Re: brk1 tests the wrong thing

[Matthew Wilcox <willy@infradead.org>]

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;
> 	}
> }
>