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SubjectSlight Return (was Re: [VM] 2.4.14/15-pre4 too "swap-happy"?)
When updatedb kicked off on my 2.4.16 6-way Xeon 4GB box this morning, I
had an unfortunate flashback:

5:02am up 2 days, 1 min, 59 users, load average: 5.66, 4.86, 3.60
741 processes: 723 sleeping, 4 running, 0 zombie, 14 stopped
CPU states: 0.2% user, 77.3% system, 0.0% nice, 22.3% idle
Mem: 3351664K av, 3346504K used, 5160K free, 0K shrd, 498048K buff
Swap: 1052248K av, 282608K used, 769640K free 2531892K cached

PID USER PRI NI SIZE RSS SHARE STAT LIB %CPU %MEM TIME COMMAND
2117 root 15 5 580 580 408 R N 0 99.9 0.0 17:19 updatedb
2635 kb 12 0 1696 1556 1216 R 0 99.9 0.0 4:16 smbd
2672 root 17 10 4212 4212 492 D N 0 94.7 0.1 1:39 rsync
2609 root 2 -20 1284 1284 672 R < 0 81.2 0.0 4:02 top
9 root 9 0 0 0 0 SW 0 80.7 0.0 42:50 kswapd
22879 kb 9 0 11548 6316 1684 S 0 11.8 0.1 7:33 smbd

Under varied load I'm not seeing the kswapd issue, but it looks like
updatedb combined with one or two samba transfers does still reproduce
the problem easily, and adding rsync or NFS transfers to the mix makes
kswapd peg at 99%.

I noticed because I was trying to do kernel patches and compiles using a
partition NFS-mounted from this machine. I guess it sometimes pays to
be up at 5am...

Unfortunately it's difficult for me to reboot this machine to update the
kernel (59 users) but I will try to reproduce the problem on a separate
machine this weekend or early next week. And I don't have profiling on,
so that will have to wait as well. :-(

Andrea, do you have a patch vs. 2.4.16 of your original solution to this
problem that I could test out? I'd rather just change one thing at a
time rather than switching completely to an -aa kernel.

Grrrr!

Thanks much,
--
Ken.
brownfld@irridia.com


On Tue, Nov 20, 2001 at 06:50:50AM +0000, Linus Torvalds wrote:
| In article <20011119235422.F10597@asooo.flowerfire.com>,
| Ken Brownfield <brownfld@irridia.com> wrote:
| >kswapd goes up to 5-10% CPU (vs 3-6) but it finishes without issue or
| >apparent interactivity problems. I'm keeping it in while( 1 ), but it's
| >been predictable so far.
| >
| >3-10 is a lot better than 99, but is kswapd really going to eat that
| >much CPU in an essentially allocation-less state?
|
| Well, it's obviously not allocation-less: updatedb will really hit on
| the dcache and icache (which are both in the NORMAL zone only, which is
| why Andrea asked for it), and obviously your Oracle load itself seems to
| be happily paging stuff around, which causes a lot of allocations for
| page-ins.
|
| It only _looks_ static, because once you find the proper "balance", the
| VM numbers themselves shouldn't change under a constant load.
|
| We could make kswapd use less CPU time, of course, simply by making the
| actual working processes do more of the work to free memory. The total
| work ends up being the same, though, and the advantage of kswapd is that
| it tends to make the freeing slightly more asynchronous, which helps
| throughput.
|
| The _disadvantage_ of kswapd is that if it goes crazy and uses up all
| CPU time, you get bad results ;)
|
| But it doesn't sound crazy in your load. I'd be happier if the VM took
| less CPU, of course, but for now we seem to be doing ok.
|
| Linus
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