Re: [PATCH 4/4] thp: rewrite freeze_page()/unfreeze_page() with generic rmap walkers

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Feb 03, 2016 at 07:42:01AM -0800, Dave Hansen wrote:
> On 02/03/2016 07:14 AM, Kirill A. Shutemov wrote:
> > But the new variant is somewhat slower. Current helpers iterates over
> > VMAs the compound page is mapped to, and then over ptes within this VMA.
> > New helpers iterates over small page, then over VMA the small page
> > mapped to, and only then find relevant pte.
> 
> The code simplification here is really attractive.  Can you quantify
> what the slowdown is?  Is it noticeable, or would it be in the noise
> during all the other stuff that happens under memory pressure?

Okay, here's more realistic scenario: migration 8GiB worth of THP.

Testcase:
	#define _GNU_SOURCE
	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <sys/mman.h>
	#include <linux/mempolicy.h>
	#include <numaif.h>

	#define MB (1024UL * 1024)
	#define SIZE (4 * 1024 * 2 * MB)
	#define BASE ((void *)0x400000000000)

	#include <time.h>

	void timespec_diff(struct timespec *start, struct timespec *stop,
			struct timespec *result)
	{
		if ((stop->tv_nsec - start->tv_nsec) < 0) {
			result->tv_sec = stop->tv_sec - start->tv_sec - 1;
			result->tv_nsec = stop->tv_nsec - start->tv_nsec + 1000000000;
		} else {
			result->tv_sec = stop->tv_sec - start->tv_sec;
			result->tv_nsec = stop->tv_nsec - start->tv_nsec;
		}
	}

	int main()
	{
		char *p;
		unsigned long ret, node_mask;
		struct timespec start, stop, result;

		node_mask = 0b01;
		ret = set_mempolicy(MPOL_BIND, &node_mask, 64);
		if (ret == -1)
			perror("set_mempolicy"), exit(1);
		p = mmap(BASE, SIZE, PROT_READ | PROT_WRITE,
				MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE,
				-1, 0);
		if (p == MAP_FAILED)
			perror("mmap"), exit(1);

		system("grep thp /proc/vmstat");
		clock_gettime(CLOCK_MONOTONIC, &start);
		node_mask = 0b10;
		ret = mbind(p, SIZE, MPOL_BIND, &node_mask, 64, MPOL_MF_MOVE);
		if (ret == -1)
			perror("mbind"), exit(1);
		clock_gettime(CLOCK_MONOTONIC, &stop);
		system("grep thp /proc/vmstat");

		timespec_diff(&start, &stop, &result);
		printf("--------------------------\n");
		printf("%ld.%09lds\n", result.tv_sec, result.tv_nsec);

		return 0;
	}

Baseline: 25.146 +- 0.141
Patched:  28.684 +- 0.298
Slowdown: 1.14x

Can we tolerate this?

-- 
 Kirill A. Shutemov

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