Hello!
I have noticed that linux discards disk cache when it is not really needed.
Here is vmstat trace:
r b swpd free buff cache si so bi bo in cs us sy id wa st
1 0 150368 56112 46728 358512 0 0 832 1416 17460 18229 14 26 53 6 0
1 1 150360 54284 46940 359472 12 0 672 2760 17488 18465 15 27 53 6 0
0 0 150356 53292 47032 360504 0 0 312 2060 16735 20288 14 27 55 5 0
1 2 150356 51216 47240 362364 0 0 1104 2984 17482 20538 16 27 45 13 0
1 0 150352 49436 47356 364152 4 0 816 1484 17427 18354 15 26 50 8 0
1 1 150344 46252 47628 367136 0 0 1076 1890 17445 21731 15 27 42 16 0
0 0 150344 44268 47752 368916 16 0 776 1772 16639 16545 15 26 51 7 0
0 0 150336 42056 47864 370580 4 0 784 3348 16501 16426 13 23 58 6 0
0 2 150344 66196 46236 347336 0 8 276 3908 16305 17032 13 22 58 7 0
0 0 150420 221500 37140 199684 4 84 976 1592 16626 16129 14 27 50 9 0
1 0 150416 218972 37328 201736 0 0 704 1096 17801 16720 16 28 46 11 0
0 0 152932 251020 34572 171656 4 2528 284 3592 18838 15565 17 27 46 12 0
3 1 153728 258724 33612 165096 4 812 540 5448 17270 17582 15 26 48 11 0
0 0 154300 269484 32716 155428 0 596 1084 2372 17050 14742 15 25 40 22 0
1 0 154300 267348 32860 157520 0 0 1388 1768 17192 13229 17 26 46 10 0
0 4 154256 264840 33024 159896 40 0 1652 1704 17324 17067 18 27 40 16 0
The strange thing is that it had freed 150M of cache and *then* swapped out
another 18M. Why? This happens regularly.
Another vmstat output:
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 152740 198420 32820 217080 52 0 620 852 17606 18243 17 25 50 9 0
0 0 152720 194740 32892 219672 8 0 412 728 18256 19845 17 27 52 4 0
1 1 152712 193352 32964 220948 4 0 692 332 15686 19272 15 31 47 8 0
1 1 168064 240668 27628 178640 508 15804 7036 24344 144163 139061 14 26 45 15 0
2 5 170152 259460 26380 160896 244 2216 1316 11656 19073 16941 15 31 21 34 0
1 6 170584 263084 26096 157132 148 528 1044 31464 17949 16048 14 30 38 18 0
1 1 170996 269560 25312 151652 264 500 936 1280 17594 19059 14 26 23 37 0
It started to swap when there are 190M of free memory! No big processes were
started at the time.
kernel version 2.6.34.7-61.fc13.x86_64 (fedora13).
Here is my /proc/meminfo:
MemTotal: 4023864 kB
MemFree: 132140 kB
Buffers: 38952 kB
Cached: 284568 kB
SwapCached: 46620 kB
Active: 2360084 kB
Inactive: 997628 kB
Active(anon): 2217764 kB
Inactive(anon): 819796 kB
Active(file): 142320 kB
Inactive(file): 177832 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 8388604 kB
SwapFree: 8224428 kB
Dirty: 20416 kB
Writeback: 0 kB
AnonPages: 3010196 kB
Mapped: 5156 kB
Shmem: 3340 kB
Slab: 413676 kB
SReclaimable: 240068 kB
SUnreclaim: 173608 kB
KernelStack: 1392 kB
PageTables: 8488 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 10400536 kB
Committed_AS: 3166880 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 561196 kB
VmallocChunk: 34359125752 kB
HardwareCorrupted: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 9088 kB
DirectMap2M: 4149248 kB
--
Alexander.