Messages in this thread | | | Date | Wed, 16 Sep 1998 12:09:02 -0700 (PDT) | From | Simon Kirby <> | Subject | Re: Very bad swap bug -- 2.0, 2.1 at least |
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On Wed, 16 Sep 1998, Rik van Riel wrote:
> On Tue, 15 Sep 1998, Simon Kirby wrote: > > > This swap bug that I mentioned a while back is still happening, and this > > time seems to be much worse than before. In this particular case it is > > happening on a medium-loaded web server running 2.0.35. > [SNIP] > > 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 5376 18236 10200 52244 380 0 174 0 292 305 2 9 89 > [SNIP 13 lines] > > 0 0 0 5376 16028 10200 53596 264 0 125 0 275 471 13 9 79 > > Strange... There is more free memory than there is stuff to > swap in. This does indeed suggest a bug, unless of course > you use perl for web forms (4 simultaneous forks which glob > the machine will have left some 16 megs.
Some large perl script is probably what cleared the 16MB free. Still, it wasn't executing when I was running the vmstat (I was also running a "top"...).
> OTOH, if you have the values for freepages (/proc/sys/vm/freepages) > set to a rediculously high value, the swapping and paging might be > for real. Setting the values in this file to something like > "128 256 384" will leave you with some 1.5 MB of free memory > (which is a must when heavy forking is going on).
"For real"? I'm very sure it is actually reading from disk -- I can hear the drive working every second, and if I run my "stail" (continously-updating cat) program on the /proc/scsi/aic7xxx/0 file, I can see the disk being read as well.
[sroot@elmo:/root]# cat /proc/sys/vm/freepages 280 420 560
This is high, I know...should I try it lower?
> > As you can see, the amount of stuff swapped out is staying the same, but > > it is constantly "swapping in" something. There is obviously a lot of > > memory available to have this data swapped back in and out of swap space, > > but it isn't happening. > > The MM code in 2.0 was mostly introduced in the 1.2 (!) > days and is very well tested. Apart from the freepages > thingy or some hardware fault (very improbably, but then > again you're the only one to ever report this) I can't > think of many other errors. > > Does the error also occur with a different kernel? Or > with a kernel compiled with a different compiler?
All kernels compiled with 2.7.2.3. It seems to be happening on 2.0.33 and 2.0.55, and even on 2.1.120. I will try to write a C program that will reproduce it.
> > >From the tests I did before, it seems what is being "swapped in" is the > > executable code of a program which has been previously swapped out. I had > > Executable code is mmap()ed from the executable file, and is > not swapped in. Faulting pages from executables are shown with > the PageIn (pi) stats...
"pi"? This does not appear to be in vmstats...Where is it?
> > Issuing a "swapoff -a ; swapon -a" causes the program to go away, until > > something else forces some programs out to swap. (In this case, it seems > > to be some customer's run-away CGI script). > > Running those is always a bad idea, but the system should be > resistant against that... > > You might want to try 2.1.117 (running stably here) if > nobody has had very bad experiences with that version... > Or maybe the 2.0.36 prepatch (that one should be stable > too).
I believe 2.1.117 was also having the same problem. We're using 2.1 on a web statistics compilation server due to how much faster it runs, and I also noticed the swap thing happening on it. I believe this was with 2.1.117 when I noticed it for the first time.
Thanks for your help,
Simon-
| Simon Kirby | Systems Administration | | mailto:sim@netnation.com | NetNation Communications | | http://www.netnation.com/ | Tech: (604) 684-6892 |
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