Den 18-Mar-00 02:20:40 skrev Matija Nalis fĝlgende om "Re: Overcommitable memory??":
> On 17 Mar 2000 00:27:42 +0100, Rask Ingemann Lambertsen
> <rask-linux@kampsax.k-net.dk> wrote:
>>Den 15-Mar-00 11:49:45 skrev James Sutherland fĝlgende om "Re:
>>Overcommitable memory??":
>>
>>> As well as *destroying* performance (you'd effectively eliminate COW
>>> capability on fork - if your 500Mb simulation wants to fork a 100k mailer
>>> process to send you an update, the kernel has to allocate and copy 500Mb
>>> of RAM/swap first, then discard it all again.)
>>
>> Not at all. COW is a performance optimisation which does not depend on
>>overcommitment of memory in any way. Why would you want to turn it off?
> Say you have 800MB virtual RAM, and you simulation currently uses 500MB.
> And now, it tries to fork(2). Do you allow it, or does fork fail with
> -ENOMEM ?
If you don't overcommit there is only once choice: Let it fail with
ENOMEM because there is not enough memory: 2*500MB > 800 MB.
> If you allow it, and one copy later just writes different data to
> say 400MB of itself, it will run out of memory (because system will need to
> create new page for COW, and would need 500MB+400MB, which is more than
> 800MB virtual RAM your machine have). Ka-boom, you process will die on
> simple memory access (no syscall to check error from, since you allowed that
> fork() which was last place to check it).
That's why overcommitment of memory is a bad idea. It breaks programs
that do nothing wrong.
Regards,
/ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻTŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ\
| Rask Ingemann Lambertsen | E-mail: mailto:rask@kampsax.dtu.dk |
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