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SubjectRe: Memory pressure handling with iSCSI
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On Tue, 2005-07-26 at 16:33, Martin J. Bligh wrote:
> >> > After KS & OLS discussions about memory pressure, I wanted to re-do
> >> > iSCSI testing with "dd"s to see if we are throttling writes.
> >>
> >> Could you also try with shared writable mmap, to see if that
> >> works ok or triggers a deadlock ?
> >
> >
> > I can, but lets finish addressing one issue at a time. Last time,
> > I changed too many things at the same time and got no where :(
>
> Adam is working that one, but not over iSCSI.

I wrote a simple/ugly C program to demonstrate the MAP_SHARED,PROT_WRITE
case. I was able to saturate the system with 75% of all memory in dirty
pages before I got bored.

To reproduce:
- Create a 3GB file with dd
- ./map-shared-dirty bigfile <number of chunks>

I break up the mmap & dirty operation into chunks in case the system is
tight on memory. Choose a large enough number of chunks so the
individual mmaps will be small enough for your system to accomodate.

--

MemTotal: 4092492 kB
MemFree: 786988 kB
Buffers: 6372 kB
Cached: 3211388 kB
SwapCached: 0 kB
Active: 3197428 kB
Inactive: 36696 kB
HighTotal: 3211264 kB
HighFree: 1024 kB
LowTotal: 881228 kB
LowFree: 785964 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 3117300 kB
Writeback: 3568 kB
Mapped: 24780 kB
Slab: 59316 kB
Committed_AS: 49760 kB
PageTables: 780 kB
VmallocTotal: 114680 kB
VmallocUsed: 32 kB
VmallocChunk: 114648 kB

/*
* map-shared-dirty.c - Demonstrate a loophole in dirty-ratio when
* heavily dirtying MAP_SHARED memory.
*
* Usage: (I know it's ugly)
* ./map-shared-dirty <large file> <number of chunks>
*/
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>
#include <stdio.h>

size_t page_size;

void dirty_file(int fd, unsigned long bytes, size_t map_offset) {
char *addr;
addr = mmap(NULL, bytes, PROT_READ|PROT_WRITE, MAP_SHARED, fd, map_offset);
if (addr == MAP_FAILED) {
fprintf(stderr, "Failed to map file\n");
fprintf(stderr, "bytes: %i offset: %i\n", bytes,map_offset);
exit(1);
}
/* Dirty the pages */
memset(addr, map_offset%255, bytes);
munmap(addr, bytes);
}
int main(int argc, char **argv)
{
char *filename = argv[1];
int chunks = atoi(argv[2]);
int fd;
unsigned long i, chunk_size, bytes;
struct stat file_info;
fd = open(filename, O_RDWR|0100000); /* O_LARGEFILE */
if (fd <= 0) {
fprintf(stderr, "Failed to open file\n");
exit(1);
}
fstat(fd, &file_info);
bytes = file_info.st_size;
page_size = getpagesize();
chunk_size = (bytes / chunks) & ~(page_size - 1);
printf("Chunk size = %i\n", chunk_size);
for (i = 0; i < bytes; i+=chunk_size)
dirty_file(fd, chunk_size, i);

exit(0);
}

--
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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