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SubjectDelaying writes to disk when there's no need
Date
In all kernels I've tested writes to disk are delayed a long time even when
there's no need to do so.

A very simple test shows this: on an otherwise idle system, create a tar of
a NFS-mounted filesystem to a local disk. The kernel starts writing out the
data after 30 seconds, while a slow and steady stream would be much nicer
to the system, I think.

On 2.4.x this can block the system for several seconds. 2.5.6x and
2.5.6x-mm (with AS) also show this behaviour, but the system doesn't block
anymore. I'm using a preemtable kernel.

I only started to notice this behaviour when I upgraded from 256 MB ram to
512 MB. In other words: Linux behaves more nicely with 256 MB.

Attached is a vmstat trace, it starts at the moment the tar process starts
too. It's a simple tar cf /local/test.tar /home (/home being mounted over
NFS).

Further data:
SIS5513: IDE controller at PCI slot 00:02.5
SIS5513: chipset revision 208
SIS5513: not 100% native mode: will probe irqs later
SiS745 ATA 100 controller
ide0: BM-DMA at 0xff00-0xff07, BIOS settings: hda:DMA, hdb:DMA
ide1: BM-DMA at 0xff08-0xff0f, BIOS settings: hdc:DMA, hdd:DMA
hda: MAXTOR 6L080J4, ATA DISK drive

It's an 80 GB ATA 133 drive.

AMD Athlon XP 1800+, 512 MB ram.

Attached vmstat was created on:
Linux bender 2.5.66-mm1 #10 Wed Mar 26 15:16:17 CET 2003 i686 unknown

tar output was going to a logical volume formatted with reiserfs.

procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
5 0 0 0 123720 45068 200044 0 0 9 10 1038 766 73 1 26
0 1 0 0 144640 45488 178568 0 0 420 0 1587 2139 9 15 76
0 1 0 0 137976 45496 184624 0 0 4 0 4543 3415 17 28 54
0 1 0 0 134848 45496 188112 0 0 0 0 2877 2311 21 29 50
0 1 0 0 124992 45500 197848 0 0 0 0 5313 2795 32 59 9
4 1 0 0 110528 45604 212088 0 0 0 496 7460 3510 24 72 4
3 1 0 0 105408 45608 217068 0 0 0 0 3594 2436 20 27 53
6 0 0 0 104192 45608 218160 0 0 0 0 2037 2068 15 18 68
3 0 0 0 99328 45608 222736 0 0 0 0 4064 3484 16 26 57
4 1 0 0 95040 45612 226888 0 0 0 0 3379 2500 24 30 45
5 1 0 0 90752 45656 230936 0 0 0 116 3497 2821 19 21 60
1 1 0 0 85056 45660 236568 0 0 0 0 3744 2369 25 45 30
1 1 0 0 83200 45660 238308 0 0 0 0 2786 3216 40 18 42
3 1 0 0 81400 45660 239876 0 0 0 0 3020 3587 22 21 57
1 1 0 0 71608 45664 249512 0 0 0 0 5742 3378 23 48 29
0 1 1 0 63872 45716 257040 0 0 0 124 5063 3405 25 36 39
2 1 1 0 56064 45720 264752 0 0 0 0 4652 2694 26 52 22
2 1 0 0 47936 45724 272660 0 0 0 0 5443 3850 27 41 32
1 1 0 0 46912 45724 273664 0 0 0 0 2300 4372 66 21 13
3 1 0 0 41728 45728 278732 0 0 0 0 3755 3370 50 37 13
2 1 0 0 36928 45780 283368 0 0 0 124 4549 4094 33 33 35
1 1 1 0 31680 45780 288508 0 0 0 0 4116 2931 23 31 46
0 1 0 0 28672 45780 291436 0 0 0 0 2694 2183 33 33 33
3 1 0 0 24448 45784 295556 0 0 0 0 3152 2322 19 30 51
0 1 0 0 24256 45784 295724 0 0 0 0 1354 1414 6 4 90
4 0 1 0 20800 45824 298184 0 0 0 21120 3267 3374 22 20 57
4 0 1 0 18304 45828 300440 0 0 0 36328 3583 4013 47 53 0
3 0 0 0 16640 45828 302468 0 0 0 0 3594 4084 16 15 69
4 1 1 0 14456 45828 304332 0 0 0 0 3531 3946 23 20 57


--
Erik Hensema <erik@hensema.net>
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