MMAP issues

MMAP issues

[Peter Zaitsev]

Hello ,

  I'm trying to write a program which uses mmap agressively to mmap
  files (really it's used as fail safe memory allocator to store data
  if application failed)
  I'm using the latest kernel 2.4.16.

  I've found a couple of problems.

  1) I can mmap only about 64K files (4K size ones) and I can't find there the limit
  is triggered. I can open huge number of files (tried 500K) or I can
  map ammoniums pages even  lager number.  May be this limit is
  compiled in kernel somehow so may be changed ?
  The error code returned is 12 -  Can not allocate memory

  2) I see the speed dramatically degrades over time with mapping
  segments:
zetta:/home/pz/mmap # ./a.out
 10000  Time: 7
 20000  Time: 22
 30000  Time: 38
 40000  Time: 61
 50000  Time: 78
 60000  Time: 90

 First 10000 of mmaps took only  7 second there  mapping 10000 of
 files after 50000 took 90 seconds.  I used the same file associated
 with many file descriptors to avoid disk related speed issues. Map
 ammoniums in the same case are runned much faster and with almost no
 speed penalty as well as open calls.  So the question is if this
 thing may be tuned somehow - hash increase or something.


 3) It looks like the speed degrades over the program runs:

 1st
 10000  Time: 7
 20000  Time: 22
 30000  Time: 38
 40000  Time: 61
 50000  Time: 78
 60000  Time: 90 

 2nd
 10000  Time: 7
 20000  Time: 28
 30000  Time: 49
 40000  Time: 71
 50000  Time: 92
 60000  Time: 104 

 3rd
 10000  Time: 9
 20000  Time: 31
 30000  Time: 52
 40000  Time: 68
 50000  Time: 87
 60000  Time: 107 





-- 
Best regards,
 Peter                          mailto:pz@spylog.ru

Re: MMAP issues

[Jeff Epler]

The difference in runtime between successive runs of your program
doesn't look terribly significant.

You open 'fd' each time, and never close it.  I die about 1000 mmap()s
into the process (-EMFILE returned by sys_open).  You may be testing
Linux' performance with huge fd sets in your test as well.

Moving the open() outside the loop, and running on a 512M, kernel 2.2
machine that's also running a full gnome desktop I get really intense
kernel CPU usage, and the following output:
 10000  Time: 12
 20000  Time: 45
 30000  Time: 79
 40000  Time: 113
[and I got too bored to watch it go on]

Unmapping the page after each map yields much better results:

 10000  Time: 4
 20000  Time: 4
 30000  Time: 4
 40000  Time: 5
 50000  Time: 4
 60000  Time: 4
 70000  Time: 5
 80000  Time: 5
 90000  Time: 5
 100000  Time: 4
[etc]

Interestingly, forcing the test to allocate at successively lower
addresses gives fast results until mmap fails (collided with a shared
library?):
 10000  Time: 4
 20000  Time: 4
 30000  Time: 4
 40000  Time: 4
 50000  Time: 4
 60000  Time: 4
Failed 0x60007000 12

So in kernel 2.2, it looks like some sort of linked list ordered by user
address is being traversed in order to complete the mmap() operation.
If so, then the O(N^2)-like behavior you saw in your original report is
explained as the expected performance of linux' mmap for a given # of
mappings.

Jeff

#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

int main()
{
    int i = 0;
    void *p;
    int t;
    int fd;
    int addr = (void *) 0x70000000;

    fd = open("test.dat", O_RDWR);
    if (fd < 0) {
	puts("Unable to open file !");
	return;
    }
    t = time(NULL);
    while (1) {
	p = mmap(addr, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
	addr = addr - 4096;
	if ((int) p == -1) {
	    printf("Failed %p %d\n", addr, errno);
	    return;
	}
	i++;
	if (i % 10000 == 0) {
	    printf(" %d  Time: %d\n", i, time(NULL) - t);
	    t = time(NULL);
	}
    }
}

mmap issues

[David Stroupe]

In the driver is:
static int S_Mmap(struct file * flip, struct vm_area_struct *vma)
{
 
<snip>
  unsigned long size = (unsigned long)vma->vm_end - vma->vm_start; 
<------size = 2000 for this case
  unsigned long start = (unsigned long)vma->vm_start;

   vma->vm_flags |= VM_LOCKED;

 
  vaddy = kmalloc(size , GFP_KERNEL);
  strcpy((char*)vaddy, "testing\0");
  mem_map_reserve(virt_to_page(vaddy));
  result = remap_page_range(start, virt_to_phys(vaddy), size, 
vma->vm_page_prot);
   if (result)
        return -EAGAIN;
 return 0;
 
in user space is:
<snip>
  pdma->dwBytes = 2000;
  pdma->pUserAddr =  mmap(0, pdma->dwBytes, PROT_READ | PROT_WRITE, 
MAP_SHARED, hWd, 0);
  str = (char*)pdma->pUserAddr;
<snip)

when I try to view the memory that I just mmap()ed using gdb I see:
$12 = 0x40018000 <Address 0x40018000 out of bounds>

If I don't do the mem_map_reserve() command I can view the memory but it 
is all zeros.

What am I missing?

TIA

-- 
Best regards,
David Stroupe
Keyed-Up Software