Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Mike Rapoport]

On Tue, May 30, 2017 at 04:39:41PM +0200, Michal Hocko wrote:
> On Tue 30-05-17 16:04:56, Andrea Arcangeli wrote:
> > 
> > UFFDIO_COPY while not being a major slowdown for sure, it's likely
> > measurable at the microbenchmark level because it would add a
> > enter/exit kernel to every 4k memcpy. It's not hard to imagine that as
> > measurable. How that impacts the total precopy time I don't know, it
> > would need to be benchmarked to be sure.
> 
> Yes, please!

I've run a simple test (below) that fills 1G of memory either with memcpy
of ioctl(UFFDIO_COPY) in 4K chunks.
The machine I used has two "Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz" and
128G of RAM.
I've averaged elapsed time reported by /usr/bin/time over 100 runs and here
what I've got:

memcpy with THP on: 0.3278 sec
memcpy with THP off: 0.5295 sec
UFFDIO_COPY: 0.44 sec

That said, for the CRIU usecase UFFDIO_COPY seems faster that disabling THP
and then doing memcpy.

--
Sincerely yours,
Mike.

----------------------------------------------------------
{
	...

	src = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if (src == MAP_FAILED)
		fprintf(stderr, "map src failed\n"), exit(1);
	*((unsigned long *)src) = 1;

 	if (disable_huge && prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0))
		fprintf(stderr, "ptctl failed\n"), exit(1);

	dst = mmap(NULL, page_size * nr_pages, PROT_READ | PROT_WRITE,
		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if (dst == MAP_FAILED)
		fprintf(stderr, "map dst failed\n"), exit(1);

	if (use_uffd && userfaultfd_register(dst))
		fprintf(stderr, "userfault_register failed\n"), exit(1);

	for (i = 0; i < nr_pages; i++) {
		char *address = dst + i * page_size;

		if (use_uffd) {
			struct uffdio_copy uffdio_copy;

			uffdio_copy.dst = (unsigned long)address;
			uffdio_copy.src = (unsigned long)src;
			uffdio_copy.len = page_size;
			uffdio_copy.mode = 0;
			uffdio_copy.copy = 0;

			ret = ioctl(uffd, UFFDIO_COPY, &uffdio_copy);
			if (ret)
				fprintf(stderr, "copy: %d, %d\n", ret, errno),
					exit(1);
		} else {
			memcpy(address, src, page_size);
		}

	}

	return 0;
}