sbrk(2) broken

Igor Mozolevsky igor at
Fri Jan 4 03:30:24 PST 2008

On 04/01/2008, Robert Watson <rwatson at> wrote:
> On Fri, 4 Jan 2008, Igor Mozolevsky wrote:
> > Of course, if you're afraid of memory overcommit and you know in advance
> >> how much memory you need, you can simply allocate a sufficient amount of
> >> address space at startup and touch it all.  This way, you will either be
> >> killed right away, or be guaranteed to have sufficient memory for the rest
> >> of your (process) lifetime.  Alternatively, do what Varnish does: create a
> >> large file, mmap it, and allocate everything you need from that area, so
> >> you have your own private swap space.  Just make sure to actually allocate
> >> the disk space you need (by filling the file with zeroes, or at the minimum
> >> writing a zero to the file every sb.st_blksize bytes, preferably
> >> sequentially to avoid excessive fragmentation)
> >
> > Surely you can just fseek() on the file at the correct lenght?
> That will create a sparse file without file system blocks to back it, and is
> effectively also over-commit.  When the file system runs out of room, you will
> get SIGSEGV when the vnode pager discovers it can't write a page to disk.  If
> you zero-fill it, the blocks are pre-allocated.

Surely you should not be allowed to overcommit on fseek() followed by
write(,,1); zeroing out gigs of hdd space seems rather silly...


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