[PATCH 2/2] Quicklist shouldn't be proportional to # of CPUs

[KOSAKI Motohiro]

Now, Quicklist store some pageis in each CPU as cache.
(Each CPU can cache node_free_pages/16 pages)

and it is used for page table cache.
Then, exit() increase cache, the other hand fork() consumes it.

So, if apache type (one parent and many child model) middleware run,
One CPU process fork(), Other CPU process the middleware work and exit().

At that time, one CPU (on which the parent runs) don't have page 
table cache at all, Others (on which children runs) have maximum caches.

	QList_max = (#ofCPUs - 1) x Free / 16
	=> QList_max / (Free + QList_max) = (#ofCPUs - 1) / (16 + #ofCPUs - 1)

So, How much quicklist spent memory at maximum case?
That is proposional to # of CPUs because the limit of per cpu quicklist
 cache doesn't see the number of cpus.

Above calculation mean

	 Number of CPUs per node            2    4    8   16
	 ==============================  ====================
	 QList_max / (Free + QList_max)   5.8%  16%  30%  48%


Wow! Quicklist can spend about 50% memory at worst case.


My demonstration program is here
--------------------------------------------------------------------------------
#define _GNU_SOURCE

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sched.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/wait.h>

#define BUFFSIZE 512

int max_cpu(void)	/* get max number of logical cpus from /proc/cpuinfo */
{
  FILE *fd;
  char *ret, buffer[BUFFSIZE];
  int cpu = 1;

  fd = fopen("/proc/cpuinfo", "r");
  if (fd == NULL) {
    perror("fopen(/proc/cpuinfo)");
    exit(EXIT_FAILURE);
  }
  while (1) {
    ret = fgets(buffer, BUFFSIZE, fd);
    if (ret == NULL)
      break;
    if (!strncmp(buffer, "processor", 9))
      cpu = atoi(strchr(buffer, ':') + 2);
  }
  fclose(fd);
  return cpu;
}

void cpu_bind(int cpu)	/* bind current process to one cpu */
{
  cpu_set_t mask;
  int ret;

  CPU_ZERO(&mask);
  CPU_SET(cpu, &mask);
  ret = sched_setaffinity(0, sizeof(mask), &mask);
  if (ret == -1) {
    perror("sched_setaffinity()");
    exit(EXIT_FAILURE);
  }
  sched_yield();	/* not necessary */
}

#define MMAP_SIZE (10 * 1024 * 1024)	/* 10 MB */
#define FORK_INTERVAL 1	/* 1 second */

main(int argc, char *argv[])
{
  int cpu_max, nextcpu;
  long pagesize;
  pid_t pid;

  /* set max number of logical cpu */
  if (argc > 1)
    cpu_max = atoi(argv[1]) - 1;
  else
    cpu_max = max_cpu();

  /* get the page size */
  pagesize = sysconf(_SC_PAGESIZE);
  if (pagesize == -1) {
    perror("sysconf(_SC_PAGESIZE)");
    exit(EXIT_FAILURE);
  }

  /* prepare parent process */
  cpu_bind(0);
  nextcpu = cpu_max;

loop:

  /* select destination cpu for child process by round-robin rule */
  if (++nextcpu > cpu_max)
    nextcpu = 1;

  pid = fork();

  if (pid == 0) { /* child action */

    char *p;
    int i;

    /* consume page tables */
    p = mmap(0, MMAP_SIZE, PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
    i = MMAP_SIZE / pagesize;
    while (i-- > 0) {
      *p = 1;
      p += pagesize;
    }

    /* move to other cpu */
    cpu_bind(nextcpu);
/*
    printf("a child moved to cpu%d after mmap().\n", nextcpu);
    fflush(stdout);
 */

    /* back page tables to pgtable_quicklist */
    exit(0);

  } else if (pid > 0) { /* parent action */

    sleep(FORK_INTERVAL);
    waitpid(pid, NULL, WNOHANG);

  }

  goto loop;
}
----------------------------------------

When above program which does task migration runs, my 8GB box spends
800MB of memory for quicklist.  This is not memory leak but doesn't seem
good.

% cat /proc/meminfo

MemTotal:        7701568 kB
MemFree:         4724672 kB
(snip)
Quicklists:       844800 kB

because

- My machine spec is
	number of numa node: 2
	number of cpus:      8 (4CPU x2 node)
        total mem:           8GB (4GB x2 node)
        free mem:            about 5GB

- Then, 4.7GB x 16% ~= 880MB.
  So, Quicklist can use 800MB.

So, if following spec machine run that program

   CPUs: 64 (8cpu x 8node)
   Mem:  1TB (128GB x8node)

Then, quicklist can waste 300GB (= 1TB x 30%).  It is too large.

So, I don't like cache policies which is proportional to # of cpus.

My patch changes the number of caches
from:
   per-cpu-cache-amount = memory_on_node / 16
to
   per-cpu-cache-amount = memory_on_node / 16 / number_of_cpus_on_node.


Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
CC: Keiichiro Tokunaga <tokunaga.keiich@xxxxxxxxxxxxxx>
Acked-by: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
Tested-by: David Miller <davem@xxxxxxxxxxxxx>

---
 mm/quicklist.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: b/mm/quicklist.c
===================================================================
--- a/mm/quicklist.c
+++ b/mm/quicklist.c
@@ -26,7 +26,10 @@ DEFINE_PER_CPU(struct quicklist, quickli
 static unsigned long max_pages(unsigned long min_pages)
 {
 	unsigned long node_free_pages, max;
-	struct zone *zones = NODE_DATA(numa_node_id())->node_zones;
+	int node = numa_node_id();
+	struct zone *zones = NODE_DATA(node)->node_zones;
+	int num_cpus_on_node;
+	node_to_cpumask_ptr(cpumask_on_node, node);
 
 	node_free_pages =
 #ifdef CONFIG_ZONE_DMA
@@ -38,6 +41,10 @@ static unsigned long max_pages(unsigned 
 		zone_page_state(&zones[ZONE_NORMAL], NR_FREE_PAGES);
 
 	max = node_free_pages / FRACTION_OF_NODE_MEM;
+
+	num_cpus_on_node = cpus_weight_nr(*cpumask_on_node);
+	max /= num_cpus_on_node;
+
 	return max(max, min_pages);
 }
 


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