, on fork
decrement the available pages of swap. Then later on when at the point
the process actually dirties the page the kernel could whatever more
extensive housekeeping is needed.
i suppose i'm talking about better accounting, and having swap ==
backing store, amn't i?
> forks, it doesn't get a seperate copy of pages until it writes to them.
> This is a termendous speed increase (no copies until needed, cache, etc)
> and quite a bit of memory savings, but you lose this memory saving if you
> turn off overcommit.
>
as i understand you can't turn off overcommit in linux at the
moment. Yet it would be a useful option to have. I can easily imagine
certain situations where i would choose to turn off overcommit at the
expense of more swap if it would guarantee that OOM would never
happen. (hard disk space is cheap compared to downtime)
actually: if i configure linux with swap == address space now, would
this guarantee that OOM would never happen? ie on ia32, would 2GB of
swap be effectively equivalent to backing store?
regards,
Paul Jakma.
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