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SubjectSubtle MM bug
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I'm trying to get more familiar with the MM code in 2.4.0, as can be
seen from lots of questions I have on the subject. I discovered nasty
mm behaviour under even moderate load (2.2 didn't have troubles).

Things go berzerk if you have one big process whose working set is
around your physical memory size. Typical memory hoggers are good
enough to trigger the bad behaviour. Final effect is that physical
memory gets extremely flooded with the swap cache pages and at the
same time the system absorbs ridiculous amount of the swap space.
xmem is as usual very good at detecting this and you just need to
press Alt-SysReq-M to see that most of the memory (e.g. 90%) is
populated with the swap cache pages.

For instance on my 192MB configuration, firing up the hogmem program
which allocates let's say 170MB of memory and dirties it leads to
215MB of swap used. vmstat 1 shows that the pagecache size is
constantly growing - that is swapcache enlarging in fact - during the
second pass of the hogmem program.

...
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 1 1 131488 1592 400 62384 4172 5188 1092 1298 353 1447 2 4 94
0 1 1 136584 1592 400 67428 5860 4104 1465 1034 322 1327 3 3 93
0 1 1 141668 1592 388 72536 5504 4420 1376 1106 323 1423 1 3 95
0 1 1 146724 1592 380 77592 5996 4236 1499 1060 335 1096 2 3 94
0 1 1 151876 1600 320 82764 6264 3712 1566 936 327 1226 3 4 93
0 1 1 157016 1600 320 87908 5284 4268 1321 1068 315 1248 1 2 96
1 0 0 157016 1600 308 87792 1836 5168 459 1293 281 1324 3 3 94
0 1 0 162204 1600 304 92892 7784 5236 1946 1315 385 1353 3 5 92
0 1 0 167216 1600 304 97780 3496 5016 874 1256 301 1222 0 2 97
0 1 1 177904 1608 284 108276 5160 5168 1290 1300 330 1453 1 4 94
0 1 2 182008 1588 288 112264 4936 3344 1268 838 293 801 2 3 95
0 2 1 183620 1588 260 114012 3064 1756 830 445 290 846 0 15 85
0 2 2 185384 1596 180 115864 2320 2620 635 658 285 722 1 29 70
0 3 2 187528 1592 220 117892 2488 2224 657 557 273 754 3 30 67
0 4 1 190512 1592 236 120772 2524 3012 725 760 343 1080 1 14 85
0 4 1 195780 1592 240 125868 2336 5316 613 1331 381 1624 2 2 96
1 0 1 200992 1592 248 131052 2080 2176 623 552 234 1044 3 23 74
0 1 0 200996 1592 252 130948 2208 3048 580 762 256 1065 10 10 80
0 1 1 206240 1592 252 136076 2988 5252 760 1314 309 1406 7 4 8
0 2 1 211408 1592 256 141080 5424 5180 1389 1303 395 1885 3 5 91
0 2 0 214744 1592 264 144280 4756 3328 1223 834 327 1211 1 5 95
1 0 0 214868 1592 244 144468 4344 5148 1087 1295 303 1189 11 2 86
0 1 1 214900 1592 248 144496 4360 3244 1098 812 318 1467 7 4 89
0 1 1 214916 1592 248 144520 4280 3452 1070 865 336 1602 3 3 94
0 1 1 214964 1592 248 144580 4972 4184 1243 1054 368 1620 3 5 92
0 2 2 214956 1592 272 144548 3700 4544 1081 1142 665 2952 1 1 98
0 1 0 214992 1592 272 144588 1220 5088 305 1274 282 1363 1 4 95
0 1 1 215012 1592 272 144600 3640 4420 910 1106 325 1579 3 2 9

Any thoughts on this?
--
Zlatko
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