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SubjectKernel 2.4 diskcache pushes applications into swap?
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Hi all,

The need for Large File Support drove us to upgrade a dual CPU server to
RedHat 7.1 (kernel 2.4.2-2smp) to get a distribution with kernel 2.4. While
working with 2-4GB files on the system, we found some odd behaviour. The
server is equipped with 512MB RAM and 1GB swap. When copying large files
around, all RAM is consumed by the diskcache which is fine and natural.
Unfortunately the kernel seems very reluctant to release this memory to
applications. After running large file copying operations for a minute or
two, interactive performance of the server is horrible, with a simple
console login taking several minutes.

I looked at top output, and couldn't help but notice that the kernel was
starting to eat into swap. I made a similar test on my laptop (256MB RAM,
128 MB swap) running Mandrake 8.0 (kernel 2.4.3-20mdk).

After boot the machine uses 42MB RAM, and 0B swap.

Running top in one console and "dd if=/dev/hda of=/dev/null" in another, I
observe RAM usage climbing rapidly. It peaks out at about 3MB free RAM, and
then it slowly starts eating into _swap_. After it's been running now for 10
minutes, it's using 5.8MB swap.

Interactive performance is not as bad as on the SMP server, so I'm not sure
I've identified the problem, but I would like to know if this is really the
intended behaviour?

I tried the same dd test on a completely different machine with 64MB RAM and
kernel 2.2.19. On this machine no swap was consumed by the test.

Here's some memory statistics from the laptop while running the dd test, let
me know if you need anything else.

/proc/meminfo:
total: used: free: shared: buffers: cached:
Mem: 261873664 258998272 2875392 0 224747520 4030464
Swap: 139788288 4308992 135479296
MemTotal: 255736 kB
MemFree: 2808 kB
MemShared: 0 kB
Buffers: 219480 kB
Cached: 3936 kB
Active: 222184 kB
Inact_dirty: 260 kB
Inact_clean: 972 kB
Inact_target: 64 kB
HighTotal: 0 kB
HighFree: 0 kB
LowTotal: 255736 kB
LowFree: 2808 kB
SwapTotal: 136512 kB
SwapFree: 132304 kB

/proc/slabinfo:
slabinfo - version: 1.1
kmem_cache 58 78 100 2 2 1
urb_priv 0 0 32 0 0 1
uhci_desc 1038 1062 64 18 18 1
clip_arp_cache 0 0 128 0 0 1
ip_mrt_cache 0 0 96 0 0 1
tcp_tw_bucket 0 0 128 0 0 1
tcp_bind_bucket 1 113 32 1 1 1
tcp_open_request 0 0 96 0 0 1
inet_peer_cache 0 0 64 0 0 1
ip_fib_hash 10 113 32 1 1 1
ip_dst_cache 18 24 160 1 1 1
arp_cache 3 30 128 1 1 1
blkdev_requests 512 520 96 13 13 1
dnotify cache 0 0 20 0 0 1
file lock cache 1 42 92 1 1 1
fasync cache 0 0 16 0 0 1
uid_cache 2 113 32 1 1 1
skbuff_head_cache 140 144 160 6 6 1
sock 17 18 1280 6 6 1
inode_cache 219 536 480 67 67 1
bdev_cache 5 59 64 1 1 1
sigqueue 0 0 132 0 0 1
kiobuf 0 0 128 0 0 1
dentry_cache 208 960 128 32 32 1
dquot 0 0 128 0 0 1
filp 350 360 96 9 9 1
names_cache 0 1 4096 0 1 1
buffer_head 219909 220720 96 5504 5518 1
mm_struct 24 24 160 1 1 1
vm_area_struct 642 708 64 11 12 1
fs_cache 23 59 64 1 1 1
files_cache 23 27 416 3 3 1
signal_act 28 30 1312 10 10 1
size-131072(DMA) 0 0 131072 0 0 32
size-131072 0 0 131072 0 0 32
size-65536(DMA) 0 0 65536 0 0 16
size-65536 0 0 65536 0 0 16
size-32768(DMA) 0 0 32768 0 0 8
size-32768 0 0 32768 0 0 8
size-16384(DMA) 0 0 16384 0 0 4
size-16384 0 0 16384 0 0 4
size-8192(DMA) 0 0 8192 0 0 2
size-8192 0 0 8192 0 0 2
size-4096(DMA) 0 0 4096 0 0 1
size-4096 32 33 4096 32 33 1
size-2048(DMA) 0 0 2048 0 0 1
size-2048 63 64 2048 32 32 1
size-1024(DMA) 0 0 1024 0 0 1
size-1024 30 32 1024 8 8 1
size-512(DMA) 0 0 512 0 0 1
size-512 43 48 512 6 6 1
size-256(DMA) 0 0 256 0 0 1
size-256 81 90 256 6 6 1
size-128(DMA) 0 0 128 0 0 1
size-128 473 480 128 16 16 1
size-64(DMA) 0 0 64 0 0 1
size-64 103 118 64 2 2 1
size-32(DMA) 0 0 32 0 0 1
size-32 1273 1808 32 16 16 1

--
Troels Walsted Hansen, M.Eng. Mail: troels@fast.no
System Architect Web: http://www.fast.no
Fast Search & Transfer ASA Phone: +47 21 60 11 84
P.O. Box 1126 Fax: +47 77 69 66 89
NO-9261 Tromsø, NORWAY Mob: +47 99 25 16 43

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