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SubjectHow to tune swapping in 2.4.21?
Hi,

on a i386 SMP server with 2.5GB RAM I'm seeing permant swapping activity
during times with a lot of IO (many concurrent processes accessing the
RAID quite randomly). This is the output of "vmstat 10":

procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 2 0 57108 9572 386592 1517792 0 0 0 0 3 0 1 3 2
1 2 0 57092 11504 380084 1526244 0 0 3982 816 1610 1434 10 5 85
0 0 0 57084 9728 379964 1514116 0 0 4376 755 1801 1615 13 7 80
2 5 0 57044 10736 381272 1513240 2 0 4317 284 1565 1354 14 7 80
8 3 0 57012 8492 383244 1498728 45 0 4166 828 1795 1738 15 8 77
1 1 0 56180 9928 388664 1498920 74 0 6695 770 2136 2239 35 17 49
0 3 0 56060 7736 393356 1507672 142 0 2922 752 2529 2478 21 10 69
1 3 0 56092 8560 394020 1515752 216 0 5031 531 1867 1686 12 6 82
5 2 0 56684 9452 395332 1520212 114 4 3974 357 1764 1538 10 5 84
2 0 0 57172 9168 390048 1524348 1 9 4027 1290 1669 1546 14 7 79
[...]
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
1 0 0 57172 9592 390064 1524364 0 0 0 0 3 0 1 3 2
1 1 0 51884 8712 391392 1531232 35 3 2177 630 1554 1373 9 5 85
1 3 0 51152 8836 389544 1528080 35 7 2497 538 2080 1836 10 6 84
1 1 0 51256 11776 387724 1522688 0 10 5100 763 1980 1886 12 6 82
2 12 2 49724 9912 388968 1489428 13 22 2673 537 1838 1711 22 10 67
0 1 0 49836 8488 394824 1494352 12 21 6845 1020 2287 2190 21 9 69
0 0 0 49940 10304 396400 1491932 0 17 3690 545 1674 1507 12 6 82
2 3 0 50932 9664 392576 1472600 0 22 4846 352 1668 1660 24 10 66
1 1 0 50896 15596 398612 1485900 0 3 4356 1103 2073 1903 19 10 72
0 0 0 51004 8588 399684 1484036 145 135 3866 507 1796 1743 10 4 86

As you see, there's a large amount of cache available but the kernel decides
to swap in and out all the time.

I'd rather prefer to avoid swapping since the swap partition is on the same
RAID system as part of the application data. So valuable disk performance
is eaten by needless swapping.

Are there any parameters for tuning/optimizing the kernel's bevahiour
regarding this? /proc/sys/vm/kswapd seems to be only a fake interface
(documented, implemented, but the values contained therein aren't used
inside the kernel). Kernel 2.6 seems to add a parameter called "swappiness"
but is there a 2.4 counterpart? Or is there a backport to 2.4?

Any hints how to change this behaviour are appreciated :-)

Sven
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