ten thousand small processes

Jeff Roberson jroberson at chesapeake.net
Mon Jun 23 16:29:14 PDT 2003

On Mon, 23 Jun 2003, Jeff Roberson wrote:

> On 21 Jun 2003, D. J. Bernstein wrote:
> > FreeBSD 4.8. Test program: malloc(360); malloc(80); malloc(180);
> > malloc(16); malloc(440); sleep(10); _exit(0). Compile statically.
> >
> > The program ends up with 44KB RSS. Where is all that DRAM going? The
> > program also ends up with 168KB VSZ. Where is all that VM going?
> >
> > I don't care much about the 3-page text segment. But I do care about the
> > 39 extra pages of VM, and the 8 extra pages of DRAM. There's no obstacle
> > to having a small program fit into _one_ page per process; two or three
> > can be excused, but 39 is absurd. (Yes, I know that Solaris is worse.)

The following should read "you need at least three pages"

> Even small programs need page tables.   On x86 unix you need at least
> pages for page tables for any process, if I'm counting correctly.  One for
> the page directory, one page table for text, data, bss, and the guard
> page, and one page table page for stack.
> 32 of those 'pages of VM' are your initial stack size.  They don't really
> consume any resources other than preventing anyone else from allocating
> overlapping pages.  It's just the initial upper limit on the stack map
> which is allowed to grow.  I haven't looked closely enough to find out
> what the other 7 might be.
> There is an obstacle to having a small program fit into one page.
> Actually, a significant one.  First of all, you need protections on
> different sections of the actual executable image.  Text must be read only
> since it is shared.  Data is read write and bss is read-write.  BSS is a
> pseudo section and not actually mapped from the file.  Text and data both
> can be paged in from the binary in a demand paged system such as freebsd.
> Data can not be written out to its backing object and neither can text.
> Text can be shared while data changes are private and so the two must be
> placed in seperate pages.  This topic is explored quite well in any modern
> operating systems book.  I suggest you pick up "The Design and
> Implementation of the 4.4BSD Operating System".  It is a little out dated
> but provides a good introduction to these topics.  I really didn't do them
> justice with this paragraph.
> If demand paging, shared libraries, and the like are not suited for your
> problem perhaps you should look at an embedded operating system? Or DOS
> even.
> Cheers,
> Jeff
> >
> > At least 2 pages appear to be wasted by exit(), because it brings in a
> > chunk of stdio, which uses 84 bytes of data and 316 bytes of bss. The
> > libc implementors clearly don't care about 316 bytes of memory, so why
> > don't they make those 316 bytes static? Why doesn't the compiler
> > automatically merge some bss into data when that saves a page? Why can't
> > I omit exit(), manually or automatically, when it's unreachable?
> >
> > Furthermore, malloc() appears to chew up a whole new page of DRAM for
> > each allocation, plus another page---is this counted in VSZ?---for an
> > anonymous mmap. Would it really be that difficult to fit 1076 bytes of
> > requested memory into the 3000-odd bytes available at the end of bss?
> >
> > I sure hope that there's some better explanation for the remaining 32
> > pages than ``Well, we decided to allocate 131072 bytes of memory for the
> > stack,'' especially when I'm hard-limiting the stack to 4K before exec.
> >
> > ---D. J. Bernstein, Associate Professor, Department of Mathematics,
> > Statistics, and Computer Science, University of Illinois at Chicago
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