On Thu, 01 Oct 2009 14:23:48 +0300, bearophile <[email protected]>
wrote:
Time ago Jon Harrop has found a memory leak in a F# program running on
Mono, he has reduced the program to a minimal test case. I have
translated that code to D2 and Python2:
Sorry for the late reply. I wanted to get to the bottom of this, and
facilitate others to do the same easily, thus this motivated me to improve
Diamond to make that possible.
What basically happens here is that the program creates an infinite
contiguous linked list. Even though there is no reference to the original
node, a stray reference to any of the nodes in the list will prevent any
nodes following it to be freed. The program's behavior was inconsistent on
my machine (it either leaked or didn't leak memory), because the memory
pools were allocated at different memory addresses.
*** SHAMELESS PLUG ALERT ***
Diamond ( my post-mortem memory debugger -
http://dsource.org/projects/diamond ) should allow to diagnose these
situations easily, more so with the latest additions. Here's an example of
how it's possible to easily analyze this program's memory behavior and
find the source of the leak:
Firstly, the program was backported to D1 (I don't use D2, thus Diamond
doesn't support it ATM). Its execution time is limited, to allow a clean
exit. The final source is as follows:
-------------------------------------------
import diamond;
struct Node {
int data;
Node* next;
}
void main() {
Node* lh = new Node;
lh.data = 1;
lh.next = lh;
for(int i=0;i<1000000;i++) {
Node* lh2 = lh;
lh = new Node;
lh.data = 2;
lh.next = lh2.next;
lh2.next = lh;
lh2 = lh;
lh2.next = lh2.next.next;
}
}
-------------------------------------------
The diamond runtime module was configured with only the MEMLOG and
MEMLOG_CRC32 options.
The program was compiled and executed; the runtime module created a .mem
file which contained a log of memory operations, and full contents for
every garbage collect. This file was loaded into the memory log analyzer:
Diamond Memory Log Analyzer, v0.2
by Vladimir "CyberShadow" Panteleev, 2008-2009
Using the most recent memory dump file.
Loading diamond_2009-09-04_02.25.49.mem...
1000013 events loaded.
Using the most recent map file.
1047 symbols loaded from test1final.map.
> dumps
65023 @ 02:25:49: MEMDUMP ( 256/ 256/ 256)
89503 @ 02:25:49: MEMDUMP ( 256/ 256/ 256)
155041 @ 02:25:49: MEMDUMP ( 512/ 512/ 512)
286115 @ 02:25:49: MEMDUMP ( 1024/ 1024/ 1024)
482725 @ 02:25:50: MEMDUMP ( 1792/ 1792/ 1792)
744871 @ 02:25:51: MEMDUMP ( 2816/ 2816/ 2816)
MEMDUMP events are generated automatically before garbage collects.
Judging by the history, the GC failed to free up space starting with the
second collect. Let's look at the memory map before and after the first
collect:
> nextdump
D65023> map
Page map for pool 02090000 - 02190000 ( 256/ 256/ 256 pages):
(page size = 0x1000) +10000 +20000 +30000
02090000: 6744444444444444 4444444444444444 4444444444444444
4444444444444444
020D0000: 4444444444444444 4444444444444444 4444444444444444
4444444444444444
02110000: 4444444444444444 4444444444444444 4444444444444444
4444444444444444
02150000: 4444444444444444 4444444444444444 4444444444444444
4444444444444444
D65023> n
M65024> map
Page map for pool 02090000 - 02190000 ( 162/ 256/ 256 pages):
(page size = 0x1000) +10000 +20000 +30000
02090000: 674............. ................ ................
................
020D0000: ................ ................ .444444444444444
4444444444444444
02110000: 4444444444444444 4444444444444444 4444444444444444
4444444444444444
02150000: 4444444444444444 4444444444444444 4444444444444444
4444444444444444
We can see the list's nodes filling up all available memory, before
trigging a collect (4 - pages containing objects <=16 bytes in size).
However, the collect stopped at the page at 020F1000. Let's get a closer
look:
M65024> binmap 020F1000
Bin map for page 020F1000 - 020F2000:
(object size = 0x010) +100 +200 +300
020F1000: DDDDDDDDDDDDDDDD DDDDDDDDDDDDDDDD DDDDDDDDDDDDDDDD
DDDDDDDDDDDDDDDD
020F1400: DDDDDDDDDDDDDDDD DDDDDDDDDDDDDDDD D...............
................
020F1800: ................ ................ ................
................
020F1C00: ................ ................ ................
................
Note that inside a page, objects are allocated in reverse order (due to
how freelists are constructed). We can conclude that the rest of the
linked list is not freed due to a stray reference to the node at 020F1600.
To find the stray reference, we use the "refs" command:
M65024> refs 020F1600 020F1610
00433D2C -> 020F160D - in root range 0042F000 - 00438A00
00433D2C is located within the program's static data segment. Let's see if
it's in the map file:
M65024> symbolat 00433D2C
00433D2C - ___rtl_criticalsection +30
This symbol is from snn.lib, the standard C library.
We can also dump the memory region around that address to examine it:
M65024> dump 00433D00
00433D00: FF FF FF FF 00 00 00 00 00 00 00 00 00 00 00 00 | ....
00433D10: 00 00 00 00 01 16 02 02 03 02 04 18 05 0D 06 09 |
............
00433D20: 07 0C 08 0C 09 0C 0A 07 0B 08 0C 16 0D 16 0F 02 |
................
00433D30: 10 0D 11 12 12 02 21 0D 35 02 41 0D 43 02 50 11 |
......!.5.A.C.P.
00433D40: 52 0D 53 0D 57 16 59 0B 6C 0D 6D 20 70 1C 72 09 | R.S.W.Y.l.m
p.r.
00433D50: 76 16 80 0A 81 0A 82 09 83 16 84 0D 91 29 9E 0D |
v............)..
00433D60: A1 02 A4 0B A7 0D B7 11 CE 02 D7 0B 00 00 00 00 | ............
00433D70: 7F 13 00 00 25 73 3A 20 00 00 00 00 25 73 0A 00 | .. %s:
%s.
00433D80: 4E 6F 20 65 72 72 6F 72 00 4F 70 65 72 61 74 69 | No error
Operati
00433D90: 6F 6E 20 6E 6F 74 20 70 65 72 6D 69 74 74 65 64 | on not
permitted
00433DA0: 00 4E 6F 20 73 75 63 68 20 66 69 6C 65 20 6F 72 | No such
file or
00433DB0: 20 64 69 72 65 63 74 6F 72 79 00 4E 6F 20 73 75 | directory
No su
00433DC0: 63 68 20 70 72 6F 63 65 73 73 00 49 6E 74 65 72 | ch process
Inter
00433DD0: 72 75 70 74 65 64 20 66 75 6E 63 74 69 6F 6E 20 | rupted
function
00433DE0: 63 61 6C 6C 00 49 6E 70 75 74 2F 6F 75 74 70 75 | call
Input/outpu
00433DF0: 74 20 65 72 72 6F 72 00 4E 6F 20 73 75 63 68 20 | t error No
such
=== Conclusion ===
This problem isn't something that can be easily fixed. Any stray reference
to any node in the list will prevent nodes following it from ever being
deallocated. It can be avoided by avoiding memory structures that could be
vulnerable to one stray pointer causing the entire structure "leaking".
For this particular case, the stray reference came from a C library's
static data. Can the D compiler be modified to distinguish between C and D
object files, and only add D object files' data to the list of GC roots?
--
Best regards,
Vladimir mailto:[email protected]
