Last data page not being filled

Last data page not being filled

[D. J. Bernstein]

When a program has (to take a real example) 220 bytes of initialized
data and 444 bytes of zero-filled data, the GNU tools put the 220 bytes
into the data segment and the 444 bytes into the bss segment, frittering
away a page of memory per process. They should instead put all 664 bytes
into the data segment.

The general rule is simple: as long as there's something in bss that
fits into the last page of data, move it to the last page of data and
repeat, unless the user explicitly asks for different behavior.

(The reason I care about a page per process is that I'm building systems
on the scale of ten thousand processes. Each process has its own uid,
and memory protection is crucial, so please don't suggest threads.)

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago

Re: Last data page not being filled

[DJ Delorie]

> When a program has (to take a real example) 220 bytes of initialized
> data and 444 bytes of zero-filled data, the GNU tools put the 220 bytes
> into the data segment and the 444 bytes into the bss segment, frittering
> away a page of memory per process. They should instead put all 664 bytes
> into the data segment.

Doesn't it already do that?  The bss section should be right after the
data section, so they should share the last page.

Of course, you failed to tell us which target you're using, so maybe
your specific target is asking binutils to do otherwise.

Re: Last data page not being filled

[Ian Lance Taylor]

"D. J. Bernstein" <djb@cr.yp.to> writes:

> When a program has (to take a real example) 220 bytes of initialized
> data and 444 bytes of zero-filled data, the GNU tools put the 220 bytes
> into the data segment and the 444 bytes into the bss segment, frittering
> away a page of memory per process. They should instead put all 664 bytes
> into the data segment.

GNU ld is supposed to already work as you describe.  I just verified
that on i386 GNU/Linux it appears to work.

What is the target?  Can you provide a simple test case?

Ian

Re: Last data page not being filled

[D. J. Bernstein]

My apologies; I was misled by a few people on the freebsd-performance
mailing list. Now that I investigate more carefully, I see that the GNU
tools do the right thing, keeping bss adjacent to data in the same VM
segment. Please ignore my initial report.

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago