Re: brk1 tests the wrong thing

[Anton Blanchard <anton@ozlabs.org>]

Hi Willy,

Thanks for this, I've merged it.

Anton

> ping
> 
> On Thu, Dec 10, 2020 at 08:07:36PM +0000, Matthew Wilcox wrote:
> > Linux has this horrendously complicated anon_vma structure that you
> > don't care about, but the upshot is that after calling fork(), each
> > process that calls brk() gets a _new_ VMA created.  That is, after
> > calling brk() the first time, the process address space looks like
> > this:

> > 
> > 557777fab000-557777ff0000 rw-p 00000000 00:00 0
> >      [heap] 557777ff0000-557777ff1000 rw-p 00000000 00:00 0
> >                  [heap]
> > 
> > so what brk1 is actually testing is how long it takes to create &
> > destroy a new VMA.  This does not match what most programs do --
> > most will call exec() which resets the anon_vma structures and
> > starts each program off with its own heap.  And if you do have a
> > multi-process program which uses brk(), chances are it doesn't just
> > oscillate betwee zero and one extra pages of heap compared to its
> > parent.
> > 
> > A better test starts out by allocating one page on the heap and then
> > throbs between one and two pages instead of throbbing between zero
> > and one page.  That means we're actually testing expanding and
> > contracting the heap instead of creating and destroying a new heap.
> > 
> > For realism, I wanted to add actually accessing the memory in the
> > new heap, but that doesn't work for the threaded case -- another
> > thread might remove the memory you just allocated while you're
> > allocating it. Threaded programs give each thread its own heap
> > anyway, so this is kind of a pointless syscall to ask about its
> > threaded scalability.
> > 
> > Anyway, here's brk2.c.  It is not very different from brk1.c, but
> > the performance results are quite different (actually worse by
> > about 10-15%).
> > 
> > 
> > #include <assert.h>
> > #include <sys/types.h>
> > #include <unistd.h>
> > 
> > char *testcase_description = "brk unshared increase/decrease of one
> > page";
> > 
> > void testcase(unsigned long long *iterations, unsigned long nr)
> > {
> > 	unsigned long page_size = getpagesize();
> > 	void *addr = sbrk(page_size) + page_size;
> > 
> > 	while (1) {
> > 		addr += page_size;
> > 		assert(brk(addr) == 0);
> > 
> > 		addr -= page_size;
> > 		assert(brk(addr) == 0);
> > 
> > 		(*iterations) += 2;
> > 	}
> > }
> >   
>