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SubjectRe: dead mem pages

hey Wayne,

thx for the long answer.

On Tue, 10 Jul 2001, Wayne Whitney wrote:

> Hi Dirk,
>
> How's things at Renaissance?

good, currently only with one exception ;-) how is it in Berkeley?

> On Mon, 9 Jul 2001, Dirk Wetter wrote:
>
> > why does the kernel have 2.8GB of cached pages, and our applications
> > have to swap 1.5+1.1GB of pages out?
>
> While I can't answer your question, one thing to know is that "cached"
> does not mean file cache. In fact, there was a thread on LKML on I think
> exactly this in the last couple of days. Anyway, I believe "cached" means
> the page cache, which can include lots of things; I gather it's a general
> kernel memory pool, and that everything in the kernel will be moving to
> the page cache.

> Although with 2.8GB of "cached" memory, alot of that must
> be from I/O.

we do some i/o. but still there shouldn't be do a harm to the OS which is
dedicating or more clearly speaking leaking (because we also don't get
it really back) memory for page- or whatever-cache.

> It would be good to know what these 2.8GB of cached pages are.

believe me, i would like to know too where all the $$$ memory
went to. ;-)

> I believe just for filesystem data and metadata.

(/proc/slabinfo doesn't reveal anything weird in our cases)

> I think
> that ext2, for example, has moved from using buffers for both data and
> metadata to using the pagecache for metadata. Assuming you are using
> ext2, it would be interesting to see what the numbers look like with an
> older kernel that still uses buffers for ext2 metadata (something like
> kernel 2.4.2, I think, it will say in the ChangeLog)--it's vaguely
> possible that a leak was introduced in moving the ext2 metadata into the
> pagecache. Or perhaps the VM balancing on cached filesystem metadata was
> broken when moving ext2 to the pagecache.

the data where the "real" file i/o is coming from are reiserfs partitions
with the exception of one machine (xfs), which is behaving in the same
manner. also, lvm is running under those fs.

> > i don't know how to collect more information, please let me know what
> > i can do in order send more info (.config is CONFIG_HIGHMEM4G=y).
>
> One thing you can do is capture the output of "vmstat 1" from a fresh
> reboot through the machines becoming unusable. Depending on how long it
> takes to fail, you might want to increase the period from 1 second.

kind of that i did. i saw a lot a lot of "so" activity, *if* there was
an output and the machines weren't too busy with swapping.

sar -B on the file from yesterday tells me:

12:00:00 pgpgin/s pgpgout/s activepg inadtypg inaclnpg inatarpg
[...]
14:00:02 0.00 1.02 24003 180843 0 2
14:20:02 0.00 1.03 24003 180846 0 0
14:40:02 0.00 1.07 24003 180848 0 0
15:00:02 0.50 1.31 25132 180885 0 55
15:20:52 317.22 1534.27 466283 29531 287829 6639
15:40:52 213.80 1.97 513702 171317 2360 292
16:02:41 1.54 0.75 590263 32258 88140 323
16:22:41 33.68 1.62 537667 83592 93147 72
16:42:41 5.79 1.55 541736 97796 73838 9
17:00:00 4.92 1.68 523394 166445 21797 44
Average: 46.43 144.11 225926 136494 37807 538

sar -r:

12:00:00 kbmemfree kbmemused %memused kbmemshrd kbbuffers kbcached kbswpfree kbswpused %swpused
[..]
14:00:02 3157092 901036 22.20 0 62400 756984 14337736 0 0.00
14:20:02 3157464 900664 22.19 0 62400 756996 14337736 0 0.00
14:40:02 3157452 900676 22.19 0 62400 757004 14337736 0 0.00
15:00:02 3141724 916404 22.58 0 62400 761668 14337736 0 0.00
15:20:52 4704 4053424 99.88 0 2848 3131724 10966232 3371504 23.51
15:40:52 18580 4039548 99.54 0 3236 2746280 10959228 3378508 23.56
16:02:41 43060 4015068 98.94 0 3240 2839404 10825980 3511756 24.49
16:22:41 44640 4013488 98.90 0 3240 2854384 10810852 3526884 24.60
16:42:41 40352 4017776 99.01 0 3248 2850232 10810852 3526884 24.60
17:00:00 45088 4013040 98.89 0 3260 2843284 10811436 3526300 24.59
Average: 1950288 2107840 51.94 0 38673 1562233 12948280 1389456 9.69

so the machine was basically memorywise ok, upto the point when the
jobs were submitted (15:20), which used up all the userspace memory
(kbmemused=4GB), but simultaneously the kernel claimed more pages for
it's cache (kbcached=3.13GB), so the machine had to start swapping
heavily (kbswpused=3.3GB). and there is exactly my point: why didn't
my app get the memory, but the kernel?

> On a general note, you can try the -ac series of kernels, but I don't
> believe that they have any significant VM changes.

ac is up to -ac2 for 2.4.6. i don't see either that my points are
addressed there.

> Again on a general note, the 2.4 kernel's VM is new and hence not fully
> mature. So the short and unhelpful answer to your query is probably that
> the current VM system is not well tuned for your workload (4.3GB of memory
> hungry simulations on a 4GB machine).

concerning the maturity that's also the answer i got from the kernel
guru's at last USENIX in boston. but ihmo it *should* become soon
better for the future if Linux intends to become bigger in the server
business. (my $0.02)

> Will the simulations still run if you ulimit them to 1.75GB? If so, it
> would be interesting to know, for another data point, how things behave
> with 2 1.75GB processes.

with one process i get:

1:49pm up 20:16, 2 users, load average: 1.13, 1.02, 0.97
55 processes: 52 sleeping, 2 running, 1 zombie, 0 stopped
CPU0 states: 99.0% user, 0.1% system, 99.0% nice, 0.0% idle
CPU1 states: 0.1% user, 4.0% system, 0.0% nice, 94.0% idle
Mem: 4058128K av, 4012536K used, 45592K free, 0K shrd, 3644K buff
Swap: 14337736K av, 1870764K used, 12466972K free 3510436K cached

PID USER PRI NI SIZE SWAP RSS SHARE D STAT %CPU %MEM TIME COMMAND
5747 uid1 18 4 2290M 1.4G 864M 414M 0M R N 99.9 21.8 709:42 ceqsim
13297 uid2 14 0 1048 0 1048 820 56 R 5.6 0.0 0:00 top
1 root 8 0 76 12 64 64 4 S 0.0 0.0 0:01 init
2 root 9 0 0 0 0 0 0 SW 0.0 0.0 0:00 keventd
3 root 9 0 0 0 0 0 0 SW 0.0 0.0 0:09 kswapd
4 root 9 0 0 0 0 0 0 SW 0.0 0.0 0:00 kreclaimd
5 root 9 0 0 0 0 0 0 SW 0.0 0.0 0:00 bdflush
6 root 9 0 0 0 0 0 0 SW 0.0 0.0 0:01 kupdated
7 root 9 0 0 0 0 0 0 SW 0.0 0.0 0:00 scsi_eh_0

the of total of VSZ + RSS (in ps alx:) 2520012 899304

cat /proc/meminfo

MemTotal: 4058128 kB
MemFree: 45628 kB
MemShared: 0 kB
Buffers: 3644 kB
Cached: 3510712 kB
Active: 1824092 kB
Inact_dirty: 1523012 kB
Inact_clean: 167252 kB
Inact_target: 1076 kB
HighTotal: 3211200 kB
HighFree: 13532 kB
LowTotal: 846928 kB
LowFree: 32096 kB
SwapTotal: 14337736 kB
SwapFree: 12466972 kB

so again, top shows me that 1.4GB are swapped out, so does /proc/swaps:

Filename Type Size Used Priority
/dev/sda5 partition 2048248 1870764 -1
/dev/sdb1 partition 2048248 0 -2
[..]

and this looks also like the same bug to me.

> Well, hope this helps.

a little bit, yes. thanks!


any official word from a guru?


~dirkw




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