On Thu, Mar 16, 2000 at 11:22:17AM +0000, Paul Jakma wrote:
> On Thu, 16 Mar 2000, James Sutherland wrote:
>
> > On Wed, 15 Mar 2000, Paul Jakma wrote:
>
> > That doesn't happen. malloc() ALLOCATES the memory to the process. It is
> > *NOT* overcommitted. It may be backed by swapspace rather than physical
> > memory, but that block of memory *IS* available to the process.
> >
>
> so malloc() isn't overcommited? malloc()'ed memory is guaranteed to be
> available - ie the memory is reserved and accounted for at malloc()
> time?
>
> (i always thought malloc()'s were noted by the kernel ie in the page
> table for the process, but that the kernel didn't actually allocate a
> real page until the process tried to access that page - ie malloc() is
> overcommited)
Looking at the sources of do_brk(), it faults in the pages (thereby
assuring they will exist for the process) only if the process' VM is
locked (mlockall()). Also, it returns with ENOMEM only[1] if
vm_enough_memory() says there isn't enough mem, but this function only
examines a snapshot of mem usage at the time of the call (and is
always successful if overcommit is set to on).
Unless I misunderstand the sources (I'm not a Linux VM expert), malloc
will overcommit.
[1] Well, not only, it also checks for rlimits and a few other things
not related to real memory.
--
Andreas E. Bombe <andreas.bombe@munich.netsurf.de>
http://home.pages.de/~andreas.bombe/ DSA key 0x04880A44
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